### Oscilloscopes Blog

September 1, 2016

# Advanced Oscilloscope Measurements and Analysis

Posted by mike1305 Sep 1, 2016

Modern digital oscilloscopes are so capable it’s often impossible to use every feature available. But that doesn’t mean that you shouldn’t be aware of what your scope is capable of. As my father used to say, “can’t hurt, might help”. Today’s post is a preview of a webcast posted on our YouTube channel, the link can be found at the end of the article. The topic: advanced measurements and analysis using a Keysight InfiniiVision oscilloscope.

Measurements are critical to any oscilloscope user who is looking to analyze on their device under test. Whether it’s characterizing a functional design or characterizing bugs and glitches, the measurements menu is something with a lot of functionality. Additionally, our current oscilloscopes have dozens of features that are not traditionally found or expected from an oscilloscope – so we’ll review those as well.

Measurements

There are dozens of automated measurements included in Keysight InfiniiVision oscilloscopes, organized by type of measurement: voltage, time, mixed, and counting. Many of these measurements are tightly related – for example, peak-to-peak is a function of maximum and minimum, while amplitude is a function of base and top. While these may seem like identical measurements (isn’t peak-to-peak and amplitude the same thing?), our handy help text can define each one for you. Press and hold on any selected measurements and a diagram like this will appear. The webcast details each measurement and how it is made, for all four measurement types.

Math Functions

Measuring a signal as it exists is important, but what if you want to modify a signal? Examples of this might include “what would this signal pair look like after being passed through a differential amplifier?” or “what if I added a 5 MHz low pass filter to the circuit?” Our oscilloscopes have up to 27 math functions designed to manipulate signals in software to simulate many physical circuits. There are operators, transforms, filters, and visualizations.

Depending on the oscilloscope model, up to 4 of these can be displayed on screen simultaneously, and can save you significant development time. The subtract operator can be used on two analog channels to display the differential output, while the low pass filter can be used to emulate a 5 MHz filter on your signal, all without having to develop anything in hardware! The image below shows the simulated output of a signal through a 5 MHz low pass filter.

Analysis Tools

Some of the most powerful tools in the oscilloscope live in the analysis menu. Things like histograms, mask testing, eye diagrams, and frequency counters come as options in many models. While advanced triggers and measurements can help with isolating and characterizing a signal, the analysis tools in the oscilloscope are part of the final step of troubleshooting, which is root cause analysis. As an example, histograms can be used to visualize the distribution of measurements or waveforms. In this screenshot, we are looking at the distribution of period measurements over the course of a few minutes on a clock signal with some timing jitter. The shape of the histogram tells us the signal has periodic jitter due to its bimodal shape. Learn more about jitter here!

Summary

After exploring each measurement, math function, and analysis tool, we take some time at the end of the webcast to show you real world examples of how all these functions can be combined to solve problems. The core purpose of an oscilloscope is to help you identify, isolate, and determine the root cause of physical layer issues in your design, and we sincerely believe that our scopes can help you do that easier and faster than anything out there! Check out the Advanced Oscilloscope Measurements and Analysis webcast video .

Thanks and happy watching!

# Cool Oscilloscope Software You Might Not Know Exists

Posted by rlashlee Sep 1, 2016

With all of the recent advances in oscilloscope capability, it can be hard to keep up with the latest features and enhancements available. However, what you don’t know may hurt you since many of these were developed to make your job easier. Two recent examples of these types of enhancements are offline analysis software and MultiScope software.

Offline Analysis

Keysight engineers work closely with their fellow engineers to understand their needs and what makes their job difficult. This helps them decide what to work on next to help Keysight oscilloscope users like you. One example of this is the N8900A Infiniium Offline Oscilloscope Analysis Software. When the Keysight development team spoke to engineers, they heard three main reasons for wanting the ability to analyze data captured by an oscilloscope offline:

• Documentation – The number one thing we heard from customers was the need for a more efficient documentation process. Many of them were fine in terms of using the oscilloscope and making measurements, but found documenting on the oscilloscope painful. It usually involved taking a bunch of screenshots, saving them off somewhere, importing them into a word processor, and then taking notes. The top request from these customers was to make offline software a tool people could use to document more quickly and easily. So, when Keysight engineers designed the N8900A offline software, they worked to accomplish just this and included many features that make documentation a breeze.
• Time Savings – Oscilloscopes are expensive pieces of equipment, so the ability to use the oscilloscope just to capture data and then freeing it up for another team member while the data is analyzed on a PC or laptop saves a lot of time and makes the group more efficient. Plus, it can be easier to do all of your analysis from your laptop or PC instead of your oscilloscope.
• Team collaboration – Teams that are geographically dispersed can find it difficult to share data. With offline software, one person can easily capture the data and then send it to anyone else who has the offline software so they see the exact same data, measurements, etc. Additionally, the offline software enables bookmarks and annotations that can be used to point to specific parts of the waveform as you share the data.

The Keysight N8900A offline software gives you the entire oscilloscope’s GUI on your PC, allowing you to easily analyze, document, or share data from the comfort of your laptop or PC. If you have not heard of this capability before, I highly encourage you to look into it. Learn more about how offline software can help you with your job in this application note: Three Reasons to Complement Your Scope Investment with PC-based Analysis Software.

MultiScope Software

Another request Keysight’s engineers heard was the need to use more than four channels with an oscilloscope. For example, engineers or technicians working on three phase power required more than four channels, but the oscilloscope industry lacked this capability. Two approaches emerged in the recent past. One approach was to create 8 channel oscilloscopes. The second one was to create oscilloscope software, known as MultiScope, where you could connect multiple oscilloscopes together. Keysight opted for the second approach for several reasons.

1. We found that while engineers wanted the capability to use more than four channels, they did not always need this many channels and would often prefer the flexibility to bring oscilloscopes together when they needed them and split them apart when they did not. This allows better use of their equipment.
2. Many customers needed even more than 8 channels and using MultiScope enables them to connect together up to 40 channels.

In other words, MultiScope software offers an engineer or technician more flexibility in terms of how their equipment is used. They can bring the scopes together when they need more than four channels (up to 40) and split them apart when multiples people need an oscilloscope.

After talking to many engineers, the Keysight N8834A MutiScope Application was developed. This software enables you to connect up to 10 oscilloscopes with any combination of 2000X, 3000T, 3000A, 4000X, 6000X, 7000B, 9000A, S-Series, 90000A, 90000X, Q, V, or Z-Series. The signals can be viewed on a single display either using the offline software described above or on the leader scope. This can be an incredibly powerful tool if you need more than four channels.

Summary

It is extremely useful to stay up-to-date on the latest capabilities of any test and measurement gear. Our engineers are constantly talking to fellow engineers regarding how we can make their lives easier or enable measurements not previously possible. Having access to this software, measurement, feature, or enhancement can enable you to finish your job quicker or perform it better. We will continue to update you on this blog when exciting new capabilities are available, and feel free to comment with your suggestions or feedback!

# High Bandwidth Oscilloscope + Analysis Software = Powerful, Wideband RF Measurement Suite

Posted by KeysightOscilloscopes Sep 1, 2016

A change is coming in the tools for measurements in both pulsed RF aerospace/defense and I/Q vector-modulated communications application. Whether for multi-channel analysis or for wider analysis bandwidth, high-bandwidth oscilloscopes are taking the place of traditional spectrum and signal analyzers. That’s because they can handle signals with spectral content beyond 1 or 2 GHz. These signals are being created to support the higher resolution requirements in radar systems and move the vast amounts of information in new communications systems.

So how do you create a powerful, wideband RF measurement suite? By coupling a high-bandwidth real-time oscilloscope with RF analysis software. Once you’ve married the two, you achieve a number of enhancements:

• Noise reduction through digital down-conversion
• A wide range of vertical scaling options, including linear and log magnitude
• Key RF measurements including occupied bandwidth (OBW) and power spectral density (PSD)
• Vector demodulation options for communications formats like QAM16
• Analog demodulation options including AM, FM and PM
• Set-up of segmented memory capture
• Statistical pulse analysis

Pulse amplitude, frequency, phase, and FFT measurements

For radar and electronic warfare applications, it’s helpful to perform a variety of measurements on many pulses. This includes things like amplitude variation, frequency, and phase shift across pulses, and a view of the spectrum of signals. For applications such as aircraft warning receivers, you also want the capability to measure time difference and phase difference between pulses associated with the capture of a wave front by multiple antennas on an aircraft. Let’s consider some of these measurements.

In the simplest case, you can measure the basic pulse amplitude, frequency shift, and phase shift across the measured RF pulse. The RF pulse train is sampled by the oscilloscope and then digitally down-converted to reduce noise and allow further signal processing.

For example, in Figure 1, a 15-GHz carrier, 2-GHz-wide linear FM chirped RF pulse signal is shown after vector signal analysis (VSA) processing. Here’s what the image shows:

• 2 GHz-wide spectral content of the signal (upper left);
• Real part of the down-converted I/Q data (lower left);
• 2-GHz-wide linear FM frequency chirp seen across the RF pulse (upper right);
• parabolic phase shift seen across the RF pulse (lower right).

These measurements are taken in the “Vector” measurement mode.

Basic vector mode analysis of FFT, real part of I/Q, FM chirp, and phase shift across pulse seen

Single channel, segmented memory capture, statistical RF pulse analysis

The next level of analysis requires a shift into “Pulse Analysis” mode. Here we use multiple oscilloscope channels to capture RF pulse signals into segments of oscilloscope memory. These are digitally down-converted into baseband I/Q signals, and then evaluated for single and multiple channel pulse analysis. For single-channel measurement, you can make three comparisons:

• the linear FM frequency chirp to an ideal, best-fit linear FM chirp signal;
• the phase shift across a pulse to a best-fit parabolic phase shift profile;
• the amplitude of the pulse envelope to a best fit ideal straight-line best fit reference.

In Figure 2, you’ll see these comparisons being made between measured to reference, and then the “error” between the measured and reference is expanded in vertical scale for a close view.

A pulse table also displays RF pulse parameters, including an RMS error calculation between the measured frequency or phase across the pulse, compared to a best-fit reference signal. It’s also possible to show statistics for the measurements over all the pulses.

Single-channel spectrum, amplitude, phase, and frequency measurements vs. best-fit reference signals

Dual-channel delta pulse amplitude, frequency, and time-delay measurements

You can also make “two-channel delta” measurements, as shown in Figure 3. These measurements are becoming increasingly important in applications such as aircraft warning receiver testing, where multiple signals are being captured from multiple antennas. The time delay and frequency difference of arrival between wave fronts must be measured for angle-of-arrival calculations.

Notice in this example a 1 nsec time delay being measured between two RF pulses. You’ll also see a 0.2-dB difference in amplitude and a 16-kHz difference in frequency, on average.

Pulse analysis is also performed on three of ten captured pulses that are being placed into oscilloscope memory segments. The parabolic phase shift across pulses (lower left), the linear FM chirp frequency shift across pulses (middle right), and the pulse envelope of pulses (upper left) are superimposed for signals coming into two oscilloscope channels. As in the previous example, each scope-channel measured signal can have the measured, reference, and error signal calculations made. Finally, the FFT spectral content for both scope-channel captures of the two pulse trains (center left) is also shown.

Two-channel measurements of RF pulse characteristics including time, amplitude, and frequency difference between two channels

Cross-correlation between pulses for precise time-delay measurements between RF pulses

In the aircraft warning receiver example mentioned previously, you can determine very precise measurements of time delay between RF pulses captured on different antennas on an aircraft by using a cross-correlation measurement between pulses. In Figure 4, a 50-psec time difference of arrival (TDOA) is being measured between two RF pulses captured on two scope input channels. Here pulses have a 10-GHz carrier, 100-MHz-wide linear FM chirp modulation, and a 1-usec width. In this measurement, you can first remove the channel-to-channel skew between oscilloscope channels, including cable delays at the temperature measurements will be taken, through de-embedding. Then a measurement can be made to see the actual time shift between the captured signals. Measurements show a mean delay of 50 psec, with a peak-to-peak variation in delay between 47 psec and 53 psec.

Two-channel cross-correlation measurement for precise time delay between pulses

Math function used to measure phase shift between two RF pulses

The difference in phase between two RF pulses is also critical in a variety of radar/EW/warning receiver-oriented applications. Through the use of math functions, the measured phase across one pulse can be subtracted from the measured phase across a second pulse, measured on two oscilloscope input channels. We can measure the same two linear FM chirp signals from the last example to view the phase shift between the two pulse trains by comparing related pulses. Again this might be seen from two antennas on an aircraft. The time shift has now been set to zero on an arbitrary waveform generator, but a 25-degree phase shift is being introduced between the two signals. A capture shown in Figure 5, top center trace C, and related blue marker 1, show this 25-degree phase shift in a mean measurement in lower right Trace D, as well as only a 0.8-degree standard deviation and a 0.7 variance.  These are average values over the width of the pulses.

Two-channel phase difference measurement between two RF pulses

Summary

More radar/EW/warning receiver applications are driving toward wider modulation bandwidths to increase range and angle-of-arrival precision capability in related systems. At times, this extends beyond 1-GHz modulation bandwidths. Designers increasingly use wideband oscilloscopes as RF receivers to evaluate related wideband signals when validating their hardware prototypes. Although scope measurements directly are of interest, it’s often advantageous to use analysis software to digitally down-convert captured wideband signals to reduce noise and allow more in-depth analysis of baseband I/Q signals. By combining a wideband scope and VSA software with appropriate techniques, you can readily make angle-of-arrival calculations for a variety of systems.

# Fourier Series of a Square Wave (and Why Bandwidth Matters)

Posted by KeysightOscilloscopes Sep 1, 2016

By: Drew Hanken

Have you ever wondered, “What is this ringing I am seeing in my signal? Why is there preshoot and overshoot on a simple square wave? How is a preshoot possible when it appears to be precausal to downstream information?”

I have been an R&D engineer here at Keysight/Agilent Technologies for the last five years. Fourier series is a topic that was covered in a recent graduate class as a method for solving partial differential equations. I’ll explain the occurrence of this ringing from the perspective of the underlying theory, and then relate it back to using an oscilloscope.

In short, this ringing is a phenomenon that presents itself because of the method an oscilloscope uses to construct a signal – by summing the frequency components of the signal. This method of building a signal is known as Fourier series. It is the most useful way for an oscilloscope to process and measure a signal because it deconstructs the signal into its frequency components for analysis. The inherent discontinuity of a square wave presents some problems with this reconstruction method that can be understood by exploring the mathematical theory. Understanding this can help you to select the right oscilloscope for your measurement needs. It’s also worth noting that this method is used by all oscilloscopes, it’s not just a Keysight method.

First off, you need to understand that any arbitrary function f(x) can be constructed by the sum of simple sine or cosine functions that vary in amplitude and frequency. More specifically, any function of x, f(x), can be built using an infinite series of sine waves with coefficients An and increasing frequencies (n*pi).

Sine and cosine functions can both be used for Fourier series because they both have the property of orthogonality. There are a few other functions that satisfy the orthogonality requirement including Bessel functions and Legendre polynomials. While these functions are useful for problems in cylindrical or polar coordinates, they do not apply to this discussion. The definition of orthogonality for a function φ is that it must satisfy the following condition:

Using the above definition, it is possible to solve for the coefficients (An) for any function, and build the function, using orthogonal functions.

You can see that any function can be constructed using an infinite series of terms, and approximated by a finite number of sine waves. Every function has a unique set of coefficients, An, that are substituted into the sum. The values of these coefficients determine the function that will be reconstructed.  It is easy to picture changing the amplitude of a sine wave by multiplying it by a coefficient. This would be a Fourier series with only one term, and would return the desired function with the magnitude changed. Let’s look at constructing a linear line using sine and cosine functions. This is tougher to picture because a line is not oscillatory, but the addition of multiple sine or cosine terms will begin to take the shape of a line.

You can see that the more terms that are summed, the closer the approximation becomes to the actual function. Theoretically, if an infinite number or terms are used, the Fourier series will cease to be an approximation and take the exact shape of the function.

Now, let’s take a look at a square wave and how it appears when constructed using Fourier series the same way an oscilloscope would. We will first write a step function of length (L) that, when repeated periodically, will be our representation of a square wave.

As stated earlier, this function can be rewritten as an infinite series of an orthogonal function φ:

With choosing a sine wave as the orthogonal function in the above expression, all that is left is to solve for the coefficients to construct a square wave and plot the results.

One important takeaway from this formula is that the series composition of a square wave only uses the odd harmonics. This stems from the fact that a square wave is an odd function, which has important implications on measuring signals of this sort. Given a 1 Gb/s square wave, the bandwidth of the measurement device must now exceed 3 Gb/s to capture more information than the primary frequency. Each incremental harmonic that is captured will start to look more like a square wave with faster rising and falling edges as seen below.

The rise time of the plotted signals gets smaller as the number of terms increases. The highest term in the Fourier series will correspond to the highest frequency that is used to construct the signal. Thus, the rise time is dictated by this last term, which in turn dictates highest frequency. An ideal square wave will have a zero rise time – but that would take infinite bandwidth to reproduce with this method. This square wave’s discontinuity is the heart of the problem, and is the reason for the preshoot and overshoot seen above. Taking a closer look at these areas of the wave, you can see that the ringing in the signal does not change in magnitude with the number of terms and remains at roughly 18% of the amplitude. It will, however get thinner as more terms are used and be a smaller source of error. This ringing caused by a discontinuity is referred to as Gibbs Phenomenon, and is unavoidable when the signal is properly constructed using Fourier series. That is not to say that other sources of ringing are not present, but it is important to be aware of this behavior at discontinuities.

Given this information, you can see why it is important to select the right oscilloscope based on your measurement needs. If you need to capture the slew rate of your transceiver, or to open the area of your eye diagram, you may need a higher bandwidth oscilloscope to capture the frequency content of the higher order harmonics and to reduce the effects of ringing on your capture.

# Take Your Measurements and Analysis to the Next Level with New Features

Posted by melissakeysight Sep 1, 2016

Infiniium Oscilloscopes 5.60 Software Release

The latest Keysight Infiniium oscilloscope software release, version 5.60, offers new capabilities and enhancements that make measuring and analyzing your designs easier and more comprehensive than ever before.  With additions like a new crosstalk app, bit error rate measurements for PAM-4 signals, enhancements to MultiScope, and eye contour software, you will get even more insight to your designs.  Check out what’s new with the Infiniium 5.60 release:

Crosstalk Analysis Application (N8833A/B)

Figure 1 – Keysight Crosstalk Analysis

The Keysight Crosstalk analysis application is the industry’s first and only application to measure and analyze crosstalk.  It allows you to probe up to four signals on your board at once – this means one victim (the signal of interest) and three aggressors (signals that could be causing crosstalk on the victim).  The application can be used to analyze both NEXT (near end crosstalk) and FEXT (far end crosstalk).  With the crosstalk app, you can measure the amount of crosstalk appearing on the victim and even remove the crosstalk from measurement.  This can help you decide whether or not it is worth redesigning your board to remove the crosstalk.

New Bit Error Rate (BER) Measurements for the PAM-4 Measurement Application (N8836A)

Figure 2 – Cumulative BER measurement setup

The addition of BER measurements to the PAM-4 measurement application software can help you quickly identify and analyze design flaws in your digital systems.   With this functionality you can measure the BER across multiple acquisitions, similar to the standard statistical BER measurement you would get from a BERT.  This is helpful for determining if your design passes a required specification.

Figure 3 – Per Acquisition BER measurement setup

In addition to this cumulative BER measurement, the oscilloscope can also measure BER per acquisition. In this mode you can look at how many errors happen in a single acquisition.  If you also have InfiniiScan and EZJIT on your oscilloscope, you can even graphically display where the errors are happening in the signal. This is helpful for understanding the errors at a more detailed level than a cumulative measurement.

For example, let’s say the cumulative measurement indicates the BER is 10e-5 and that passes your design specification. But when looking at the per acquisition measurement, you find that all the errors are happening in quick succession. This could be bad news.  With four different levels in a limited amplitude range, it is likely that a bit may be lost here and there, and forward error correction (FEC) algorithms can correct standalone, faulty bits.  But when you see several errors all together – what we call burst errors – your FEC may no longer be able to handle this.  So the design has passed the general specification, but is still faulty.  Without this type of analysis tool, it would be much harder to debug such a situation.

Keysight N8834A MultiScope Enhancements

Figure 4 – MultiScope

The MultiScope application gives you visibility of up to 40 channels simultaneously.  You can connect 2, 5, or 10 scopes together.  This is especially helpful for power system designers, or anyone with a need to look at more than four analog channels at once.

Using MultiScope with the Infiniium 5.60 update, you can see all of your signals on and work directly from the leader oscilloscope. This release also extends the triggering capabilities available in MultiScope to the oscilloscope’s full range of triggering capabilities when triggering from the leader scope.

Eye Contour Software added to EZJIT Complete (N8823A)

Figure 5 – Eye Contours

Reduce your test time by using the Keysight eye contour software on any digital signal.  Originally designed for DDR4, Keysight is the only company whose eye contour algorithm has been approved by JEDEC.  This software extrapolates noise and jitter trends from the measured signal to predict how they eye will close over time, eliminating the need to run the eye test for days or weeks to find out.

Figure 6 – Custom Mask Editor

Looking for a glitch? With the new custom mask editor, it is quick and easy to set up a custom mask test.  The editor provides fifteen points to drag and drop on the screen, allowing you to design the mask test you want in seconds.

Save Time with the Easy Analysis Gallery

Figure 7 – Analysis Gallery

Not sure where to find the measurement you’re looking for?  Check the analysis gallery.  It is a one-stop-shop of analysis options and measurements, represented graphically so you can easily find and run the test you’re looking for.  Your testing time is valuable, don’t spend it searching through menus to find a measurement.

And More…

There are a number of additional enhancements with the Infiniium 5.60 upgrade, including:

– CAN-FD protocol and decode is now included in the automotive bundle option (N8803C). This bundle includes protocol decoding for CAN, LIN, FlexRay and CAN-FD.

Figure 8 – Quick Jitter and Quick Eye Diagram menu

– Quick jitter and Quick eye diagram options.  Instead of going through lengthy set up menus, you can do the quick setup option and set up your jitter or eye diagram analysis in just two clicks.

The Infiniium oscilloscopes now have more capabilities than ever. Armed with 5.60 you are equipped to test and impress like never before.  Whether you’re looking for digital memory analysis, designing power systems, or work in the automotive industry, we have test solutions for you.

Check out the Infiniium software, version 5.60 update!

# Understanding Oscilloscope Trigger System Basics & Why You Should Care

Posted by colinmattson Sep 1, 2016

You sit down at your lab bench to debug some funny behavior in a 10 MHz clock. You fire up your oscilloscope, get your probing in place and hit the almighty Auto Scale button, after which you’re presented with something like this:

Figure 1: 10 MHz square wave after Auto Scale

and then it strikes you; there are ten million clock cycles occurring every second! How in the world is the scope able to accurately display such a clean representation of your signal? How is it that the middle of the rising edge of your clock is perfectly aligned at center screen? The answer is the trigger system.

The trigger system is both one of the most commonly-used and least-understood sub-systems in real-time oscilloscopes. In this article I’m going to pull back the curtain just a bit and explain what the trigger system does, how it works and why you should care.

What Oscilloscope Triggers Do

The sole responsibility of the trigger system is to tell the rest of the oscilloscope what data to care about. It decides when the acquisition system begins acquiring, which means that by default it decides what is displayed on screen and what data is available to make measurements on. It can make these decisions with a very simple set of conditions or very complex conditions, based on user input.

Let’s consider the example above; a 10 MHz square wave. The reason the signal in the image above looks so clear and well-positioned on-screen is that the trigger is set up, appropriately, to look for a rising edge on Channel 1:

Figure 2: Trigger setup as configured by Auto Scale

Remember that to start we used Auto Scale, which was kind enough to pick an appropriate trigger source and threshold based on our input signal. But what would our signal look like without an appropriate trigger configuration? I’m glad you asked:

Figure 3: 10 MHz square wave, auto-triggered. Infinite-persistence is enabled on Channel 1 to better illustrate what’s going on

In Figure 3 above I have changed the trigger condition to look for an edge on Channel 2 (which has no signal connected at the moment). The auto-trigger feature, which we can see is enabled in the “Sweep” section of the trigger configuration dialog, is automagically kicking off acquisitions on a regular time interval giving us a smear of yellow signal trace across the screen. The signal itself appears to be clearly visible in this image, superimposed over the “smeared” signal trace, unfortunately however this is just an artifact of the screen shot; that’s where the signal happened to line up at the instant I took the screen capture. In practice, auto-triggered acquisitions aren’t good for much of anything other than to determine the relevant DC parameters to use in setting up your trigger condition. Note that if the auto-trigger feature is disabled, without an appropriate trigger configuration the oscilloscope simply won’t acquire:

Figure 4: Disabling the auto-trigger feature without an appropriate trigger configuration means the oscilloscope won’t acquire at all.

The trigger system will always place the trigger point (the instant at which all conditions present in the trigger configuration are met) at t = 0.0 s on screen. Later on, we’ll see how using advanced trigger configurations can help capture infrequent and hard to find events, in addition to the simple rising-edge trigger we’ve seen so far.

How Oscilloscope Triggering Works

Most real-time oscilloscopes have an “analog” trigger system. This system is actually a mishmash of analog circuitry and digital counters but it relies on input from analog comparators fed from the scope pre-amp. Some oscilloscopes now feature a “digital” trigger, meaning that the trigger system is entirely digital and is fed with integer data from the ADC output. Both types of systems perform the same function; evaluating whether or not all of the configured trigger conditions are met at a given moment in time. Because fully digital trigger systems are fairly rare, we’ll focus on analog trigger systems.

Figure 5: Generic four-channel DSO front-ends and trigger system

Figure 5 above shows a generic representation of the parts of a four-channel DSO (digital-storage oscilloscope) that we’re concerned with; the analog channel front-ends and the trigger system. The trigger system takes inputs from comparators on all of the analog channels and provides a single output. For convenience, let’s focus on a simplified diagram with only one analog channel:

Figure 6: Generic DSO front-end and trigger system

Figure 6 is a simplified, one-channel view of the same systems depicted in Figure 5. When a signal is connected to the channel input it goes through a series of transformations before it ends up on-screen:

• First, the signal is scaled appropriately and offset if necessary by the pre-amp. The pre-amp output is sent to the ADC to be digitized.
• Trigger comparators observe the output of the preamp and fire if it exceeds the threshold they have been set to. This threshold is set based on user input, or by helper routines like the almighty Auto Scale.
• The trigger system observes all of the trigger comparator outputs in the system and combines them in such a way as to monitor for a given set of conditions. These conditions can be very straight-forward (IE: rising-edge on channel one) or quite complex (IE: pulse-width greater-than 2.4 ns on channel 3, followed by a pattern of channel one high, channel two low and channel four low, held for a duration of greater-than 30.0 ns and less than 50.0 ns).
• When the trigger system sees that all of the conditions are met for the specified trigger, it sends a pulse on its output. We call this signal “System Trigger” or “SysTrig” for short. SysTrig is monitored by the acquisition system as well as a special sub-system known as the “time-base interpolator”.
• When the acquisition system sees a pulse on SysTrig, it begins to digitize, process, store, measure and finally display data. We refer to this entire process in general as “acquisition”.
• Before the acquisition data (the waveform) which is now stored in memory can be displayed on-screen, we need to know how to orient it horizontally; this is where the time-base interpolator comes in. The interpolator monitors SysTrig, just like the acquisition system. When it goes high, it’s the interpolator’s job to figure out what address in waveform memory matches up with the instant the trigger occurs. It communicates this information to the acquisition system and voila, the result is the desired waveform on-screen, with the trigger point placed perfectly at t = 0.0 s!

Why You Should Care about Oscilloscope Triggering

Although some of you may find the inner workings of oscilloscope sub-systems interesting, it’s fair to say that most folks couldn’t care less. If you are one of those folks and you just skipped the entire “How It Works” section above, no sweat, there will not be a quiz at the end! The bottom line is that you should care about the trigger system and take the time to understand it because it can help you debug difficult issues and save you quite a bit of time and frustration.

Using an oscilloscope in a basic way, that is to say, pushing the Default Setup and Auto Scale buttons can tell us a little bit about our signal and is a quick and convenient way to get started. However, if you’re interested in capturing an infrequent event, as is often the case when debugging common issues like runts, glitches and setup & hold violations, the trigger system is a powerful tool.

In addition to edge trigger mode discussed at the beginning of this article, most real-time oscilloscopes have a number of advanced trigger modes designed to detect common problems. When used in conjunction with “Triggered” sweep (aka, auto-trigger disabled) these modes will ensure that only the behavior you’re looking for is acquired and displayed. As an example, let’s look for a runt in a square wave. We connect our signal, use our trusty Auto Scale button and all we see is a square wave, no runt in sight!

Figure 7: Looking for runts in all the wrong places…

Figure 7 shows us that although we suspect a runt is present in our signal, finding it with edge trigger will prove difficult. Maybe we’ll be able to see it if we zoom out a bit?

Figure 8: Looking for a runt with edge trigger, zoomed out horizontally

As shown in Figure 8, by zooming out horizontally we can tell that there’s something fishy going on, but it’s not entirely clear what. Now, let’s use runt trigger mode on our Infiniium S-Series oscilloscope and see what we can find:

Figure 9: Runt trigger mode setup on an Infiniium S-Series oscilloscope

Figure 10: Looking for a runt with runt trigger mode

There’s our runt! The waveform in Figure 10 is clear and steady and the event we’re interested in, our elusive runt, is right at t = 0.0 s! This is the value of learning the trigger system on your scope, it will allow you to find the events you’re interested in, and only the events you’re interested in, very quickly. Although this example focused on finding a runt, the same sort of example can be demonstrated with glitches, setup & hold violations, specific data across multiple channels, data patterns relative to clock edges, edge transition times, etc. using the appropriate trigger mode.

In addition to advanced trigger modes like runt mode discussed above, many oscilloscopes have features that can be used in conjunction with trigger modes to further refine what we want the oscilloscope to show and to instruct the scope to automatically take actions when a trigger occurs.

Figure 11: Trigger Conditioning dialog on an Infiniium S-Series oscilloscope

Figure 12: Trigger Actions available on an Infiniium S-Series oscilloscope

Figure 11 above shows some common options for trigger conditioning, while Figure 12 shows some of the things we can configure the oscilloscope to do for us automagically when a trigger occurs. My personal favorite is the “Email on Trigger” feature. If you have a really infrequent event, no problem; set up your trigger and leave for the weekend. Come back, open up your email and find all the data you need!

Thanks for reading! If you have specific questions regarding triggering or challenges triggering on a specific event let us know in the comments!

# New tools make short work of wideband RF measurements

Posted by KeysightOscilloscopes Sep 1, 2016

Wideband RF measurements are changing and along with them, the tools you need to make sense of the signals. Today’s radar systems require higher target tracking resolution, communications systems require higher data throughput—and to meet the demands, you require wider modulation schemes on related signals to validate prototypes and production units.

Gone are the days when an instantaneous measurement bandwidth of 510 MHz, the longtime standard in signal and spectrum analyzers, could handle this modulation bandwidth. Some systems have crossed beyond 1- GHz and even 2-GHz-wide formats. You need a different approach to make high-quality, insight-providing wideband RF measurements.

How different an approach? One that uses high-bandwidth, real-time oscilloscopes. Digitizers and oscilloscopes offer enough bandwidth and sample rate to directly sample the carrier plus modulation either alone or with the use of down-converters in front of the scopes.

The trick is knowing what to use when. One way to consider your options for wideband measurements is to plot the possibilities on a chart. The vertical axis represents analysis bandwidth of the solution and the horizontal axis representing carrier frequency that you can measure. Don’t worry about doing the plot—we’ve taken care of it:

Applicable tools as a function of signal carrier frequency and spectral width

As you see, classic signal analyzers offer analysis bandwidths up to 1 GHz and handle carrier frequencies up to around 50 GHz. As an alternative, mid-range oscilloscopes offer bandwidths in the 8-GHz range, letting you measure signals with carrier frequencies approaching 8 GHz, and with very wide modulation bandwidths, approaching 8 GHz. As long as the carrier plus modulation spectrum fits within the oscilloscope bandwidth, you can make meaningful measurements.

But even that may not be enough. In wideband aerospace/defense applications, including electronic warfare, radar, and surveillance, signals of interest may have carrier frequencies higher than 8 GHz. Cue the high-performance oscilloscopes. These scope families have higher bandwidths up to 33 GHz and 63 GHz and, as you might guess, corresponding higher prices. But they offer impressive performance in areas like frequency response flatness and low noise. An alternative is to place a down converter in front of a mid-range oscilloscope. You pay less but can handle high carrier-frequency signals with wideband modulation—provided you’re willing to make some tradeoffs in amplitude and phase linearity.

As a first down-converter option, you can place a standard signal analyzer in front of a mid-range oscilloscope and use the IF down-conversion path in the signal analyzer. You’ll typically need calibration to flatten the overall system amplitude and phase response over frequency. But a solution like this can address a wide range of carrier frequencies, typically up to 50 GHz.

A second down-converter option is to place a lower cost harmonic mixer in front of a mid-range scope. This results in a “banded” solution: Very high carrier frequencies can be analyzed, but there is generally a “band” of carrier frequencies that a particular harmonic mixer can handle. That makes this option especially convenient for applications like 5G, Wigig, and automotive radar.

Typical RF performance for high-bandwidth real-time oscilloscopes

So what do you need to know before making FFT or wideband RF measurements with an oscilloscope or scope combined with vector signal analyzer (VSA) software? You need to know that the RF characteristics can have a major influence on the measurement results—so you’ll need to evaluate this first.

Today you can find oscilloscopes that incorporate amplitude and phase correction for excellent absolute amplitude accuracy and low deviation from linear phase across their frequency range. This in turn contributes to high-quality RF measurements. These oscilloscopes also offer excellent noise densities, in the vicinity of -160 dBm per hertz, and high dynamic range and signal-to-noise ratios, considering the wide bandwidth capability they offer.

What does that do for you? You can look at wideband signals with very small amplitude adjacent in time to large signals. You can also boost scope sensitivity to measure isolated, small-amplitude signals. The time-base circuitry in these oscilloscopes also means good, close-in phase noise, which corresponds to low jitter in very deep memory traces. If you want more details, see the RF characteristics of a high-performance 33-GHz oscilloscope in Table 1.

Table 1. Typical RF performance in a high-bandwidth oscilloscope

Wideband pulsed RF time-domain measurements of envelope, frequency, and phase chirp

Now that we know what our high-bandwidth scope is capable of, let’s see how it handles time-domain measurement and analysis of wideband pulsed-RF signals with no help. The choice of which oscilloscope to use depends on the maximum frequency content of the carrier plus modulation. Consider an example where a signal under test is supposed to have 1-usec-wide pulses, with a pulse repetition interval of 100 usec. It also has an RF carrier frequency of 15 GHz and linear FM chirping that is 2-GHz wide.

Figure 2 shows a variety of measurements on a single RF pulse, including envelope parameters and the frequency chirp across the pulse. Stable triggering on this pulse is accomplished with trigger “holdoff” set to a value slightly longer than the 1-usec RF pulse width.

Figure 2. Time-domain measurements on 1-usec wide, 15-GHz carrier, 2-GHz-wide linear FM chirped RF pulse with a 33-GHz bandwidth oscilloscope

To make amplitude measurements, we use the “Envelope” math function and then pulse measurements are dropped down onto the visible RF pulse envelope. A “Frequency” measurement is dropped down onto the RF pulse (not onto the envelope), and a “Measurement Trend” math function is defined with the frequency measurement as a source. Next we perform a smoothing math function on the measurement trend with the resultant linear ramp display of the linear FM chirp modulation, also shown in Figure 2. The oscilloscope magnitude linearity over the frequency span of interest has a direct effect upon the quality of the envelope measurement. To see the effect, take a look at the magnitude plot over frequency of the 33-GHz bandwidth scope in Figure 3.

Figure 3. Typical magnitude linearity over frequency on four individual 33-GHz channels

Wideband pulsed RF-gated FFT measurement of spectrum

You can create a wideband FFT by defining an “FFT Magnitude” math function with “Rectangular” windowing. Then create a time-gated FFT using the (you guessed it) “Timing Gate” math function. Once the time-gating math function is defined, you can define an FFT math function that is calculated from the time record within the time gate, as shown in Figure 4.

Figure 4. View of normal and time-gated FFT and display with time gate at the beginning of the RF pulse

Wideband pulsed-RF time- and frequency-domain measurements with a scope plus VSA software

But that’s not all. You can further enhance RF and FFT measurements made with high-bandwidth oscilloscopes by importing scope-captured signals into VSA software. Some advantages of using VSA software include:

• many built-in RF measurements;
• ability to bandpass-filter oscilloscope input samples and decimate prior to the FFT calculation to reduce noise and speed the calculation;
• variety of digital and analog demodulation options like QAM16 and FM demodulation;
• time-domain baseband view of pulse with reduced noise through processing gain;
• frequency and phase shift across the pulse through a demodulator.

If the oscilloscope-captured data is imported to VSA software, it can be digitally down-converted into I and Q baseband data, bandpass-filtered, and then resampled. This can greatly decrease the amount of noise in the measurement. Essentially the process is “tuning” to the center frequency of the signal and “zooming” into the signal to analyze the modulation. This is also referred to as “processing gain.”

In this example, the original 8-GHz-wide measurement with the associated noise is reduced to a 500-MHz-wide measurement, centered on the 3.7-GHz carrier with an instantaneous measurement bandwidth slightly wider than the width of the signal modulation. This corresponds to an improvement in signal-to-noise (SNR) ratio of:

10log*(ScopeBW/Span) = 10log*(8E+09/500E+6) = 12 dB.

SNR is improved by 10log*(ScopeBW/Span).

By taking advantage of this processing gain, combined with the VSA software’s capability to use a log-magnitude scale, and using averaging, you can now see the 50-dB down pulse, as shown in Figure 5. It wasn’t visible in the scope display with the 8-GHz wide measurement.

Figure 5. 50 dB down pulse seen with VSA software “Center Frequency” and “Span” set

The secret to long target-time capture and statistical pulse analysis

When an oscilloscope samples a wideband RF signal, it must do so at a fast enough rate to accurately capture the carrier plus modulation. Often a very fast sample rate is required. In a normal real-time sampling mode, the oscilloscope memory will not allow for a long capture period.

But there is a work-around: oscilloscope segmented memory. This can greatly increase the target activity time when there is a low-duty-cycle signal, such as a common pulsed RF radar signal. The scope memory is divided into smaller segments of fixed time width, chosen to be a little wider than the widest RF pulse. The scope triggers on an event, such as the beginning of the RF pulse, and then places one RF pulse in a memory segment. The scope then stops capturing data, rearms the trigger, and waits for the next RF pulse. A second RF pulse is put into the second segment of memory. This process continues until all the scope memory segments are used.

Modern pulse-analysis software can let you take advantage of the scope segmented memory and then offers built-in measurements for pulsed RF signals. Figure 6 shows a capture of many RF pulses via segmented memory, combined with pulse-parameter measurements in the pulse-analysis software. Here a 1-GHz linear FM chirp and related phase shift across pulses is compared to a best-fit ideal linear ramp and ideal parabola, respectively. A close-up view is made of the delta between measured and reference for frequency in trace S and for phase in trace J.

Figure 6. Pulse analysis software calculations based on measurements taken on oscilloscope segmented memory

Conclusion

The bandwidth limitations of signal and spectrum analyzers are driving designers to use digitizers and oscilloscopes, with or without down-converters. Math functions like envelope, measurement trend, and FFT all prove helpful in understanding target system operation and issues. Combining an oscilloscope with VSA software creates a powerful RF-measurement suite to perform measurements, including demodulation, extended SNR time-domain views, and statistical RF pulse analysis. Yes, there’s a tradeoff between dynamic range/SNR and the instantaneous bandwidth available, but you can still access many useful wideband measurements to evaluate a prototype or production unit.

# Plotting 8 Signals with 4 Channels

Posted by KeysightOscilloscopes Sep 1, 2016

By: Ryan Carlino

As you become an oscilloscope power user, you may find that there are times when the usual four oscilloscope channels are just not enough. You might want to make sure your system comes out of reset properly by observing the timing relationship of various reset and status lines. You also might need to verify that power supply rails come up and go down in the proper order. Modern FPGAs often have more than four power supplies and those supplies are required to sequence in a specified fashion (i.e. certain supplies need to come up before other ones and in a specified amount of time). Here’s an example of Altera’s specification of how fifteen power supplies for a Stratix FPGA should come up.

From page 322 of: Stratix V Device Handbook

Designing power sequencing circuits and verifying their actual behavior is something I do on most boards I work on. Here’s a trick for capturing more than four power supplies in a single scope image.

A board I am working on right now provides eight power supplies to some custom bipolar ASICs (Keysight-designed chips that have both positive and negative power supplies). Like an FPGA, the ASICs have specific power sequencing requirements. In my case, four positive supplies should come up and “peel off,” followed by four negative supplies which also “peel off.” The ASIC designers and I came to this agreement and I documented it on a post-it note, which became my design spec (Figure 1).

Now that boards are built, I want to verify that my design worked this way. Ideally, I would capture an oscilloscope plot that looked just like my drawing, but my scope only has four channels! I don’t want two plots and I don’t have time to save the waveform data and load them into a plotting program.

This is where saving waveforms into memory really shines: you can display up to eight waveforms at once. Here’s how I get the plot I want using my Keysight S-Series oscilloscope.

1. Pick a master signal to trigger on – It’s going to take three triggers to get all eight waveforms, so pick one signal to be the “master”. I like to use the first signal that turns on. Set the trigger level in the middle of that signal and keep it there.
2. Make sure the timing is consistent – Stacking waveforms is only useful if the signals aren’t moving in time with respect to each other. I like to take a few test triggers to see if the waveforms come up consistently. If some of the signals shift in time, try to figure out how far they go. Will a composite waveform make sense if the timing changes?
3. Check the vertical and horizontal scales – You want to capture the signals so they fit on the screen. Pick an appropriate vertical scale and offset for each signal. Check that the horizontal scale is big enough to capture the last signal when triggering on the first.
4. Trigger and save, then repeat as needed – Get probes on your signals and start capturing. First probe your master signal (#1) and three other signals (#2, #3, #4). Save signals #2-#4 into three waveform memories. Then, move the probe from #2 to signal #5. Trigger and save signal #5 into the last memory. Then move the probes to the final signals #6-#8 and trigger and save. Now you can view all eight of your signals!

Let’s run through an example of my eight power supplies turning on. I use two grids (positive rails on top, negative rails on bottom) with all waveforms at the same vertical scale. I use +12V as my trigger, since it comes up first. I checked that the turn-on timing is consistent during power-up and set my horizontal and vertical scales.

On my first trigger, I capture three of the negative supplies and save them into memories.

Just right-click each waveform to save and select Save Channel -> To Memory.

With three memories done, I move the CH2 probe and trigger a second time to capture the last negative supply into a memory.

On my final trigger, I move the probes to the positive supplies and save 8-waveform plot.

I like to label the waveforms with descriptive names using bookmarks.

So, there are my 8 power supplies turning on – all on one plot. I can see where things are not ideal and can react to them if needed. Now, I can repeat the process to get a turn-off plot.

# Decoding Serial Data

Posted by mike1305 Sep 1, 2016

If you are fluent in more than one language, you can appreciate how difficult it is to translate between different languages. Not only do you have to identify what languages are being spoken between two parties, but you must appreciate the nuances of each language. Often times certain words or phrases don’t translate well, creating confusion. Perhaps one person is speaking faster than you can understand, or using a local dialect you aren’t familiar with. Or what if the person you are translating for is only interested in particular words or phrases spoken by the other party? Or when the person speaks their native tongue incorrectly?

Translating for the grammar police doesn’t sound like a very fun job. But it does make for a great analogy. Engineers use test tools to translate communication protocols into readable formats to look for certain activity and errors, or to simply monitor activity on the bus. Today we’re going to talk about serial decoding, how it works, and some serious advantages for those of us who don’t speak fluent electricity. Which unless you are a superhero, is everyone!

If you’re working on design or debug of a serial bus protocol in a system, like USB, you likely have many weapons in your arsenal. A common first choice is a protocol analyzer. This tool will read out protocol layer information on the bus it’s designed to analyze. An example solution is Total Phase’s Beagle I2C/SPI sniffer. This will tell you what is being communicated and help identify improperly transmitted bits or frames. But, this tool will not tell you how or why said errors are occurring, nor show you bit timing.

A next step might be a logic analyzer with decode capabilities. Plenty of options exist at all different price ranges depending on your channel density and speeds required. This will show you the data being transmitted in the protocol layer, and show you bit timing charts using high/low views.

The ultimate tool is the oscilloscope, which provides the best view of physical layer issues that could be plaguing your bus. These physical layer problems – such as noise, jitter, glitches, or unstable edges – can’t be viewed on a protocol or logic analyzer.

How does an oscilloscope decode and trigger on a serial bus?

There are two ways an oscilloscope can decode bus traffic. The first is using a software routine. An acquisition is taken, then analyzed before the next acquisition. Second is using an ASIC or FPGA. This is much faster and can be done in real time. Both methods accomplish the same thing: identifying high and low levels in time, and using a given bit rate, creating a sequence of 1’s and 0’s. Once a bit stream is translated by the oscilloscope (e.g. 10001001010110010), the protocol in question must be identified and defined. Every protocol has a syntax, and the decoded bits are fed into it to translate them to readable information to the user. Most protocols have options that can change how the data is transmitted – this can range from bit rate, address size, payload size, and bit order. All of this needs to be fed to the oscilloscope for proper translation. Once that’s done, the oscilloscope can translate information from the bus into readable information to the user!

Once that is complete, the trigger process becomes simpler. Tell the oscilloscope what you are looking for – a certain address, a certain payload, or an error – and then it will decode in the background waiting for it. Once found, it will be displayed on screen! If you want more information on the Keysight serial bus decoding capabilities on InfiniiVision and Infiniium oscilloscopes:

InfiniiVision Serial Bus Options Data Sheet

See all of the Keysight oscilloscope serial bus software options

# Inventing the MSO

Posted by Daniel_Bogdanoff Sep 1, 2016

A look into the history of the mixed-signal oscilloscope

1996 was a year to remember, it brought us the Macarena, the Nintendo 64, and the first Motorola flip phone.  But, also making its debut that year was the HP54645A mixed signal oscilloscope.  Today mixed signal oscilloscopes (MSOs) are an industry standard, but this was new and exciting technology 20 years ago.  Here’s an excerpt from the HP journal from April 1997:

“This entirely new product category combines elements of oscilloscopes and logic analyzers, but unlike previous combination products, these are “oscilloscope first” and logic analysis is the add-on.”

At this point in the tech industry, microcontrollers dominated the landscape. Gone were the 1980s and the days of microprocessors and their dozens of parallel signal lines, in was the 8-bit or 16-bit microcontroller. As the need to test dozens (or hundreds) of channels decreased, the thriving logic analyzer industry began to shift in favor of oscilloscopes.  As a result, Hewlett Packard released the 54620A; a 16-channel timing-only logic analyzer built into a 54645A oscilloscope frame. This was a big hit for engineers who only needed simple timing analysis from a logic analyzer and liked the simplicity and responsiveness of oscilloscopes.

These tools were all coming out of Hewlett Packard’s famed “Colorado Springs” division, which focused heavily on logic and protocol products. In hindsight it’s clear that the shift from a logic analyzer-focused landscape to an oscilloscope-focused landscape was inevitable.  But, when the project funding decisions had to be made the logic analyzer was king.

A few R&D engineers, however, saw it coming.  They strategized amongst themselves to get a new oscilloscope project underway.  However, they knew it was going to be a hard fought battle. Following the old adage “if you can’t beat them, join them,” the engineers proposed a new project combining the oscilloscope and the logic analyzer into one frame. The thought was that if an oscilloscope project wouldn’t get funding, then surely integrating a logic analyzer into the scope would do the trick. Below is a picture of Bob Witte’s (RW) original notes from the 1993 meeting in which the MSO was conceived. (Follow Bob Witte on Twitter: @BobWEngr) This product was internally code named the “Logic Badger,” stemming from the 54620A oscilloscope’s “Badger” code name and the 54645A’s “Logic Bud” code name.

One thing led to another, and the 54620A and the 54645A were combined into the paradigm shifting 54645D. A new class of instrument was introduced into the world: the mixed signal oscilloscope. For the first time ever, engineers could view their system’s timing logic and a signal’s parametric characteristics in a single acquisition using the two analog oscilloscope channels and eight logic channels.

From its somewhat humble beginnings, the MSO has become an industry standard tool globally, with some estimating that up to 30% of new oscilloscopes worldwide are MSOs. Logic analyzers are also still sold today and are an invaluable tool for electrical engineers thanks to their advanced triggering capabilities, deep protocol analysis engines, and state mode analysis. If you’re debugging FPGAs, DDR memory systems, or other high-channel-count projects you’ll want to consider using a logic analyzer. However, mixed signal oscilloscopes dominate today’s bench for their ability to quickly and easily trigger and decode serial protocols.

Finally, it’s worth noting that the Hewlett Packard division is still alive and strong in its current form here at Keysight Colorado Springs. In fact, many of the same engineers from the very first MSO project are still here working on today’s (and tomorrow’s) MSOs.

To learn more about how the digital channels on an oscilloscope work, check out this 2-Minute Guru videoon the Keysight Oscilloscopes YouTube channel.

Learn more about MSOs or view the mixed signal oscilloscopes available today from Keysight Technologies at Keysight.com.

# Why Would you Want a Mixed Signal Oscilloscope?

Posted by schoenecker Sep 1, 2016

As an oscilloscope user you understand the importance of analyzing analog signals in a digital circuit. However, many users are missing out on one of the most powerful features of today’s oscilloscopes: the mixed signal oscilloscope (MSO). An MSO adds up to 16 digital channels to your 4 analog channel oscilloscope. This greatly expands the types of analysis that can be performed by this versatile engineering tool. Digital signals can be a simple chip select, or a communication bus. The ability to monitor these digital signals is often critical to properly analyze system operation.

Debugging a mixed signal design can be a difficult and somewhat daunting task to the engineer who is armed with a 4-channel oscilloscope since you often need to capture more than four signals. An mixed signal oscilloscope provides that capability, with the ability to examine the state of up to 20 signals all on the same timescale, while using the familiar controls of a basic oscilloscope.

When needed, MSO digital channels provide just enough logic analysis for users whose home base is an oscilloscope. MSO digital channels serve as an extension to the oscilloscope capabilities and can provide valuable insight into the operation of your design.

Correlation of input/output of ADC/DAC is simple and straightforward. An MSO adds some very powerful and useful tools for analyzing a digital bus. In this one simple view we can see the state of the analog signal, the state of each of the digital signals, the hex representation of this digital bus, and I still have all of the measurements available on the oscilloscope to evaluate the signal quality. The controls and measurements are all still based on the oscilloscope operation, making it very easy to navigate and control.

At the same time the MSO provides the means to help analyze your digital bus. The grouping of digital signals to create a “bus” with easy-to-read hex values can be used in decoding the signals, or for triggering on specific addresses or values.

MSOs and logic analyzers have fundamental architectural differences in how they acquire and display signals. MSOs exclusively use asynchronous sampling, just like an oscilloscope. For many users, this makes setting up an acquisition on digital channels simpler, because it feels like a scope.

By applying some additional logic to the digital bus you can create a visualization of the bus operation. By using one of the signals as a clock, the MSO can chart the bus “state” to display a logical representation of the digital data.

Displaying an analog equivalent of the data that is being transferred across the bus can quickly identify errors in the digital data.

Low-Speed Serial Bus Support

Today’s designs incorporate digital communications between components and systems by using high- and low-speed serial digital communication, and microprocessor buses. Serial buses like I²C and SPI are frequently used for chip-to-chip communication, but cannot replace parallel buses for all applications. But here again, oscilloscopes add powerful troubleshooting capability by providing just enough protocol analysis.

A key difference between logic analyzers and MSOs is the latter’s ability to trigger on and decode serial buses. Low-speed serial buses are ubiquitous in electronic designs because of their ease and low cost of implementation. In fact, it is hard to find a design that does not include at least one I2C or SPI bus, or USB.

Mixed signal oscilloscopes excel at debug that includes low-speed serial buses. All good MSOs come with both triggering and decode options for serial buses. However, logic analyzers have not incorporated similar triggering and decode technology. Without protocol triggering, it’s impossible to set up the oscilloscope to trigger on specific packets. For example, you can set the MSO to trigger when an I2C read to a certain address with a certain data value happens. Alternatively, you can trigger on a certain SPI data packet or at the start of USB enumeration.

Since the blending of analog and digital information is so prevalent, oscilloscope users can take advantage of the ability of current MSOs to improve their troubleshooting capabilities and simplify their mixed-signal design debugging.

Keysight oscilloscopes have the added benefit that most existing DSOs (Digital Storage Oscilloscope) can be easily upgraded to add MSO capabilities.

# Insider Tips for Using an Oscilloscope with Touch

Posted by KeysightOscilloscopes Sep 1, 2016

Author: Chris Felder

As one of the Keysight R&D engineers who developed Project Echo, the touch screen and interface for Keysight InfiniiVision oscilloscopes introduced on the 4000 X-Series in 2012, I know these oscilloscopes from the inside out; literally. Here are a few creative shortcuts we have built into the oscilloscope interface to help you get more out of the scope.

As Keysight was designing the first touch interface, which is used on the Keysight 3000T, 4000-X, and 6000-X Series scopes today, we conducted extensive usability testing to ensure the touch screen and interface design enhanced the existing interface and the scope could be entirely driven using the touch screen. While touch can provide many benefits, we also wanted to be sure that it did not impair the usability for those not using the touch feature. Even if you prefer to drive the scope using the front panel keys and knobs, using touch in minor ways may greatly accelerate your tasks.

All of the oscilloscope’s menus and dialogs are accessible through this menu.  There are some handy shortcuts along the left side, and you can manipulate several feature states directly through this menu (channels, cursors, measurements, etc.).  The Applications menu gives a list of your licensed and installed oscilloscope applications, but also lists unlicensed applications – handy if you’d like to explore and read about all the capabilities built into your scope.

From the main menu, we move on to the status area along the top of the graticule; this area hold lots of readouts that show the state of the oscilloscope, and all of them are touchable.  Touch the scale or delay values in the Horizontal grouping, for instance, and you’ll get this handy popup:

From here, you can step the values using buttons, or touch the values once more to get a numerical keypad for direct entry.  If you want to change other timebase settings, you can press the gray ‘H’ button in the status area for a quick shortcut to the Horizontal softkey menu.

In some areas, we’ve added more significant shortcuts for the most common tasks.  Touch the trigger status indicator, for example, and you immediately toggle from Auto mode to Normal mode, and vice-versa:

The sidebar along the right side of the screen is another area we’ve really optimized for touch.  Any dialog box with a series of dots in the upper left (what we call a “gripper”) can be repositioned by dragging it from the title bar area; the same is true of sidebar tabs.  Any tab can be grabbed using the grippers, undocked, and positioned anywhere you like.  You can even re-dock the tab in a half-height mode, allowing you to see two tabs at once:

Like the status area, sidebar tabs are filled with touch shortcuts.  You can touch the analog channel input information in the Summary tab to quickly perform a slew of front end and probe configuration settings:

Titles in the sidebar tab look a bit like buttons for a reason – they all have handy shortcut menus when you touch them.  Touching the title in the Cursors sidebar, for example, lets you directly change mode and source settings without needing to travel to the Cursors Menu:

In the Measurements tab, you can touch individual installed measurements to track, clear, or reset them:

The softkey menu area along the bottom of the screen frequently includes readouts for status items related to the current menu, and…you guessed it…they’re all touchable! If you have the WaveGen (waveform generator) option enabled on your scope, the Waveform Generator Menu contains a particularly handy shortcut; if you touch the “Gen Out” area, you get a comprehensive control stack for the selected WaveGen, from which you can change a variety of settings without bouncing between multiple softkey menus:

Like all dialog boxes, this dialog can be re-positioned by dragging it within its title bar area.  You can also use the blue Menu button to configure dialog boxes to use a transparent background.  Now you can position and configure dialog boxes and sidebar tabs as you wish!

We strive to follow the rule, “everything is touchable” and we’re constantly adding new shortcuts and convenience menus with every software release.

We always welcome your suggestions and feedback – comment here to let us know what we can do to make your oscilloscope experience more efficient.

# Getting Started with Jitter: The Best Jitter Glossary I’ve Ever Written

Posted by Daniel_Bogdanoff Sep 1, 2016

# (Also the only Jitter Glossary I’ve ever written)

Does jitter have you all shook up? This quick overview should help ease those jitters (puns intended, sorry). Learning this list of key terms will give you the confidence you need to start tackling the jitter bugs in your design.

Buckle up! Here’s the exhaustive list of terms you need to know:

Jitter: Essentially a measurement of where your signal’s edges actually are compared to where you want them to be. If your edges are too far off, bad things happen. Really, really bad things. Or sometimes just marginally bad things. Bit errors, timing errors, the works. You can hope it’s just marginally bad, or you can use the right equipment and know for sure.

Jitterbug: An old-school dance.  Note: you don’t actually need to learn this to talk intelligently about jitter. It will probably just have the opposite effect.

Probability Distribution Function (PDF): Remember your statistics class in college? Me neither.  But, you’ll probably remember the term “bell curve” because that affected your grades. A bell curve is just one type of probability distribution function and is simply another way to describe a “normal” or “Gaussian” distribution. A PDF is simply a chart of possible values based on their likelihood of occurring. The x-axis represents a possible value (sometimes marked by standard deviations away from zero) and the y-axis represents the possibility of that value occurring. We use PDFs to visualize and interpret jitter measurements.

Gaussian Distribution: or “normal distribution,” it’s unbounded and continuous. That’s a fancy way of saying that basically any value is possible. But the farther away from the middle of the PDF you go, the less likely it is that that value will occur.

Random Noise: Also “random jitter,” is 100% random and has a Gaussian distribution. It’s caused by physics (yay science!) and has three components: thermal noise, shot noise (or Poisson noise if you’re a math major), and pink noise. If you want to geek out more on this, just look it up on Wikipedia. So, you expected your clock to have a 60 ns period? Well, because you can’t get rid of random noise (earplugs don’t help) you could end up with a rogue 500 ns period every once and a while.  But you probably won’t unless you have a few years to run the test. But you could. This is why we like to measure and analyze jitter! You can analyze jitter on your oscilloscope using histograms.

Histograms: A tool that visually describes how a signal varies over time.  Figure 1 shows a jitter histogram on the Keysight InfiniiVision 6000 X-Series oscilloscope.  Because it looks like a bell, you can say “That’s Gaussian!” (and get smarty-pants points from your cubicle-mate). Because there’s only one peak on the histogram, you can say “Psh, it’s only random jitter so there’s nothing we can do about it!” (and get double bonus smarty-pants points from your cubicle-mate). But, look at Figure 2.  That looks a little bit scarier. Because the histogram has two peaks it means that there’s “deterministic” jitter.

Figure 1: A histogram of a signal that just has random noise

Figure 2: a “Bimodal” histogram shows that there’s deterministic jitter

Deterministic Jitter (DJ): It’s not random.  It’s usually bounded, so it can’t go off to infinity even if it wants to. This is when it starts to get scary, because deterministic jitter is caused by system phenomena. Notice that there are two peaks with a random distribution around each of those peaks. Random and deterministic jitter are both in play here.  Deterministic jitter can be broken down into a few sub-categories:

Bounded Uncorrelated Jitter (BUJ): Gives engineers night terrors.  It’s bounded but isn’t really related to anything in that same system.  It could be something like cross talk or just interference from the wall.  (The wall? Yeah, there’s noise everywhere. Check out this awesome video: https://youtu.be/SJefUNAJZNA)

Data Dependent Jitter (DDJ): Can be one of two things.  The first is “duty cycle distortion” (DCD). This is when one bit value tends to have a longer period than the other (like when you can get one kid out of bed way easier than the other). The second is “Inter symbol interference” (ISI). This is caused by long strings of a single bit value. This is sort of like when you’ve been sitting too long in a weird position and one leg doesn’t work right when you get up and try to walk.

Periodic Jitter: can be correlated or uncorrelated, but is always periodic.  This means it’s pretty easy to identify like we’ve done in figure 2. Take your jitter measurement, and plot a trend of the measurement.  Then measure the frequency of the trend, and that will point you directly to the culprit (probably Professor Plum in the library with the candlestick).

“Whoa Daniel, that was too much at once. Remind me again how they all relate to each other?” I’m glad you asked; here’s a nice family tree (Figure 3).

Figure 3: Jitter and its components

Jitter Measurements: This probably doesn’t need defining; I just needed a segue. Ok, fine. Jitter measurements are measurements you make to get a better understanding of the jitter you’re dealing with. Here are a few jitter measurements you might care to make:

Time Interval Error: The mother of all jitter measurements. It’s usually measured as an RMS value and describes the difference between the ideal clock period and the actual clock period. Like I said, it’s the mother of all jitter measurements. You might think this is all you need to measure, but there are some other helpful measurements out there.

Period Jitter: Is usually measured as a peak-to-peak value, and yields the difference between the longest and shortest clock periods over a specified amount of time.

Cycle-to-Cycle Jitter: Is also usually measured as a peak-to-peak value, and is the maximum difference between adjacent clock periods. The longer you measure this, the larger it’ll get, so if you want to characterize this for posterity, use a set number of cycles that you measure. Basically, period jitter tells you how bad it is in the long run, and cycle-to-cycle jitter tells you how fast you are going to get there.

All of this should be enough to get you started if you want to measure (or just discuss) jitter. If your interest was piqued or you felt cheated because I didn’t talk about eye diagrams, clock recovery, or phase lock loops, check out this app note on written by Johnnie Hancock. It’s really good, but doesn’t have as many jokes. Although fewer jokes are probably a welcome relief by this point. You can also learn more about jitter  and jitter measurement tools at Keysight.com.

Also, check out some of our other posts! We’ve talked about probing techniques with Kenny Johnson: Splurge, get an active probe and Measure ripple and noise on power supply voltage railsconfusion in Australia and normal triggering with Johnnie Hancock; signal modulation and DIY oscilloscope Bode Plots with Mike Hoffman and  measuring system bandwidth and measuring oscilloscope and probe bandwidth with Taku Furuta.

And of course, Melissa Spencer’s oscilloscope zombie apocalypse survival guide.

# Zombie Apocalypse Survival Guide: Oscilloscope Edition

Posted by melissakeysight Sep 1, 2016

As electrical engineers there are certain rules that we live and die by each day.  Probably, the most common of these is Ohm’s law.   No matter what we are doing it always seems to come back to V = I*R, doesn’t it? That silly little equation we learned way back in our youth, maybe in Engineering 101 or our first Physics class, lays the foundation for everything in our field.  But what about the other equations and rules that we worship and obey like a zombie survival guide during the apocalypse?  Likely, names like Nyquist and Kirchhoff come to mind. And as we delve further into our specific fields, the more specific and sometimes diversified these rules become – after all, there are many different zombies out there and each needs its respective weapon.   Sometimes, that weapon is an oscilloscope.

So what rules or guidelines are you thinking about when you are planning to purchase a scope?  Probably, you’ve thought again of things like Nyquist in terms of sample rate.  You’re thinking about memory depth and waveform update rate.  And the noobs, less likely to survive a zombie attack, might be thinking, “Give me the highest possible bandwidth!”  But you, seasoned veteran of the apocalypse, obviously know that more bandwidth is not necessarily better.

A good rule of thumb for selecting the bandwidth of your dreamy new oscilloscope is to choose a bandwidth that is 3 times the fastest frequency content in the signal you are looking to analyze.  This rule of thumb is for analog signals.  If you are on the digital side, then your rule of thumb is 5 times the clock rate of your digital signals. There is a great blog post below (What is oscilloscope system bandwidth and how do I find the bandwidth of the scope + probe) and app note (Evaluating Oscilloscope Bandwidths for Your Application) that go into more details on this if you want to get into the nitty gritty of the nerdy and work out some equations.

But in general, the 3x for analog and 5x for digital BW rule of thumb guarantees that you will have enough bandwidth to properly observe your waveform without taking in too much high frequency content, which will show itself as noise on your desired measurement. Noise is Gaussian, so a higher bandwidth scope sees higher frequency noise.

But what if, you have been bitten by the Maximum Bandwidth Zombie?  You came down with the fever and couldn’t turn your mind away from that crazy high bandwidth scope, even though most of your applications are really only operating around 2 MHz or so.  Or maybe you simply purchased a scope for a higher bandwidth application than what you need in this very moment.  Perhaps, in most cases you are after Runner zombies so you purchase a high bandwidth scope, but occasionally you have to deal with the standard Walkers.  Don’t worry, your oscilloscope is a many facetted weapon.  This is probably a situation in which you will want to apply bandwidth limiting.

So you turn on bandwidth limiting and suddenly you’ve gone from having a noisy signal and may be experiencing ghosting (a situation in which you’re seeing an additional waveform capture on the screen) to having a nice clean waveform capture.

Here’s an example.  Below is a screenshot from a Keysight MSO-X 3104T.  This scope has a bandwidth of 1 GHz.  On channel 1, I’ve input a 1 MHz sine wave and, for demonstration purposes, mixed it with noise from an 80 MHz function generator.   Because I’m using a 1 GHz scope, I’m observing my desired 1 MHz sine wave distorted with the noise from the function generator and any noise in the environment that the scope or probe configuration might be picking up. You’ll also observe the extra, faint signal on screen. This is the ghosting effect I referred to earlier.  This is happening because the scope is sometimes triggering on what appears to be the falling edge of the desired signal but actually a rising edge in the noise.  This is not pretty measurement, am I right?

Figure 1 – 1 MHz signal with noise

Now, I select the Channel 1 menu and I turn on BW Limit.  Bandwidth limiting can be applied to each channel separately.  The Bandwidth Limit feature on this scope reduces the maximum bandwidth to 20 MHz.

With the bandwidth limit turned on, the high frequency noise content has been filtered out, and the desired crisp waveform is what remains. See figure 2.  One zombie down.

Figure 2 – 1 MHz signal with noise + BW Limit turned ON

Make sure to check out the bandwidth limiting capabilities of your oscilloscope. Keysight has a wide range of options depending on the scope you are using.  For example, the InfiniiVision 6000 X-Series oscilloscope lets you select a 200 MHz BW Limit in addition to the 20 MHz BW Limit option shown above on the 3000T X-Series.  The Infiniium scopes offer even more possibilities.  For example, the MSO-S804A is an 8 GHz scope and allows you to emulate a 6 GHz, 4GHz, 2.5 GHz, 2 GHz, 1 GHz, and even a 500 MHz scope.

As I said before, Keysight is here to help you slay all forms of zombies.

# How does oscilloscope signal integrity impact your measurements and analysis?

Posted by Paul Capozzoli Sep 1, 2016

By looking for an oscilloscope with good signal integrity you not only can impress your colleagues, but you also get:

• More accurate wave shapes
• More accurate and repeatable measurements
• Wider eye diagrams
• And less jitter

Signal integrity is the primary measure of signal quality.  When you need to view small signals, or small changes on larger signals; it is critical that you see those signals the way the components in your design see those signals.

Oscilloscopes themselves are subject to the signal integrity challenges of distortion, noise, and loss.  Scopes with superior signal integrity attributes provide a better representation of signals under test, while oscilloscopes with poor signal integrity attributes show a poorer representation of signals under test.  This difference impacts your ability to gain insight, debug, and characterize designs.

Results from oscilloscopes with poor signal integrity can increase risk in development cycles times, production quality, and components chosen.  To minimize this risk, you will want to choose an oscilloscope that has high signal integrity attributes.

Let’s take a look at some of the error attributes that effect signal integrity

The Oscilloscope’s Noise Floor

Having a scope with low noise (high dynamic range) is critical if you really want visibility to small currents and voltages, or to see small changes on larger signals.  You cannot see a signal smaller than the noise floor of the oscilloscope.

Noise can come from a variety of sources, including the front end of the oscilloscope, the analog to digital converter (ADC) in the scope and the probe or cable used connected to the device. The ADC itself has quantization error. For oscilloscopes, quantization noise typically plays a lesser role in contribution of overall noise than the front end of the oscilloscope which plays a more significant role.

Most oscilloscope vendors will characterize noise for a specific model number and include these values on the product datasheet. If not, you can find out yourself.  It’s easy to measure in just a few minutes. Disconnect all inputs from the front of the oscilloscope and set the scope to 50 Ω input path.  Set the sample rate at high.  Run the scope with infinite persistence and see how thick the resulting waveform is. The thicker the waveform, the more noise the scope is producing internally.

Each oscilloscope channel will have unique noise qualities at each vertical setting. You can view the noise visually just by looking at wave shape thickness, or you can be more analytical and take a Vrms AC measurement to quantify.   These measurements will enable you to know how much noise each oscilloscope channel has at various vertical settings to measure signals that are less than the noise of the scope. All acquired vertical values are subject to deviation up to the noise value of the oscilloscope. Noise impacts both horizontal as well as vertical measurements.

The lower your oscilloscope’s noise, the better the measurement results will be.

Figure 1:  Keysight S-Series oscilloscope and a competitive scope analyzing the same signal.  Which would you like for your signal measurements?

Frequency Response

Each oscilloscope model will have unique frequency response that is a quantitative measure of the scope’s ability to accurately acquire signals up to the rated bandwidth. These requirements must be kept in order for oscilloscopes to accurately acquire waveforms:

• Capture signals must be within the bandwidth of the oscilloscope
• the scope should have a flat frequency response
• And a flat phase response

Missing any one of these requirements will cause an oscilloscope to inaccurately acquire and draw a waveform and provide misleading measurement results.

Fast signal edges contain multiple harmonics, and scope users expect the oscilloscope to accurately measure each harmonic component using the correct magnitude. Ideally oscilloscopes would have a uniform flat magnitude response up to the bandwidth of the scope, with the signal delayed by precisely the same amount of time at all frequencies (phase). Flat frequency responses indicate that the oscilloscope is treating all frequencies equally, without a flat phase response the scope will show distorted waveforms.

Frequency-response correction filters produce flat responses for both magnitude and phase for more accurate waveforms.  Some oscilloscopes have strictly analog front-end filters that determine frequency response, while others apply correction filters in real time. Combining correction filters with front-end analog filters creates flatter magnitude and phase responses verses raw analog filters alone. High-quality oscilloscopes include both analog as well as correction filters to create a uniform and flat frequency response.

Figure 2: The flat frequency response of the Keysight S-Series oscilloscope.

Bits of Resolution and Effective Number of Bits

The ADC is the most recognized component on the oscilloscope. It converts the analog data to digital data. It drives the oscilloscope’s bits of resolution.  It is defined by its sample rate and its signal to noise ratio.  Typically most scopes have 8 bits of resolution, although recently oscilloscopes have added 10 and 12 bit ADCs

Effective number of bits (ENOB) is a measure of the dynamic performance primarily associated with signal quantization levels of your oscilloscope. While some oscilloscope vendors may give the ENOB value of the oscilloscope’s ADC by itself, this figure has no value. ENOB of the entire system is what is important. While the ADC could have a great ENOB, poor oscilloscope front-end noise would dramatically lower the ENOB of the entire measurement system.

Oscilloscope ENOB isn’t a specific number, but rather a series of curves. ENOB is measured as a fixed amplitude sine wave that is swept in frequency. Each curve is created at a specific vertical setting while frequency is varied. The resulting voltage measurements are captured and evaluated. Using time-domain methods, ENOB is calculated by subtracting the theoretical best fit sine wave from what was measured. The error between these curves can come from the front-end of the oscilloscope from attributes such as phase non-linarites and amplitude variations over frequency sweeps.

ENOB values will always be lower than the oscilloscope’s ADC bits.  In general, a higher ENOB is better. However, a couple cautions need to accompany engineers who look exclusively at ENOB to gauge signal integrity quality. ENOB doesn’t take into account offset errors or phase distortion that the scope may inject.

Figure 3: ENOB of the Keysight S-Series DSOS104A 1 GHz real-time oscilloscope from 100 MHz to 1 GHz.

Intrinsic Jitter (time interval error)

An oscilloscope jitter measurement floor impacts your time interval error, decreases your eye width, can cause timing violations, and compounds accuracy of correlated measurement across channels.

Measured in picoseconds rms or picoseconds peak-to-peak.  Contributions to jitter naturally occur in high-speed digital systems. Jitter sources include thermal and random mechanical noise from crystal vibration.  Excessive jitter is bad.  If you need to make jitter measurements, understanding how well your oscilloscope will make those measurements is critical to interpreting your jitter measurement results. Oscilloscopes sample and store digitized waveforms. Each waveform is constructed of a collection of sample points. A perfect oscilloscope would acquire a waveform with all sample points equally spaced in time. However, in the real world, imperfections in the internal scope circuitry horizontally displace the ADC sample points from their ideal locations and this value is represented in the jitter measurements that the oscilloscope makes. Oscilloscopes themselves have jitter and when they make a jitter measurement, they can’t determine which portion of the jitter measurement result came from the device under test versus the scope itself.

Oscilloscope jitter can come from interleaving errors, the jitter of the ADCs sample clock input signal, and other internal sources. This is also called the intrinsic source jitter clock (SJC). Oscilloscope vendors shorten the term to “intrinsic jitter” and use this term to mean the minimum intrinsic jitter value over short time period.  Jitter measurement floor is a function of noise, signal slew rate, and intrinsic jitter.  The term “jitter measurement floor” refers to the jitter value that the oscilloscope reports when it measures a perfect jitter-free signal. The scope’s circuitry that is associated with horizontal accuracy is known as the time base. The time base is responsible for time scale accuracy as well as the horizontal component of jitter. Oscilloscopes with well-designed time bases contribute less to horizontal jitter component of jitter and hence will report a lower value.

Figure 4: Measuring the Jitter using a histogram of a TIE measurement.

And of course don’t forget probing

Consider probe noise and its effects on measurement accuracy.  Choose a probe with a lower attenuation ratio for lower noise measurements.  Lower attenuation means higher signal-to-noise ratio (less noise), but lower input resistance, lower dynamic range, and lower common mode range.

Conclusion

Your oscilloscopes’ signal integrity makes a big difference in measurement results.  So choose a scope with superior signal integrity.  Evaluate noise, frequency response, ENOB, and jitter measurement floor. An easy way to do this is to ask a scope manufacturer to supply you with the data they’ve already taken.

As unit intervals continue to shrink, every picosecond matters.  You can’t afford to have your test and measurement equipment impact your measurement and analysis.  Understanding an oscilloscope’s characteristics and how they can impact your measurements is imperative.

Want to learn more about oscilloscope signal integrity? Check out the Evaluating Oscilloscope Signal Integrity application note.

# Do yourself a favor, read this.

Posted by KennyJ Sep 1, 2016

I hope to impart on you a bit of wisdom I have learned from my years of travel and talking with well over 1,000 oscilloscope users. If you do yourself the favor of reading through this you will have gained enough insight to not shoot yourself in the foot like so many of the scope users I have visited. They are not to be judged, they were doing the best they knew how at the time, and we all make mistakes or could do better—I know this to be true for me. Once I pointed out the mistake and the solution to these users they usually all had the same reaction—“oh, that makes sense”, followed by the classic palm slap to the forehead.

These users I speak of all made the same mistake. They spent valuable time and money selecting the best oscilloscope to buy or use for their measurement task. Then they connected a high quality probe to their scope. In some cases the probe was the nice passive probe that came with their scope, other times they had sprung for a snazzy active probe (smart move going for the active probe upgrade, more on this in another blog post). Then, and this is the crux of the matter, they put a bunch of long, dangly connection accessories onto the end of the probe. Maybe it was something innocent looking like a nice convenient long ground lead or one of those super helpful looking long red input wires that make it easy to connect the probe to a grabber that they could clip onto a part on their board.  In the end, the result was the same, the signal on screen “looked bad” or the device they were testing started to misbehave. This is usually when they grabbed me and said “Hey, you designed this probe. It’s not working right”.

What these users were experiencing was what I like to call the “weakest link” phenomenon. There are three links in the typical oscilloscope measurement chain—the scope, the probe and the physical connection to the target. You can have the best scope and probe that money can buy but if you put some crazy long wires on the end of the probe to make the connection to the target easier you have limited the performance of the measurement system to be equal to the performance of those crazy long wires. The connection accessories are the weakest link. They will limit the measurement bandwidth and they can excessively load your target.

Shorter is Better

At this point I can almost hear you saying “if those connection accessories are ‘bad’ why do you include them with your probes?” We include those accessories for convenience. The idea is that you use those accessories for qualitative measurements, things like “is the clock toggling”, is there “data on the bus”, “is the 5V up”. They make it easy to poke around your circuit quickly to check for functionality. If you want to make quantitative measurements like rise-time, over-shoot, noise levels, et cetera, then we intend for you to remove the convenience accessories and use the shortest connection possible. That’s it, that’s the punch-line, shorter is better.

Consider this example. I take my fancy 2 GHz active probe and I configure it three different ways, long wires connected to grabbers, long wires only and short input pin and ground contact. Notice how the bandwidth increases as the length of accessory in front of the probe gets shorter. By the way, we try to make it easy for you and we publish these bandwidth limitations in the product manuals.

Notice too how the probe loading (how the physical presence of the probe changes the way your circuit functions) decreases as the length of the connection accessory decreases. In this example the original circuit is producing a rising edge with a rise-time of 1.1 ns (the green trace). Connecting the probe to the circuit using the long wires and grabbers loads the circuit and the rise-time changes to 1.7 ns. When I remove the grabbers and just use the long wires the rise-time gets better, 1.5 ns, though you can see the connection accessories are still affecting the circuit. Finally, I remove all the wires and go with the shortest connections for this probe and the circuit rise-time is back to its original 1.1 ns.

Don’t feel bad if you’ve been making the mistake of using long connection accessories when making important measurements. You’re in good company, a lot of oscilloscope users have made this mistake and to be honest, I have too. Just remember, it’s ok to use those long, convenient connection accessories for a quick peek but if the signal looks strange or you are not getting the answer you expect, you’d do best take them off and go with the shortest connection possible. Shorter is better.

See all of the Keysight Oscilloscope probes.

# DSO versus DPO – What’s the difference?

Posted by JohnnieHancock Sep 1, 2016

DSO stands for Digital Storage Oscilloscope. DPO stands for Digital Phosphor Oscilloscope. A DPO is also a DSO. And a DSO can also be a DPO. So what exactly is a DSO and what is a DPO?

A DSO is typically a real-time sampling oscilloscope. Real-time sampling simply means that the scope is able to capture signals in a single acquisition utilizing a high sample rate analog-to-digital (ADC). In other words, a DSO doesn’t utilize repetitive acquisitions to “build-up” sufficient samples to represent the signal under test (equivalent-time sampling), although this is a not a hard-and-fast rule.

As mentioned before, a DPO is also DSO. But a DPO adds one additional element that allows it to better represent the signal’s third dimension. The first two obvious dimensions are voltage and time. The third and less obvious dimension is frequency-of-occurrence, which is represented by trace intensity on a scope’s display. If you can beckon back to the old analog scope days you may recall that these oscilloscopes were able to display a range of trace intensities. This can provide valuable insight into the true analog characteristics of a signal under test. This is especially true for complex-modulated analog signals as shown below, as well as for digital signals that contain varying degrees of noise and/or jitter.

With older analog scope technology, trace intensity variation was a natural phenomenon based on how much time the electron beam remained within a XY region on the inside face of the cathode ray tube (CRT). The inside face of CRTs of analog oscilloscopes are coated with a material called phosphor. When electrons strike the phosphor, the phosphor begins to glow. The more electrons that strike the phosphor in a given region of the CRT for a given amount of time, the brighter the phosphor glows.

When DSOs were birthed in the early 1980’s, this third dimension of trace intensity was initially lost as shown in the screen image below.

As technology progressed, oscilloscope vendors developed a technique that could closely emulate the display quality of analog oscilloscopes utilizing digital signal processing to bring the third dimension back from the grave as shown in the screen image below.

Basically, by counting the number of hits (digital samples) in particular XY regions of a bit map — sometimes called buckets — pixel intensity could be digitally modified to represent trace intensity modulation of phosphor. This is where the term Digital Phosphor Oscilloscope (DPO) came from.

So why doesn’t Keysight have DPOs? Actually, we do. But we don’t call them DPOs. Nearly all of Keysight’s DSOs employ trace intensity modulation. In fact, Keysight’s oscilloscope display technology provides the highest quality trace intensity modulation due to the fact that Keysight scopes have the industry’s fastest waveform update rates with deep memory acquisitions. This provides more hits in XY regions (buckets) in a shorter amount of time to provide a higher level of statistical information for which to base pixel intensity upon.

So why doesn’t Keysight call them DPOs? Keysight believes there is enough confusion concerning different names for the same basic instrument. My large screen flat-panel television that I watch Rockies baseball games on is still just a TV even though it utilizes an entirely different technology than older CRT-based televisions. And besides, why use an old analog technology term when many of today’s younger engineers have never used an analog oscilloscope and don’t have a clue what phosphor has to do with an oscilloscope? Same goes for the term “sweep”. Refer to one my previous blog posts titled, “Oscilloscope Triggering: When is Normal not so Normal?”.

Maybe we should call them DSOWDPTMSCDSAAMDMCs (Digital Storage Oscilloscope with Digital Phosphor Technology, Mixed Signal Channels, Digital Signal Analysis, and Mixed Domain Measurement Capabilities). But in my eyes, it’s still just a scope! And if you are an Aussie, it will always be a CRO (pronounced “crow”).

# Capturing Intermittent or Infrequent Events?

Posted by rlashlee Sep 1, 2016

Oscilloscopes have a lot of specifications – some more readily understood than others. One specification that has recently become more frequently discussed is update rate. Despite its importance, there may still be many oscilloscope users who do not understand update rate nor why it impacts their measurements.

When you debug new designs, waveform and decode update rates can be critical—especially when you are attempting to find and debug infrequent or intermittent problems. These are the toughest kinds of problems to solve. Faster waveform and decode update rates improve a scope’s probability of capturing elusive events. To understand why this is true, you must first understand what is known as oscilloscope “dead time”.

Every oscilloscope has an inherent characteristic called “dead time” or “blind time”. This is defined as the time between each repetitive acquisition of the scope when it is processing the previously acquired waveform. Unfortunately, oscilloscope dead times can sometimes be orders of magnitude longer than acquisition times. And during this dead time, any signal activity that may be occurring will be missed by the oscilloscope, as shown in the figure below. The waveform update rate specification tells you the number of acquisitions by the oscilloscope per second. The larger the waveform update rate, the more acquisitions per second, and the shorter the dead time.

This issue of large dead times becomes particularly problematic when capturing random or infrequent events as you are essentially rolling the dice on whether you will capture these or not. The shorter this dead time, the more likely you are to successfully capture an elusive event. As an example, many of the Keysight InfiniiVision oscilloscopeshave waveform update rates of up to 1,000,000 wfms/sec. When capturing an infrequent metastable state (glitch) that occurs approximately 5 times per second using a oscilloscope with an update rate of 1,000,000 waveforms per second, this scope has a 92% probability of capturing this glitch within 5 seconds. In comparison, other oscilloscopes in this class may update waveforms only 2000 to 3000 times per second. These scopes would have less than a 1% probability of capturing and displaying an infrequent glitch such as this within 5 seconds.

It is also useful to understand how an oscilloscope’s update rate is impacted by other features or functionality. For example, some scopes spec a best-case scenario, but when features such as MSO, protocol decode, or math functions are turned on, the update rate can drop significantly. Therefore, it is important to know how the features you will be using for your applications impact the update rate of your oscilloscope.

Here is an application note that goes into greater depths on this topic, including how to measure your oscilloscope’s update rate: Oscilloscope Waveform Update Rate Determines Ability to Capture Elusive Events.

See update rate in action with Keysight’s video on debugging infrequent events.

# You Have Anything That Can Measure Ripple And Noise On Voltage Rails?

Posted by KennyJ Sep 1, 2016

Being an oscilloscope probe design engineer I get the chance to get out of the cave several times a year and talk with our users so that I can better understand the measurements they want to make and what they need from us to make their lives easier.  In a typical conversation we would be discussing the next type of DDR memory or CPU (or whatever) that the users need to probe/measure and inevitably the question would come up—you got anything to measure ripple and noise on my power supplies? Initially the answer was no. These users wanted to measure mV ripple and noise riding on top of their 1.8 V, 3.3 V…24 V supplies. This is kind of a specialized measurement. To turn that answer into a yes I would need to design a specialized probe. But before I could do that I had to understand the application and measurement need better. Here is what I learned and what we came up with.

Thanks to Moore’s law, the doubling of gates on an IC every 18-24 months, the electronics that we encounter everyday are packed with more functionality in ever smaller, denser packaging. Consider your mobile phone. It wasn’t that long ago they were a brick that performed a single function—they made a phone call, and now they are elegantly thin machines that can give turn-by-turn directions, shoot high-definition video, monitor some of my biological functions and respond to my voice control. With this increased functional density comes some power related problems—power density and power supply induced jitter (PSIJ—power supply induced jitter can be the single biggest cause of clock and data jitter in a digital system). Folks learned that if they put tighter tolerances on the ripple and noise on their supplies and reduced the supply voltages where possible, they could reduce their power and jitter issues. It is not uncommon today to see supplies with tolerances of 1-3%. I saw an LPDDR with a 0.6 V supply with a 1% tolerance—this means measuring 6 mVpp ripple and noise.

Based on what our users shared with me I distilled the measurement challenges down to this: the need to measure a small AC signal riding on top of a large DC signal. If the AC signal exceeds the tolerance, the design failed to meet its requirements. This illuminated the challenges we needed to overcome. Users needed a very low noise probe & oscilloscope combo so that their ripple and noise were not overshadowed by the noise of the measurement system. They also needed a way to remove the large DC offset so they could put the signal in the center of the screen and zoom-in on it (get down to 1 mv/div if needed). The measurement tools also had to have enough bandwidth to capture the high frequency noise caused by the switching of the digital circuits. Since this is a function of clock speeds many users needed up to 2 GHz of bandwidth (frequencies above this are attenuated quickly by the circuit board fairly close to the noise source). And in addition, they had to have a probe that would not load the supply. For example, some users would connect a 50 Ω cable to the 50 Ω input of the scope to measure ripple and noise—for a 3.3 V supply the scope would sink 66 mA which can change the behavior of the supply.

Here is what we came up with, the Keysight N7020A Power Rail probe. The first and only probe designed specifically for making ripple and noise measurements on supplies. The probe has 1:1 attenuation ratio so that full size signals make it to the oscilloscope. This creates a very favorable signal:noise ratio. There is ±24 V of probe offset. This means the probe can remove up to 24 V of DC content from the signal so that signal can be placed in the center of the screen and be scrutinized at high sensitivity settings.

Desperate users had been making use of DC blocks/AC coupling/DC reject to remove the DC content. They told us they disliked this because if filters the signal. A DC block is a ‘big’ capacitor so it also blocks low frequency supply drift and supply compression from being seen on the scope. Since the probe is active it has a DC input impedance of 50 kΩ which means it won’t change the behavior of the supply when it is attached. Finally the probe has 2 GHz bandwidth so that users can capture high frequency digitally induced noise on their supplies. Not everyone needs this much bandwidth so for those that don’t we recommend using the oscilloscope’s built-in bandwidth limiting capabilities so as to minimize oscilloscope/probe noise. And, I almost forgot, the probe comes with a lot of cool connection accessories so that you can easily probe a variety of locations on your target.

If you are curious to learn more, there is a great teardown video of the probe at The Signal Path:

# How to measure your oscilloscope (and probe)’s bandwidth yourself

Posted by KeysightOscilloscopes Sep 1, 2016

By: Taku Furuta

Visiting engineers that use our gear is exciting and it's often an excellent learning experience. My visits with engineers cover a range of topics from answering questions on specific technical capabilities to presenting the latest technologies to researching product use models.

In these visits, I often demonstrate product features and benefits when using oscilloscopes. However, it is also common that customers show me their oscilloscope measurement tips that blow my mind.  My talked about the formula to figure out system bandwidth (the bandwidth of the scope + the probe). Now let me share a neat measurement where you can quickly find the “true” bandwidth (or system bandwidth) of your oscilloscope yourself, a tip that an engineer taught me about 15 years ago.

“Let me test it to see if your new scope REALLY has 6 GHz bandwidth” said a customer in the very first VIP visit disclosing Keysight’s (then Agilent) first 6 GHz real-time oscilloscope.  This caught me by surprise as no previous customers I had ever met had made such a statement.

“I need to connect my fastest step response generator to your scope’s front end first”.  He plugged in his faster-than-50-ps edge rate step signal generator and then differentiated the signal using the “differentiate” math function to derive the impulse response signal.  He continued and applied the “FFT” (Fast Fourier Transform) math function to the calculated impulse response signal in order to plot the frequency content from DC all the way up to 6 GHz (and beyond).  Finally, he nodded, smiled and told me, “Excellent and congratulations!  Your scope has more than 6 GHz of analog bandwidth” by pointing out the FFT plot where the FFT value finally attenuated down by -3 dB (the bandwidth point).

In theory, a perfect step response has an instantaneous (zero) rise time, and therefore you can mathematically derive the perfect impulse response by differentiating it.  And in theory, the perfect impulse response signal has an infinite amount of frequency content, so it is a perfect signal to check the “finite” bandwidth limit of an oscilloscope’s front end.  No signals are perfect, but the fast edge rate step generator can serve this purpose well.  Wow, what a quick and clever way to test the system!  Ever since this customer visit, this has become my favorite method to demonstrate an oscilloscope’s front end bandwidth performance.

Alright, enough of a nostalgic story. Here are the step by step instructions with oscilloscope screenshots for you to duplicate this “measure of your oscilloscope’s true bandwidth” procedure.  As I wrote in my first blog, most oscilloscopes come with a little “more” bandwidth than what’s specified in their datasheet, so this may be a fun exercise!  The procedure will be a little simpler if you have a Keysight InfiniiVision X-Series (3000A/TX, 4000X or 6000X) oscilloscope because you can generate a pretty fast step response using the trigger out function of the X-Series.  You can alternatively use your favorite step generator, but be sure the edge rate is fast enough to contain enough frequency to test your oscilloscope.  I recommend using an edge rate more than twice as fast as the calculated rise time specified in the scope’s datasheet.  Also, note that the accuracy of your measurement heavily depends on the cleanness/flatness (signal integrity) of the input step response.

 InfiniiVision Series Oscilloscopes Trigger out edge rate 6000 X-Series (DSO/MSO-X 6000A) ~ 700 ps 4000 X-Series (DSO/MSO-X 4000A) ~ 1.4 ns 3000 X-Series (DSO/MSO-X 3000A/T) ~ 1.7 ns

Table 1:  The summary of the trigger out signal’s edge rate

Step 1: Connect a fast edge rate step response signal.  The below example uses the trigger out signal of the InfiniiVision 6000X (~700 ps edge rate).  Scale the signal so the edge gets placed at the center of the screen.  Make sure to vertically maximize the signal without clipping it in order to use full 8 bit resolution of your scope’s analog to digital converter (see the blog post “This Quick Trick Makes Your Oscilloscope Measurement 1,000 Times Better” for more detail).  Change the channel’s input impedance to 50 Ω to match your source.  Usually, a fast step generator has an output impedance of 50 Ω.  The output impedance of the InfiniiVision X-Series oscilloscope’s trigger out signal is also 50 Ω.

Step 2:  Apply the differentiate math function to your step response signal (channel 1 in this example).  For the Keysight InfiniiVision oscilloscopes, the differentiate math function is available on the 3000AX, 3000TX, 4000X and 6000X.

Figure 1: The step response signal (yellow) and the math function impulse response signal (purple)

Step 3: Apply the FFT math function to your impulse response signal (math function 1 in this example).  For the Keysight InfiniiVision oscilloscopes, the FFT math function is available on all models.

Figure 2: The step response signal (yellow) and the math function FFT plot (purple) of the impulse response signal

Step 4:  Turn on cursors to measure the frequency where the signal is attenuated by -3 dBm.  You can read the ΔY value in the cursor readout to precisely determine this point.  This is the true measured bandwidth of your oscilloscope.  In this example, it measured the “true” bandwidth of a 200 MHz InfiniiVision MSO-X 4024A to be around 250 MHz while the product’s specification says 200 MHz.  A nice bonus of an extra 50 MHz.

Figure 3: Measuring the oscilloscope’s bandwidth using the cursor

Now, let’s expand the same concept to measure the system bandwidth of your oscilloscope and probe. A similar connection can be used, however, it will require a probing point for the probe to pick up the signal.  What I usually use is a 50 Ω microstrip line fixture like the ones shown below.

Figure 4a: Handmade microstrip line 50 ohm fixture

Insert the fixture between the cable and the scope’s BNC channel input and then probe the signal with the probe you want to measure the system bandwidth for.

Figure 5: Connecting the probe to the fixture

Once you have the probed signal on screen, just repeat the steps describe above.  The next section shows two screenshots from the system bandwidth measurements done on the 200 MHz scope; 200 MHz scope + 100 MHz passive probe and 200 MHz scope + 200 MHz passive probe.  You will note the “true” bandwidth is higher than the calculated bandwidth, since both oscilloscope and probe usually have a slightly more bandwidth than they specify.

Figure 6: 200 MHz oscilloscope + 100 MHz probe system bandwidth

For example, the measured system bandwidth in Figure 6 is around 140 MHz.  If you were to use the formula from the previous blog, the theoretical system bandwidth of a 200 MHz and a 100 MHz probe should be around 90 MHz, so you are getting ~ 50 MHz more due to “bonus” bandwidths on the scope and the probe.  In fact, because you already know the true bandwidth of this 200 MHz scope is around 250 MHz, you can easily find that this 100 MHz passive probe actually has around 170 MHz bandwidth using the same formula!  Note, I’m assuming both the scope and probe have the Gaussian response filter.

Figure 7: 200 MHz oscilloscope + 200 MHz probe system bandwidth

In the final example, it measured system bandwidth to be around 200 MHz, as seen in Figure 7.  Applying the same formula, you can calculate quickly that this 200 MHz probe actually has more than 300 MHz bandwidth, 100 MHz additional bandwidth beyond the specified value!!

Posted by KennyJ Sep 1, 2016

Why, you may ask, would I want to spend money on an active probe for my oscilloscope when it came with free passive probes? As an oscilloscope probe designer I’m going to share with you a reason why you should consider upgrading to an active probe—and it’s not about bandwidth. I think a lot of folks who upgrade to an active probe do it because they need more bandwidth. Most passive probes top out at about 500 MHz so if you need more bandwidth than that you’ll need to buy an active probe. An active probe offers other benefits that should be considered even if you only feel you need a 100 MHz of so of bandwidth. I’m going to point out one that I think is most often overlooked.

Now we will put this to the test. I’ve got a circuit that contains a 1.1 ns edge. Traditional guidance would suggest that I need about 300 MHz of bandwidth in my measurement system (scope and probe) to measure this signal. This is well within the capabilities of our free passive probe. I first probe the signal with my active probe and I measure the rise time. Just like I expected, 1.1ns. Now I remove my active probe and probe the signal with my passive probe. Oops, I’m measuring 1.5 ns. Is my measurement wrong? No, my measurement is correct. That is what the edge speed of my target signal has become due to the loading of my passive probe. The large capacitance to ground of the passive probe is creating a low impedance path to ground for the higher frequency content of my signal and my target cannot drive this load so the actual signal on my target is distorted.

What you can conclude is that a passive probe is good for making qualitative measurements and an active probe is good for making quantitative measurements. Qualitative measurements are things like: is the patient’s heart beating, is the 5V up, is the clock toggling..? Quantitative measurements are things like: what is the patient’s heart rate, how much ripple/noise is on the 5V supply, what is the rise time..? Do yourself a favor, next time you get a chance, splurge and buy an active probe.

# “Hello Scope” – Oscilloscopes Past and Present

Posted by KeysightOscilloscopes Sep 1, 2016

By: Taku Furuta

I bought my first car navigation system, or “nav”, back in 2000. I believe I was still one of the “early adopters”; however, the car nav was already becoming a popular car electronic in Japan by then.  Actually, it was a pretty fancy one with a retractable display, 3D virtual map, built in gyro-compass (so it would still provide guidance even when satellite signals were lost), altitude meter and more.  However, what impressed me the most was the “full voice control system”.  “Take me home”, “call my mom”, “100 m scale”, “avoid traffic jam” were some of the popular commands I used back then.  Oh, yeah, and the nav even spoke different Japanese dialects depending on the location and setting I chose.

Coincidently, year 2000 is when I joined Keysight, (Agilent at the time). Joining one of the most technically savvy companies in the industry as an oscilloscope product line manager, I had a high hope of “what if oscilloscopes can hear my voice, too”.

Well, Agilent (Keysight) certainly did not disappoint me. In fact, did you know that Agilent has had a voice control enabled oscilloscope since 1999?  The product was called “Option 200: VoiceControl for Agilent Infiniium Oscilloscopes (E2635A)”.  Here is the picture from the original datasheet.

The option understood popular scope operating commands like “Run”, Stop”, “Default Setup” and “Auto Scale”. It controlled the vertical setting (like volts/div), horizontal controls (like time/div or delay sweep), and trigger and storage commands.  In another words, the most popular operations were possible via voice commands… in 1999!  In fact, many of my customers back then were asking for a scope “foot switch” for those operating it hands-free in the manufacturing line or engineers holding two probes in both their hands.  “Wow, this must be a perfect solution, just like I loved my car nav voice commands!”  At least, this was my first reaction as a first year product line manager.  Well, it did not come out to be exactly that way.

First it understood English, to be specific American English, but nothing else. Growing up in the US, I had no problem using it. Believe it or not, it didn’t understand my good-old colleague’s British English!  Obviously it did not understand Japanese and perhaps had a lot of trouble with “Asian pronounced English” as well.  There was no “Siri” back then and I guess I don’t need to talk about the sales results.  However, I thought it was a brilliant idea as the fundamental needs were there.  Second, as you can see on the image above, one must use an included special “microphone” when talking to the scope, which was just one more device to lose.

Now, let me fast forward the clock to the year 2014, 15 years after a great but crazy innovation. If my memory is correct, no other oscilloscope vendors released another voice control enabled scope since the Infiniium option 200. And so Keysight tries again, in the era where voice control is a lot more pervasive, again thanks to Siri and Google devices in the market.

So, the new InfiniiVision 6000 X-Series oscilloscopes released in April 2014 comes with the world’s only voice control system, but this time with 14 different languages and dialects, including English (American), English (British) and English (Indian)! And yes, it understands Japanese as well.  Furthermore, no dorky microphone is needed this time.  And off course, now it is powered by the Nuance Communications, Inc. voice recognition engine (the company who build the Siri voice recognition system).

So, the next time, you see the InfiniiVision 6000 X-Series scope, make sure to say “Hello Scope” and it will gladly listen and respond to your commands in most languages around the world.

What’s the next crazy and innovative idea? What should all scopes have as a standard feature in 2030, another 15 years from now?  As a Keysight oscilloscope planner, my job is to help realize your craziest oscilloscope dreams!  Let me know!!

# How to Create Bode Plots on an Oscilloscope

Posted by mike1305 Sep 1, 2016

I discovered a great video the other day by Dave Jones of EEVBlog, in which he “tricks” our oscilloscope into creating a bode plot on screen. So today I’ll share a version of the experiment I ran and some additional improvements to bode plots on oscilloscopes that Keysight has made since 2012.

First, connect a waveform generator to the input of your device under test, and connect the output to the oscilloscope. In this simple example I breadboarded a low-pass RC circuit, so the waveform generator supplies voltage across the RC, channel one is measuring the voltage across RC, and channel two is the output across C. I default-setup the scope and offset the channels so you can see each (yellow and green).

Next, I turned on the first waveform generator to configure the input signal. I want a 200 kHz sine wave with 5 volts peak to peak. These two shots are from the first level WaveGen1 menu.

Then I pressed Settings > Modulation to enter the modulation configuration. To do a bode plot, we need an input signal that sweeps the desired frequency range. I select “ramp” as the modulation waveform, and change symmetry to 100% (making the ramp into a sawtooth waveform, in essence) which will cause frequency to sweep from 0 Hz (200 k-200 k) to 400 kHz (200 k+200 k) in a linear fashion. Then I tapped the modulation key on the left to turn it on.

Whoa, that’s trippy dude.

The FM frequency is set to 1 Hz so that this sawtooth frequency modulation occurs once per second (one sweep per second).

This can be observed when you kick the horizontal timebase to 100 ms/div, or one second across the screen.

The distinctive line in the input (yellow) is where the modulation starts the sweep over. The green waveform is the output of the filter, which is clearly attenuating as the frequency passes its 3 dB point.

Now let’s make it look like a bode plot! At this point the triggering is still setup to rising edges of channel one, which there are millions of on screen, so the trigger is anything but stable at this point. Let’s make a unique trigger, taking advantage of the output’s visible waveform characteristic. Open the trigger menu and turn it to channel 2. Also adjust the trigger level to be towards the crest of the green signal, like so (note the green arrow and T on the left of the screen).

Then, pop into the trigger mode menu, change it to “normal” (it only triggers once a second, causing the oscilloscope to trigger automatically when in auto mode, messing up our display). Also add a holdoff of 200-500 ms so that it waits for the next period before finding another edge.

Now all you have to do is scale the channel 2 waveform as such to make it appear bode-like. Of course this is a linear bode plot, not logarithmic, which is an unfortunate limitation of the waveform generator’s FM modulation schemes. If there were a logarithmic output, the output would be much more familiar. This is another reason Dave Jones used an external generator in his video.

Finally, check the references below for a link to Dave Jones’ video, as well as a white paper on a new application for Keysight InfiniiVision oscilloscopes that can create frequency response plots with the scope and some external hardware. Thanks!

References:

Make Power Supply Control Loop Response (Bode Plot) Measurements Using a Keysight Oscilloscope

http://www.eevblog.com/2012/12/08/eevblog-396-bode-plotting-on-your-osciloscope/

# The Crow

Posted by JohnnieHancock Sep 1, 2016

During my nearly 37 year career at HP, Agilent, and now Keysight, I have presented lots of oscilloscope seminars and workshops to our current and potential customers. Back in the late 1980’s I did my first oscilloscope seminar tour in Australia with a focus on explaining the differences between analog oscilloscopes and digital oscilloscopes. During this seminar tour I kept hearing the Aussies refer to the scopes as “crows”. In my mind I was picturing an annoying bird and thought that this must be some kind of derogatory term. During this era digital oscilloscopes were in their infancy, and as such had some quirky behaviors relative to their analog predecessors. So this made sense to me that these guys might be frustrated with digital scopes and would call them names. After all, I sometimes call my instruments names if I can’t get them to behave properly. Although it is usually a pilot error on my part when this happens. But never “crow”. Finally during one of the seminars, I asked, “Why do you guys keep calling these things “crows”? It was explained to me that they weren’t calling the them “crows”, but were referring to them by the acronym CRO, which stands for cathode ray oscilloscope.

To this day, Aussies still affectionately refer to their scopes as CROs, even though oscilloscopes no longer have cathode ray tubes. But I guess we still refer to rolling up our windows in our cars and dialing a number on our phones. So we’ll forgive the blokes down under for calling their scopes “crows”. Which brings up another thought. Why is Australia considered “down under” instead of “up over”? Who decided north was up and south was down?

What else are scopes called? There’s digital storage oscilloscope (DSO). There’s digital phosphor oscilloscope (DPO). There’s mixed signal oscilloscope(MSO). There’s mixed domain oscilloscope (MDO). And there’s sampling oscilloscope. What’s the difference?  Perhaps this will be the topic for a future blog. G’day.

# What is oscilloscope system bandwidth and how do I find the bandwidth of the scope + probe?

Posted by KeysightOscilloscopes Sep 1, 2016

By Taku Furuta

“I am using a 100 MHz oscilloscope with an included 100 MHz passive probe, I am supposed to be able to measure a 90 MHz sine wave, right?  Is the scope or probe broken?”

I hear this sort of question popping up from time to time, understandably since most oscilloscope datasheets do not discuss the “system bandwidth” or your effective bandwidth when a scope is used with a specific probe.

Both an oscilloscope and a probe have bandwidth specifications, the frequency value where the amplitude of input signal attenuates by 3 dB.  So, if your scope’s datasheet specifies its bandwidth at “100 MHz”, you are guaranteed to measure at least ~70% of your signal amplitude at its bandwidth frequency.  The same can be said for your probe as well.  The tricky part is, however, your oscilloscope + probe bandwidth, or your “system bandwidth”, may not be 100 MHz when you use them together. So, what is the system bandwidth in this case?

Before knowing your system bandwidth, you need to know the front end filter response of your oscilloscope.  You may or may not find this info in the datasheet, so call your scope’s support line if it is not stated.  If you don’t want to call/write the support line, I’ll provide you a quick tip to figure this out by just looking at the calculated rise time specifications in datasheets at the end of this blog.  However, it is a good rule of thumb to think the filter is a “Gaussian” type if the bandwidth of your scope is below 1 GHz.  For oscilloscopes with 1 GHz or more bandwidth, it could have a filter type called a “Brickwall”.

In the case of the Gaussian filter, which is the traditional front end filter type used for decades in both analog and digital storage oscilloscopes, the scope and probe’s system bandwidth is calculated using the below formula.

Let’s apply the above example to this formula.  Since your scope’s and probe’s bandwidth are 100 MHz each, your system bandwidth will be 70.7 MHz.  In other words, your signal’s amplitude is attenuated by 3 dB at 70.7 MHz.  Obviously, you will not see full amplitude of a 90 MHz sine wave!

In reality, most of oscilloscope manufacturers add some margin to the bandwidth specifications of both scopes and probes.  So, if you see the specification says “100 MHz”, it most likely has some additional bandwidth, like 110 or 120 MHz.

Now, say if you have a “Brickwall (or maximum flatness)” type filter response oscilloscope and probe instead.  It is extremely rare to see the Brickwall filter on a 100 MHz scope, but for this example say you did.  In such case, unfortunately, the former “square root of sum of squares” formula cannot be used.  In this case, the system bandwidth formula will be:

System Bandwidth = min {scope bandwidth, probe bandwidth}

So, if I apply the original example to this formula, your system bandwidth is now at 100 MHz, therefore, you should see nearly full amplitude of your 90 MHz sine wave.

I am not sure why this simple formula has disappeared from most oscilloscopes’ datasheets.  Perhaps there is more than sufficient bandwidth in most oscilloscopes today where engineers do not need to operate them at their upper limits.  Perhaps this is already taught in school.  Nevertheless, this is a quite useful tip to know, especially if you are seeing unexpected measurement results.

BTW, here is a quick and dirty way to determine if your scope has the “Gaussian” or “Brickwall” type response filter.  First, find your scope’s calculated rise time info.  The below is an example from Keysight InfiniiVision 4000 X-Series oscilloscope.

Now, divide “0.35” the calculated rise time value.  In the case of the 200 MHz oscilloscope (4022A), it will be

0.35 / 1.75 ns = 200 MHz

So, you verified the coefficient it was used to calculate the rise time was “0.35”.  0.35 is the coefficient value for a “Gaussian” response filter, so you know this 200 MHz oscilloscope has a Gaussian filter front end.  On the other hand, if you apply the same formula to 1 GHz oscilloscope (4104A),

0.35 / 450 ps = 778 MHz

The value was 778 MHz and not 1 GHz.  Well, you now know the coefficient used for this oscilloscope was not “0.35”, but was “0.45” (0.45 / 450 ps = 1 GHz).  When the coefficient value is larger than 0.35 such as 0.4, 0.45 or even 0.5, it indicates the scope’s front end has a filter response closer to the Brickwall filter.

Hope this small tip helps you to understand the scopes better.  See you all in the next blog!

# Oscilloscope Triggering: When is Normal not so Normal?

Posted by JohnnieHancock Sep 1, 2016

Oscilloscopes have two primary modes of triggering: AUTO and NORMAL. However, NORMAL is not the normally used mode of triggering. AUTO is. The default trigger mode in all of today’s oscilloscopes is AUTO. There is a lot of confusion these days among oscilloscope users as to exactly when to use which mode of triggering. Let’s first define what these terms mean and then discuss how these modes of triggering came to be called what they are.

AUTO simply means “automatic”. In the AUTO trigger mode, the scope will trigger on the signal under test if a trigger condition is met, such as a rising edge. But if a trigger condition doesn’t occur within a predetermined amount of time, the scope will begin to generate its own automatic triggers, which are not synchronous to the signal under test. This means that the scope will show of blur of waveforms when this happens. So if AUTO is always the default trigger mode, why would you ever want to see a blur of waveforms? One reason is that a blur of waveforms will show you where the signal is relative to your trigger level. Perhaps you have the trigger level set above (too high) or below (too low) the signal under test. With AUTO trigger you can see what’s wrong and make adjustments. Setting up an oscilloscope is an iterative process of seeing what’s there and then making adjustments (V/div, sec/div, trigger level, etc.) until it is right. Another reason the AUTO trigger mode is the default mode of triggering is that you may want to simply view the DC level of a power supply. Scopes can’t trigger on DC, unless the DC includes lots of switching noise, in which case it is not purely DC.

The NORMAL trigger mode means that the scope triggers if and only if a trigger condition is met. If you’ve got your trigger level set above or below the signal under test, then you’ll be looking at a blank screen on your scope. So when should you use NORMAL triggering? If the signal you want to trigger on occurs very infrequently, perhaps once every three seconds, then you should use the NORMAL trigger mode so that the scope will display synchronized representations (waveforms) of your signal only when trigger event occurs, and not generate automatic and asynchronous triggers between qualified trigger events and thereby show you blurs of waveforms.

So why is this trigger mode call NORMAL? I can only guess. Back in the old analog scope days, this trigger mode was not called NORMAL triggering. It was call the TRIGGERED sweep mode, which makes sense. When a trigger qualification was met, such as a rising edge, the analog scope would trigger a linear sweep of an electron beam across the scope’s cathode ray tube (CRT). But when digital storage oscilloscopes (DSOs) came along about 30 years ago, the representation of waveforms on the scope’s display changed from the sweep of an electron beam that excites phosphor on the CRT to the digitization and storage of discreet waveform points using an analog-to-digital converter (ADC) and then represented as pixels on a scope’s display. Since newer technology scopes stopped sweeping, most oscilloscope vendors began calling it a “trigger” mode instead of a “sweep” mode. And if they had kept using the same old analog scope terminology it would have become the TRIGGERED trigger mode, which sounds redundant. So some genius marketing guy must have said, “Let’s call it the NORMAL trigger mode — maybe because it was the trigger mode that he or she normally used.

Note that some DSOs still call it an AUTO and TRIGGERED sweep mode. I feel sorry for the younger engineers that have no idea what a sweep is.

In my opinion the AUTO and NORMAL trigger modes should be called AUTO On/Off. To me, this makes more sense. But I know that’s not going to happen, just like Australians will never stop calling their oscilloscopes their “crows”, which I think will be the topic for my next blog.

Anyone out there know for sure how this mode of triggering came to be called NORMAL?

# How Does Signal Modulation Work?

Posted by mike1305 Sep 1, 2016

To understand how wireless data transfer happens, we need to understand:

•    What is frequency?
•    Message / Data Signals
•    time representation
•    frequency representation, and why is it important?
•    How do filters work?
•    FCC Communication bands
•    Modulation and demodulation

You can spend years at University learning these subjects in depth (or on Wikipedia, if that’s your style!). This is designed to be a flash flood of knowledge. This was originally put together as a PowerPoint for non-EE students in my senior project group who were curious about our lingo when we talk about “900 MHz” or “2.4 GHz” or “Frequency Hopping”. As such, it is not complete, thorough, and skips many details that one would include in a professional analysis of a system. This is only to provide a concept of wireless transmission.

What is Frequency?

Frequency is the unit describing how often something oscillates, or goes back and forth. Units are Hertz (Hz), or the inverse of a second. Something oscillating 60 times per second has frequency 60 Hz. For our purposes, we are going to focus on audio waves (oscillation of air pressure) and how it gets broadcasted from a radio station to your car in the range of hundreds of kilohertz (or any AM radio station). Any wave has a frequency – light, for example. Generally light and other higher frequency waves (e.g. x-rays, gamma rays, microwaves) are represented by their wavelength, not frequency. For example, green light is around 400 nanometers. Here is a picture showing the relationship of units on a traveling wave:

Basic units of a sine wave.

Assuming constant speed of the signal, wavelength and frequency are interchangeable. That is outside the scope of this article, though.

Message Signals of Varying Complexity

Sending a signal that is a pure sine wave is called a “tone”. It carries no real information, and doesn’t sound that great either. Here is an image of a sine wave, with time on the X axis and voltage on the Y axis. This is 150 Hz for reference.

Single tone signal (time domain)

Okay, so why am I showing you this? Let’s take a look at increasingly complex signals in the time domain. Here is a two tone signal (two tones, added together). It is the same sine wave above, added together with another sine wave with twice the frequency, 300 Hz.

Dual tone signal (time domain)

How about a signal composed of many tones of varying frequencies:

Multi-tone signal (time domain)

It’s starting to get a bit hairy. The only real information you can gather from that is voltage level at a specified time. That’s the essence of a message, and extremely important – but makes for difficult analysis, and even more difficult for understanding the way modulation works. This is why you may want to use a different way of graphing a signal: the frequency domain. It is a representation of how strong the signal is over a range of frequencies. Let’s look.

Why is the Frequency Spectrum of a Signal Important?

There is a precise mathematical operation to convert a chunk of a signal into the frequency domain. It is dense, difficult, and takes practice to master. I even struggle with convolution of non-trivial signals regularly. Regardless, let’s see what our three signals above look like in this representation (skipping to the solution). Instead of plotting a signal’s voltage in time, we are plotting the power of the signal by frequency.

Single tone signal (frequency domain)

Dual tone signal (frequency domain)

Multi-tone signal (frequency domain)

Notice the clear spikes? That is the mathematical representation of a sine wave at that particular frequency (X-axis). Ideally, these spikes would be infinitely narrow (width) and infinitely tall, but due the techniques used by my Spice software, it is imperfect. This is called an impulse signal. Read more on this here! For the tone, we see one spike at 150 Hz. The dual tone has two spikes, 150 Hz and 300 Hz. The multi-tone signal that was unreadable in the time domain has been clearly chopped into small spikes, representing all the frequencies that were summed to create the signal.

A final example would be to show an audio signal. In the below picture, I have taken a 15 second sample of the song “White Room” by Cream. Don’t worry, no microphones were damaged during Eric Clapton’s guitar solo!

Audio Signal

This is how most signals appear, especially analog ones. The human voice and instruments do not play as discreet frequencies, and thus there is frequency content over an entire range (even though some of that content is almost inaudible). This range is taken from 3 Hz to 20 kHz, the approximate range of the human ear. Bass notes are lower in the range, while treble is higher. The Y-scale is represented in dB, which is a unitless representation of proportion. In essence, the higher the dB value, the more of that frequency is in the signal.

In theory, we can represent this analog signal as the sum of an infinite number of tones added together.

Filters!

Hopefully the graphical representation of frequency domains will shed some light on filter design. There are four types of filters:

•    Low Pass filter: all frequencies over the “cutoff” are removed.
•    High Pass filter: all frequencies under the “cutoff” are removed.
•    Band Pass filter: All frequencies outside a distance from the “center” are removed.
•    Band Stop filter: All frequencies within a distance from the “center” are removed.

Clockwise: Band Pass, High Pass, Low Pass filters

The “3dB” point is where signal output is reduced by ~30%. It has to do with how “log” magnitude is calculated (dB is a log scale):

x [dB] = 10 * log(x[linear])
x [linear] = 10^(x[dB]/10)

Based on this, a gain of 0.7 [linear] is approximately -3.0dB (and change). It’s what is referred to as the cutoff frequency of a filter. A practical example of this is your car stereo, which may include a “crossover”. This is a special filter design that routes low frequencies to your woofer, high ones to the tweeters, etc. This is very important in radio receivers.

FCC Communication Bands

The FCC and other organizations worldwide have agreed that it would be absolute chaos to allow anyone to use any frequency for their own use. Thus, there are special allocations of frequency ranges for different uses. Examples include FM radio, AM radio, WiFi, cell phones, maritime communications, air traffic control, HAM radios, walkie talkies, military communications, police radios, and the list goes on. We haven’t even talked about satellites or space communication, either! It’s a crazy world out there and thankfully the FCC helps organize it all. A quick Google search will provide you a more detailed image and tables if you’re curious.

The FCC Spectral Allocation Table

The FCC has left a few bands open for low range personal use, hobbyists, and other general use in the “ISM bands” (Industrial, Scientific, Medical). This is where WiFi, walkie talkies, wireless sensors, and other commercial devices operate. Let’s talk frequencies again! The human ear has a range of 20 Hz to 20 kHz. What if our AM talk station is 680 kHz? How does the radio tower get the sound up to that frequency? How does it not interfere with other stations? How does the receiver bring the signal frequency back to an audible range?

Modulation

Let’s step away from the frequency domain and go back into the time domain. I am again making generous use of my earlier disclaimer: this is over-simplified and skips many details! This is only to get the concept. The reason I say this is because the math works out best in the time domain, and a graphical representation is best served in the frequency domain.

Modulation is what takes a signal from low frequencies (the message) and pulls it up to a higher frequency (the carrier). The idea is simple: Multiply your message by a high frequency carrier, such as 680 kHz. Voila, that’s AM radio! Wait, is it really that easy? Let’s look at a few mathematical relationships. In this case, theta is the message (the audible stuff) and phi is the carrier (the AM radio frequency, for example).

Our AM solution involves multiplying signals, but that’s hard to imagine in the time or frequency domain, since we only have seen what tones look like. But the nifty relationships above show us that two signals multiplied can be represented as two signals added together! Now it’s easy to plot a multiplied signal in the frequency domain.

A single tone (150 Hz) modulated on a carrier (1000 Hz)

In this picture, we have multiplied a 150 Hz tone with a 1000 Hz carrier. The table above shows us to expect two, half-powered signals at 1000-150 and 1000+150 Hz, 850 Hz and 1150 Hz. What does our sound byte look like when it’s been modulated?

Modulation of a sound clip to 700 kHz

Just as expected, we see two signals. One is carrier + message, one is carrier – message (even notice how it is reversed).

Here is a crude image of an AM frequency spectrum and signal content.

Demodulation

Now let’s talk about receivers. All signals start at the antenna, which sees all signals at the same time as one big jumbled mess. It isn’t the antenna’s job to sort through the mess of data it is picking up, but that of the tuner and other hardware. The theory of demodulating a signal is identical to modulating it, conveniently enough! To bring our audio signal back to “baseband” where it can be sent to a speaker, we multiply everything by the carrier again.

That’s a bunch of math, parenthesis, and f’s all over the place. But it’s correct, and we see that there are four signals that result from it:

•    1/4 power signal, (2*carrier + message)
•    1/4 power signal, (message)
•    1/4 power signal, (2*carrier – message)
•    1/4 power signal, (-message)

Let’s immediately disregard the term with a negative frequency. It is a mathematical artifact which occurs quite often when talking about modulation and the math involved. The two signals at double the carrier (assuming the carrier is much larger than the message, they are almost the same) can be filtered out with a Low Pass Filter, which will block all higher frequency content of a signal. That just leaves us with the original message, which can be boosted with an amplifier and then sent to a speaker. Cool! Here’s a picture of it, but backwards.

Conclusion

The purpose of this post was to give a 30,000 foot view of how radio transmission and signal modulation works. By taking multiple audio (or baseband) signals and mathematically multiplying them by different higher frequencies (the carrier), we can successfully transmit multiple data streams over the same channel without interference. Multiplying it by the carrier again brings the modulated signal back to baseband, and a low pass filter and amplifier clean up and magnify the signal for our listening pleasure! Please leave a comment below if you want to join the conversation!

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