2018

February 2018

# What is Oscilloscope Sample Rate (and How Much Do You Need)?

Posted by melissakeysight Feb 28, 2018

If your sample rate is not fast enough, you won’t be able to see your signal accurately on the oscilloscope screen. Sample rate is the number of samples an oscilloscope can acquire per second. This determines the resolution of your waveform. Read on to learn why.

# The Basics

A sample is a single value at a point in time. You could think of a sample like one piece in a puzzle. The more pieces you assemble over time, the more apparent and complete the picture becomes.

But unlike a puzzle, reconstructing a waveform on an oscilloscope is not solely dependent on the number of samples that are strung together. The speed at which you sample matters too. A puzzle is a static picture. Therefore, it doesn’t matter how long it takes you to assemble a puzzle – the result will still be a complete picture in the end. However, electric waveforms change with time. So, to get a complete picture of the waveform, we need to sample fast enough to capture it. That is why we talk about the specification in terms of a rate. We need a fast sample rate to properly display our device’s signals on our test equipment.

We know from Harry Nyquist that we need to take equally spaced samples of a signal at at least twice the rate of the signal’s highest frequency component to represent that signal without errors.

Fsampling 2Fsignal

This definition is given as a minimum requirement for proper sampling. You want your oscilloscope to provide more than just a minimum requirement.

# Oscilloscope Sampling

There are two key oscilloscope specs that determine if your signal will be displayed properly on screen: bandwidth and sample rate. In my previous blog “What is Bandwidth and How Much Do You Need,” we discussed the importance of bandwidth. From that blog you’ll know that without enough bandwidth, you’ll have an attenuated and distorted signal. But, it’s also important to know that without enough sample rate, you will be without all the waveform information that is necessary to display the frequency of your signal, exact rise and fall times, the height and shape of your signal, and any glitch or anomaly that may be occurring.

When you probe your device and connect it to an oscilloscope, you are sending an analog signal into the oscilloscope. Then, the scope samples and digitizes the signal, saves it in memory, and displays it on screen.

Figure 1. Simplified block diagram of signal flow from a DUT through an oscilloscope.

The default sampling setting on your Keysight oscilloscope is automatic in real-time sampling mode. Automatic sampling will select the sample rate for you. The scope will choose the highest sampling rate possible, using as much memory as necessary to fill the display with your waveform information. In real-time sampling mode, all the samples of the waveform are taken from one trigger event and are evenly spaced in time. (If you aren’t familiar with the term trigger, that is basically the event that time-correlates your device’s waveform within the oscilloscope, allowing the waveform to be steadily displayed on screen.) The scope may also apply interpolation to fill in gaps between samples.

If you don’t want the oscilloscope to select the sample rate for you, most oscilloscopes allow you to set the sample rate yourself. If you set the sample rate yourself, remember: two times the frequency is the absolute minimum rate you should use. When it comes to oscilloscopes, I recommend choosing a sample rate faster than this. Usually choosing a sample rate that is 3 to 5 times the bandwidth of the oscilloscope will give you a high-enough sampling rate to capture the details of your signal, including its frequency of oscillation and the rise times of your waveforms. You need a sample rate that will provide enough detail to see any unexpected glitches or anomalies.

### The more samples you have in each period, the more signal detail you'll capture.

One last thing to double check is the sample rate of the oscilloscope when all channels are turned on. Typically, when multiple channels are in use, the sample rate is split up among the channels. If you are using more than one channel, you’ll want to make sure the sampling rate is still sufficient.

The Specs You Need to Know

While bandwidth is the number one oscilloscope specification, sample rate is a close second. The oscilloscope sample rate determines the amount of waveform information captured and displayed on screen. You need a sample rate that will accurately show all aspects of your signal including its standard shape, accurate rise times, and glitches. You could be missing vital design flaws without being able to view a glitch, or you could waste hours trying to determine why your signal looks differently than you expected just because your scope was under sampling.

To learn about the other need-to-know oscilloscope specs to set you up for successful measurements, check out the Basic Oscilloscope Fundamentals application note.

# The Many Custom Applications of Arbitrary Waveform Generators

Posted by BoonCampbell Feb 22, 2018

There are many cases where certain signals can cause your device to malfunction. This may be a problem your customer ends up finding if you don’t properly test during product development. Designers and test engineers frequently use an Arbitrary Waveform Generator (AWG) to simulate worst-case conditions during design verification. An AWG is the ideal tool for creating degraded or stressed signals to verify product performance limits. System or product noise susceptibility, timing problems, signal-level abnormalities, bandwidth loss, harmonic distortion, or a host of related maladies can be determined.

The AWG is a very powerful tool and can create waveforms or waveform bursts needed for your specific application. An AWG combines the capabilities of a function generator with that of a pulse generator, modulation source, noise generator, sweep generator, and trigger generator. It is a good tool for everyday use in the design lab or test environment. You can create custom solutions for a wide range of applications spanning many industries. AWG applications range from high dynamic range to high bandwidth output requirements.

Figure 1. Arbitrary Waveform Generator (AWG) applications.

Below is a list of common applications covered in this blog:

• Environment signals
• Coherent optical
• Generic Orthogonal Frequency-Division Multiplexing (OFDM)
• High-speed serial
• Simulating real-world aberrations in 100Base-T physical layer
• Dual Tone Multi-Frequency (DTMF)
• Pacemaker
• Automobile suspension testing
• Power line testing

Creating the signals required for RF conformance and margin testing is increasingly difficult. Digital RF technologies require wide-bandwidths and fast-changing signals that other generators cannot produce. These types of signals are seen in RF communications and ultra-wide band radio applications.

Radar signals demand AWG-level performance in terms of sample rate, dynamic range, and memory. AWGs can oversample the signal in instances where phase and amplitude quadrature signal generation is desired. This improves signal quality, creating a spurious free dynamic output. AWG’s also provide Linear Frequency Modulation (LFM), Barker and Polyphase codes, step FM, and nonlinear FM modulation signals. They also generate pulse trains to resolve:

• Range and doppler shift ambiguity
• Frequency hopping for electronic counter-counter measures
• Pulse-to-pulse amplitude variation

Environment Signals

Radar signals must coexist with commercial signals and not affect each other. Use your AWG to thoroughly test all the corner case issues at the design or debug stage. An AWG can be programmed to output many industry-standard signals:

• WiMAX
• WIFI
• GSM
• GSM-EDGE
• EGPRS-2A
• EGPRS2B
• CDMA
• WCDMA
• DVB-T
• Noise

You can also define the carrier frequency, power, start time, and duration of these signals. This allows control of the level of signal interaction or interference.

Coherent Optical

Today's web driven world is pushing the demand for high-speed short and long haul coherent optical solutions. Phase modulation, high baud rate, high sample rate, high bandwidth, and high resolution are all critical to optical applications. Multiple synchronized AWGs can be used to generate many desired coherent optical signals.

Generic Orthogonal Frequency-Division Multiplexing (OFDM)

OFDM has become the modulation method of choice for transmitting large amounts of digital data over short and medium distances. Wide bandwidths and multiple carriers are needed to test RF receivers in today’s wireless world. AWG OFDM packets can specifying the spacing between the symbols or frames or stressed by adding gated noise.

High-speed Serial

Serial signals are made of binary data (simple ones and zeros). These signals have begun to look more like analog waveforms with analog events embedded in the digital data. The textbook zero-rise time and flat top of the theoretical square wave no longer represent reality. Today’s serial communication environments are negatively impacted by noise, jitter, crosstalk, distributed reactances, and power supply variations. Your arbitrary waveform generator can create all these signals!

Using direct synthesis techniques, AWGs can simulate the effects of propagation through a transmission line.

Rise times, pulse shapes, delays, and aberrations can all be controlled by your AWG. You can also create a variety of digital data impairments such as jitter (random, periodic, sinusoidal), noise, pre/de-emphasis, duty cycle distortion, inter-symbol interference, duty cycle distortion, and spread spectrum clocking.

Simulating Real-World Aberrations in 100Base-T Physical Layer

To simulate physical layer test signals for 100Base-T transceivers, your AWG will create several analog parameters:

• Undershoot and overshoot
• Rise and fall time
• Ringing
• Amplitude variations
• Specific timing variations such as jitter

AWGs provide an efficient method for generating signal impairments like these for testing product margins.

Dual Tone Multi-Frequency (DTMF)

Touch-tone signals on push button telephones are created by combining a low frequency and a high-frequency signal. Simulating the superimposed frequencies creates a special challenge if the frequencies are not harmonically related. An arbitrary waveform generator can generate these signals along with controlled levels of noise and harmonic content.

Pacemaker

A simple square wave or sine-wave pulse was used to test pacemakers in the past. Today’s AWGs can create a simulated heartbeat waveform that pacemakers are designed to detect.

The arbitrary waveform generator can customize pacemaker testing for particular heart rate types.

Automobile Suspension Testing

An AWG can simulate automobile sensor outputs just as a car would when it hits a bump. The suspension’s response and reliability can be tested under virtually any simulated road condition because the size of the “bumps” can be precisely controlled.

Power Line Testing

Multichannel AWGs can simulate three-phase power. Transients or glitches can be created to simulated problematic waveforms. For example, you could simulate a transient on one phase and signal dropout on another.

In addition to all the applications above, there are many more across several different industries, and the arbitrary waveform generator will support them all:

• Sequencing and deep memory
• Creating long scenario simulations
• Leading edge physics, chemistry, and electronics research
• Validation and compliance testing of high-speed silicon and communications devices
• Stressing testing receivers with a wide array of signal impairments
• Generating high Baud rate baseband signals with higher order, complex modulation
• Radar, satellite, electronic warfare, and multilevel signals
• Jitter margin testing for analog-to-digital converters

Conclusion

We have now covered the importance of an arbitrary waveform generator to ensure your device is working properly for your specific application. As you can see, AWGs excel in creating mixed-signal waveforms that can mimic real world conditions. To learn more about arbitrary waveform generators, check out: A High-Performance AWG Primer.

# What is Bandwidth? How Much Do You Need?

Posted by melissakeysight Feb 16, 2018

If you are using an oscilloscope, make sure you are using the right bandwidth! Choosing the wrong amount could adversely affect your measurement results. Let’s look at what oscilloscope bandwidth is and why you need just the right amount.

What is Bandwidth?

Bandwidth is often regarded as the single most important characteristic of an oscilloscope. Measured in Hertz, the bandwidth of your oscilloscope is the range of frequencies that your oscilloscope can accurately measure. Without enough bandwidth, the amplitude of your signal will be incorrect and details of your waveform might be lost. With too much bandwidth, you will capture excessive noise, providing you with an inaccurate measurement. Here’s why:

You can think of an oscilloscope like a low pass filter, meaning it will only pass frequencies from 0 Hz up to a specified frequency. An oscilloscope’s bandwidth is specified as the 3 dB down point of the filter. What the heck is a 3 dB down point? Read on.

Low pass filters allow signals to pass through them at full amplitude until the signal frequency approaches the high end of the frequencies that the filter can pass. Then a filter attenuates signals passing through them until the signal’s amplitude is dampened to nothing. When the signal is attenuated by three decibels (3 dB), that is the cutoff point for an oscilloscope’s bandwidth specification. If you aren’t familiar with decibels, the 3 dB down point is when the amplitude of a sine wave is 70.7% of its actual height. Look at the diagram below to visualize the frequency response of a low pass filter, depicted in blue.

Figure 1. Frequency response of a low pass filter, depicting the 3 dB down point and cutoff frequency.

So, if you have an oscilloscope that has a bandwidth of 200 MHz, you know that the cutoff frequency of that oscilloscope’s filter is 200 MHz. Why does this matter for your measurements?

Too Little Bandwidth

You can see from Figure 1 that if you are measuring a signal that has a higher frequency than the cutoff frequency, you’ll either see an attenuated and distorted version of your signal or not much of a signal at all. Even measuring a signal as fast as the bandwidth of the scope is not a good idea. Measuring a 200 MHz signal on a 200 MHz oscilloscope will not provide you with the best representation of your signal, as the filter has already begun to roll off and distort your input.

Measuring with too little bandwidth will provide distorted results

Here is the rule of thumb for choosing the right bandwidth:

• Digital signal measurements: five times higher bandwidth than the fastest digital clock rate in your system
• Analog signal measurements: three times higher bandwidth than the maximum signal frequency on an oscilloscope with a flat frequency response

For more detail on these rules, read Evaluating Oscilloscope Bandwidths for Your Application.

So why not just use an oscilloscope with the highest bandwidth possible?

Too Much Bandwidth

Oscilloscopes can capture environmental noise. Oscilloscopes also add noise to your signal from filtering, processing, and digitizing (though a high-quality oscilloscope will do all of this properly and add less noise than a poorly-designed scope). And noise occurs at all frequencies. So if you have a 200 MHz oscilloscope, that scope is only going to show noise up to 200 MHz. But, if you have a 33 GHz oscilloscope, it will add noise to your measurement through its entire measurement range up to 33 GHz, regardless of the frequency of your signal.

Increasing bandwidth increases noise

If you want to measure a 50 MHz signal, a 200 MHz oscilloscope will give you plenty of bandwidth to clearly display your signal without attenuation and filter distortion but not so much that it adds high frequency noise content to your measurement.

Insider tip: If all you have access to is a high bandwidth oscilloscope, but you are measuring low frequencies, turn on hardware filters in the oscilloscope to eliminate that high frequency noise and get a cleaner measurement.

The other reason why you probably don’t want to buy the highest bandwidth oscilloscope out there is price. The higher the bandwidth, the higher the price. If you are worried the bandwidth you need today will not be enough for future measurements, look for an oscilloscope that lets you upgrade the bandwidth with a software license. That way you can buy the bandwidth you need now and upgrade later without having to purchase a new oscilloscope or send it in to the factory for a hardware update. (Most Keysight oscilloscopes can be bandwidth upgraded with a software license for this very reason.)

Demonstration

Don’t be afraid to be the Goldilocks of bandwidth. Did she settle for the porridge that was too hot or too cold? No. She went for the one that was just right. And lucky for us, we won’t be eaten by bears if we set our bandwidth to just the right amount. Here is an example of how even a simple sine wave can be falsely represented on an oscilloscope without the right bandwidth.

In this demonstration, I am measuring a sine wave oscillating with a frequency of 80 MHz and a peak-peak voltage of about 2 volts.

I am using an 8 GHz oscilloscope. This is an excessive amount of bandwidth for an 80 MHz signal. The rule of thumb for analog signals is to use about 3 times the frequency of the signal. While this measurement doesn’t look horrible, let’s see how much better it can get when I apply the rule of thumb.

With only 240 MHz of bandwidth, look at how much cleaner my measurement is.

If I just want a quick check on the basics like voltage and frequency, the difference might not be crucial. But if I’m proving the quality of my design or attempting to pass strict performance or compliance specs, I would want the best (and cleanest) representation of my signal.

Now, I’ll decrease the bandwidth even further. As I mentioned earlier, you shouldn’t measure a signal at the bandwidth of the oscilloscope. The signal will be passing right through the 3 dB down point of the filter.

Here I’m measuring my 80 MHz signal with 80 MHz of bandwidth. You can see that the voltage is decreased from 1.92 V to 1.36 V. This is 70.8% of the voltage we should be seeing. The signal is attenuated by the filter.

To demonstrate the effects of the filter above the cutoff frequency, here is my measurement of the same signal with only 75 MHz of bandwidth. The signal is attenuated even further to 161 mV. The period of my measured signal is displayed as 12.74 ns. This would imply that the frequency of my signal is only 78 MHz, which we know to be false.

And here I’ve measured the same signal again with only 70 MHz of bandwidth. It barely looks like there is a signal at all.

You can see how dramatically the signal is attenuated when you try to measure a signal with frequency beyond the bandwidth of the oscilloscope.

Summary

Bandwidth is the most important characteristic of an oscilloscope

While there are many important features of an oscilloscope that you’ll need to evaluate before choosing one for your measurements, clearly bandwidth is the number one spec that you must check before any other. If you don’t have enough bandwidth you’ll see distorted or attenuated signals, giving you inaccurate measurements. If you have too much bandwidth, your measurements will be noisier than necessary. You have to choose a bandwidth that can support a clean and accurate representation of your test signals.

Now that you understand why bandwidth is the most important characteristic of an oscilloscope, check out Basic Oscilloscope Fundamentals to learn the other important oscilloscope characteristics and how to use an oscilloscope.

# What is an Oscilloscope? Why Do You Need One?

Posted by melissakeysight Feb 9, 2018

Picture the heart rate monitor that you always see next to hospital beds on “House” or “Grey’s Anatomy.” You hold your breath as you wait for the next beep and jump of the line on the screen, and you dread the flat line as the TV show reaches its apex.

Well, when my family asks me what I do for a living, this is how I describe an oscilloscope. But instead of displaying the signal of a human heart, oscilloscopes show the heartbeat of electronic devices. They give us all kinds of insights into whether or not an electronic device is operating correctly, allowing us to check its vitals.

The vitals of our devices could be voltage or current. And just like we don’t want our hearts to beat too fast or too slow, we want those voltages to oscillate at the right pace or frequency. We all know heart murmurs are bad. Well, we don’t want any glitches in our electrical signals either, and an oscilloscope can help us find them. Having insights like this into your electronic devices allows you to validate it is operating as expected. And if it’s not, oscilloscopes help you diagnose the problem and correct it. If you are an electrical engineer, chances are you could use an oscilloscope ─ whether you’re a test engineer or student or work in manufacturing, repair, research, or development.

1000 X-Series oscilloscopes making a variety of measurements.

# Oscilloscope Basics

The basic operation of an oscilloscope displays voltage versus time, with voltage on the vertical axis and time on the horizontal axis. This allows you to double check that your device’s signal is as you expect, both in magnitude and frequency. And because oscilloscopes provide a visual representation of the signal, you can view any anomalies or distortion that might be occurring. But before you start testing, there are some things for you to consider.

Oscilloscopes display voltage on the vertical axis and time on the horizontal axis.

Oscilloscopes come in many flavors. You want to select an oscilloscope with the right bandwidth, signal integrity, sample rate, and channel inputs. You also want to make sure it is compatible with any applications and probes you may need. Here is a list of some of the features you should check when deciding what oscilloscope to use:

• Bandwidth – The range of frequencies the oscilloscope can measure accurately. Oscilloscope bandwidths typically range from 50 MHz to 100 GHz.
• Sample Rate – The number of samples the oscilloscope can acquire per second. The greater the samples per second, the more clearly and accurately the waveform is displayed.
• Signal Integrity – The oscilloscope’s ability to represent the waveform accurately. This is a topic I’m particularly passionate about and you’ll find me writing about this a lot. You wouldn’t want a heart rate monitor that displays incorrect information. It would do no good to declare a patient dead whose heart is still beating. The same is true for your device under test. You wouldn’t want to declare your device is malfunctioning and spend weeks trying to find the root cause when there isn’t actually a problem.
• Channels – The input to the oscilloscope. They can be analog or digital. There are typically 2 to 4 analog channels per oscilloscope.
• Probe Compatibility – A probe is the tool used connect the oscilloscope to your device under test. There are a large variety of passive and active probes, each made for specific use cases. You want an oscilloscope that is compatible with the type of probe you need for your specific tests.
• Applications – Signal analysis, protocol decode, and compliance test software can greatly reduce the time it takes to identify and capture errors in your designs. Analysis software can help you find and evaluate jitter, perform Fourier transforms, create eye diagrams, and even identify and quantify crosstalk. Protocol decoding software can identify digital packets of information, trigger on different packet conditions, and identify protocol errors. Not all oscilloscopes are compatible with every application.

# What are Oscilloscopes Used for?

Now that you’re armed with the lingo, you’re ready to get going. The most basic testing only requires an oscilloscope with 50 to 200 MHz of bandwidth, a passive probe, and sufficient sample rate, signal integrity, and channel inputs.

Armed with these basics, you can spot-check your printed circuit boards (PCBs) to find faulty parts, noisy power lines, shorts, and I/Os (inputs and outputs) that are not working; dive into different trigger modes to search for runts, glitches, and timing errors; and capture signals and data to prove the quality of your designs. Some basic oscilloscopes even provide Bode or frequency and phase response analysis. And this is just the start.

Frequency response analysis performed on an InfiniiVision oscilloscope.

Oscilloscopes are versatile and widely used instruments. Automotive technicians use oscilloscopes to diagnose electrical problems in cars. University labs use oscilloscopes to teach students about electronics. Research groups all over the world have oscilloscopes at their disposal. Cell phone manufacturers use oscilloscopes to test the integrity of their signals. The military and aviation industries use oscilloscopes to test radar communication systems. R&D engineers use oscilloscopes to test and design new technologies. Oscilloscopes are also used for compliance testing such as USB and CAN protocols where the output must meet certain standards.

# Get Started

Now that you know what an oscilloscope is and some of the crucial oscilloscope specs, it’s time to get testing. So throw on your scrubs (or maybe an ESD strap instead) and get started!