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2017

In oscilloscopes and oscilloscope probes, bandwidth is the width of a range of frequencies measured in Hertz. Specifically, bandwidth is specified as the frequency at which a sinusoidal input signal is attenuated to 70.7% of its original amplitude, also known as the -3 dB point. Most scope companies design the scope/probe response to be as flat as possible throughout its specified frequency range, and most customers simply rely on the specified bandwidth of the oscilloscope or oscilloscope probes. This often leaves them wondering if they are indeed getting the bandwidth performance at the probe tip. This article provides some step-by-step instructions on how to simply measure and verify the bandwidth of your probe with an oscilloscope you may already have.

Oscilloscope Gaussian frequency response

Figure 1 An example of an Oscilloscope Gaussian frequency response

 

To measure the bandwidth of an oscilloscope probe, a VNA (vector network analyzer) is often used, which can be very expensive and difficult to learn how to use. Also, because typical passive probes are high impedance probes that should be terminated into 1 Mohm of an oscilloscope, it makes the traditional VNA s21 method hard to implement because it is 50 ohm based system.

 

The other way to get bandwidth is to use a sine wave source, a splitter, and a power meter and sweep the response directly. If you do this, you must set this up to run using a remote interface such as GPIB or USB. Doing it manually is very laborious, subject to mistakes, and requires extensive effort every time you want to evaluate a tweak, etc.

 

An easier way of measuring probe bandwidth, especially for the lower bandwidth probes (say, <1 GHz passive probe) is the time domain approach utilizing only an oscilloscope with the built-in step signal source, the ‘differentiate’ function, and the ‘FFT’. To be able to use this method, your oscilloscope should support the function of another function output. If you don't, an alternative is to pull the time domain waveform data out of the oscilloscope, import it into the PC based analysis tool such as Mathlab or Excel, and apply the math functions on the step data there.

 

When you apply a step function to your system, then you will get the step response. If you then apply the differentiate (or derivative) to this step response, you obtain the impulse response, and then take the FFT of the impulse response to obtain the frequency response of the system.

 

Keysight’s Infiniium real-time oscilloscope is an excellent tool for this quick bandwidth testing. Here is the step by step procedure of the testing. For this bandwidth measurement example, a N2873A 500MHz 10:1 passive probe with an Infiniium MSOS804A 8 GHz oscilloscope is used.

  • Use a performance verification fixture such as Keysight’s E2655C with a 50 ohm BNC cable to connect the Aux output of the oscilloscope to the input of the oscilloscope. The Infiniium oscilloscope has an Aux output port with fast edge speed (~140 psec, 10-90% for Infinium S Series) for probe calibration. It is very important to note that the rise time of the signal source should be faster than the probe’s rise time, and the frequency response of the source is reasonably flat over frequency.

Probing 25 ohm signal source with Keysight E2655C

Figure 2 Probing 25 ohm signal source with the Keysight E2655C performance verification fixture

 

  • Connect the probe to the PV fixture to measure one edge of the source. Use as short a probe ground as possible to reduce probe loading associated with ground leads.

Ch 1 (yellow) = signal source (Aux output) as loaded by the probe

Ch 2 (green) = the measured output of the probe

 

Probing fast edge

Figure 3 Probing fast edge

 

  • Place the rising edges at center of the screen. Trigger on the measured output of the probe (ch2) and use the averaging or high resolution acquisition to reduce the noise on the waveform.
  • Use the oscilloscope’s built-in math function to differentiate the step response. Now you get the impulse response of the channel 2 where the probe is connected to. Assign the differentiated output of the step response into the F1 of the oscilloscope.

 

Built-in math function to differentiate the step response

Figure 4  -- Use the oscilloscope’s built-in math function to differentiate the step response.

 

  • Apply the built-in FFT Magnitude function on the impulse response (F1) of the measured step signal. Rescale the FFT to 100MHz/div (the center frequency at 500 MHz with the 1 GHz of frequency span across the screen) and 3dB/div vertically.

 

FFT magnitude function

Figure 5 -- Apply the built-in FFT Magnitude function on the impulse response

 

  • Now you have a plot of bandwidth. Since the vertical scale of the FFT plot is set to 3 dB/div with the horizontal scale set to 100 MHz/div, you can see the probe has ~660 MHz, as you pick the point in the FFT trace falling by 3 dB.

 

Plot of bandwidth

Figure 6 Now you have a plot of bandwidth

 

There is one catch to this. The way we do differentiate in some of the oscilloscopes is taking the best fit slope to three adjacent points and then assign this slope to the center point. This can really hose the bandwidth measurement up if you don't have enough sample density on the edge, so experiment with sample density and make sure it doesn't affect the bandwidth.

 

Conclusion

Utilizing the built-in mathematical capabilities available in modern digital oscilloscopes, it is possible to derive the frequency response or the bandwidth characteristics of a probe based on the measured step response of a fast step signal. Among those several test methods, the time domain approach is the easiest for an oscilloscope user to duplicate without having a need to use expensive test instruments.

Lab benches are many times cluttered with multiple pieces of test equipment. Keysight’s InfiniiVision Oscilloscopes are equipped with a built in digital voltmeter, frequency counter, and totalizer giving the oscilloscope user additional measurement options that can reduce the amount of test equipment needed. In addition, when you only measure the frequency of a signal, you rarely get the whole story. A repetitive signal can have spurs, intermittent spikes, and noise that you need to see during design and or debug. The oscilloscope counter will show you all of these attributes in addition to the frequency in one screen shot giving you the “big” picture. Keysight InfiniiVision oscilloscopes include both a 3-digit voltmeter (DVM) and a 5-10-digit integrated counter depending upon the oscilloscope model number (Figure 1 below).

 

 Digital Voltmeter

Figure 1 – Functionality, options and specifications across the Keysight InfiniiVision family of Oscilloscopes

 

Digital Volt Meter

The DVM and integrated counter operate through the same probes as the oscilloscope channels. However, these measurements are decoupled from the oscilloscope triggering system measuring 100 points per second. This flexibility allows engineers to make DVM and triggered oscilloscope measurements with the same connection. DVM results are presented with an always-on seven-segment display keeping these quick characterization measurements at the engineers' fingertips. You get the added flexibility of measuring four types of DVM measurements depending upon your application: Peak-Peak, AC rms, DC, and DC rms. As a user you should also note that the oscilloscope DVM is designed for quick rough measurements as needed in design or debug and not meant to replace exact measurements you would get from a calibrated external DVM.

 

Standard 5-digit counter resolution

The traditional oscilloscope counter measurements offer only five or six digits of resolution, which may not be enough for the most critical frequency measurements being made. With a 10-digit counter you can see your measurements with the precision you would normally expect, only from a standalone counter. The Keysight integrated counter’s ability to measure frequencies up to a wide bandwidth of 3.2 GHz allows it to be used in many high-frequency applications. This integrated hardware counter allows users to make much more accurate frequency measurements on signals. Four digits is one part in ten thousand, or ~0.01% of the displayed number. In addition, relative to an industry standard oscilloscope frequency measurement, the Keysight counter measurement is designed to be very easy to use. It uses the trigger level of the oscilloscope as the trigger level for the counter independent of the cycles shown on the screen.

 

Up to 8 to 10-digit resolution with external time base

If an external 10MHz reference is used, the counter is as accurate as the externally fed 10MHz signal, and the measurement resolution is increased. The 10MHz REF BNC connector on the rear panel is provided so you can supply a more accurate clock signal to the oscilloscope. To drive oscilloscope’s time base from external clock reference, connect a 10MHz square or sine wave reference signal to the 10MHz REF BNC input on the rear panel, and go to the Utility -> Options ->Rear Panel menu and select Ref signal mode to 10 MHz input. The working 10MHz input voltage is 180mV to 1V in amplitude, with a 0V to 2V offset. To get the highest resolution, the time/div setting should be at 200mS/div or slower. With this setting, the resolution is increased up to 8 digits, which is what would be displayed if an external 10MHz reference is used. When the internal reference is used, the oscilloscope displays counter measurement in 5 digits. The counter measurements can measure frequencies up to the bandwidth of the oscilloscope.

 

Accuracy

Basically, the counter is as accurate as the time base reference that is used.  The oscilloscope’s time base uses a built-in 10MHz reference that has an accuracy of 1.6 ppm to 50 ppm depending upon the oscilloscope model number. This means that the number displayed is within 0.00016% to 0.0050% respectively of the actual signal measured. For example, if you are making a counter measurement of 32,768 Hz signal using a model 6000X with 1.6 ppm accuracy, you are measuring the signal at ~0.05 Hz accuracy (see calculation below).

32,768 Hz x 1.6 ppm (0.00016%) = ±0.0524288 Hz

 

Totalizer

The totalizer feature of the DSOXDVMCTR counter option adds another valuable capability to the oscilloscope. It can count the number of events (totalize), and it also can monitor the number of trigger-condition-qualified events. The trigger-qualified events totalizer does not require an actual trigger to occur. It only requires a trigger-satisfying event to take place. In other words, the totalizer can monitor events faster than the trigger rate of an oscilloscope, in some cases as fast as 25 million events per second. Keep in mind that the number of events is a function of the oscilloscope’s hold off time.

 

Summary

The voltmeter and counter functions discussed in this article are just two of the “6 instruments in one oscilloscope” of the Keysight InfiniiVision family. The six instruments are the oscilloscope, 16 digital channels (mixed signal), serial protocol analyzer, Dual channel 20 MHz function/arbitrary waveform generator, 3-digit voltmeter, and 5 to 10-digit counter with totalizer.

 

The voltmeter operates through the same probes as the oscilloscope channels. However, the DVM measurements are made independently from the oscilloscope acquisition and triggering system, so you can make both the DVM and triggered oscilloscope waveform captures with the same connection.

 

Traditional oscilloscope counter measurements offer only five or six digits of resolution. While this level of precision is fine for quick measurements, it falls short of expectations when critical frequency measurements are needed. With the integrated counter within the five Keysight oscilloscope families summarized in Figure 1, you can select between 5 and 10 digit counter options and see your measurements with the precision you would normally expect only from a standalone counter. Because the integrated counter measures frequencies up to a wide bandwidth of 3.2 GHz, you can use it for many high-frequency applications as well.

This blog was written by Ailee Grumbine- Keysight Memory Solutions Product Manager

 

As a design engineer, your job is to design the best product. Your manager’s job is to reduce the number of redesigns and deal with engineering shortages and budget constraints. Your manager asked for test results to decide if your product is ready for release to production. You would spend days analyzing the test results to gain confidence that your product is good. You then translate the information into graphs and test reports that are presentable to your manager. Does this all sound familiar?

 

Data analytics is the answer for overcoming these challenges. In the test and measurement industry, designers use test equipment to help determine if their design meets the industry passing criteria for device certifications. Data sources include test results from compliance test software, simulation software, multiple vendors test equipment, and individual company’s proprietary measurement tools. Data collected is exported to a data repository server or cloud which is accessible by a globally distributed design team. Data analytics with visualization tools helps the decision making process more intuitive and a lot faster. The visualization tools include line and histogram charts with pass fail limits and statistical information. The image below shows an example of a measurement jitter histogram plot of different ASIC names. It reveals that the two ASICs, SERDES 700 and SERDES 701. Both have the same histogram mode and profile while SERDES 702 doesn’t have enough measurement to conclude its performance. You may want to hold off SERDES 702 for release to production.

Histogram Plot of jitter measurment

Histogram plot of jitter measurement on three different SERDES 

 

The next example is a bit error measurement against input voltage for different ASIC versions. Alpha, beta, and gamma versions have the same bit error measurements, while delta version is performing better with lower bit error measurement. You could conclude that delta version ASIC has better performance compared to the other versions. It could also be that there is discrepancy in the way the measurement is made that causes the outlier behavior.  You should also look at other possible contributing factors such as test equipment, test bench, and the engineer who made the measurement.  

 line plot of bit errors

Line plot of bit errors on four different ASIC versions

 

The visualization tool is the easy part of setting up data analytics capability. The hard part is setting up a web server that would interact with the data repository server for data upload and access. The data repository server has to be secured and has the support for backup, restore, and replication. It is highly recommended to have company’s internal IT department support in setting up the data repository server. The web server hosts the data analytics web server application software. It needs to support massive data upload via streaming or bulk transfer. It needs to be OS and programming language independent. It has to protect the data from any corruption and ensures consistency. It is recommended that the web server and the data repository server is setup using two separate servers to allow for scalability, performance, and data repository security.  You can collect the data in a .CSV file with measurements and properties information. Example of properties are temperature, test bench names, ASIC names, ASIC versions, and test engineers. Measurements can be jitter, bit error, input voltage, and power. For most measurements, there are upper and lower limits which would tell the design engineer the margins they have in their design.

 

Being ahead of the competition and doing it in the most cost efficient manner have a positive business impact. Hence, data analytics features are designed to work with all measurement data collection methods to allow for simple, quick, non-tedious integration into the design and characterization work flow. Important data analytics software features would include a web server application to enable real time huge data import and access. It would also support visualization tools with different chart options to enable fast and intuitive data analysis for making quick decisions. All of these elements should build an infrastructure that would support data analytics successfully in your company.

What does the piezoelectric effect have to do with oscilloscopes? If you follow any of the electrical engineering YouTube channels, you’re likely familiar with Dave Jones & the EEVBlog. His latest video caught my eye “EEVBlog #983 – A Shocking Oscilloscope Problem”. Now, this made me stop in my tracks. Not because he’s highlighting an oscilloscope “problem,” but because after waiting for 982 videos, Dave thought this topic was finally worth using the word “shocking” as a pun. I don’t know about you, but if I had made 982 videos, I’d probably have played that card already. Although, our Keysight Oscilloscopes YouTube channel just broke 250 videos and we haven’t done it yet, so you never know.

 

Anyways, what could be such a big deal? As it turns out the topic is actually, well (sigh) shocking. Who knew that simply bumping an oscilloscope the wrong way could cause mystery signals to appear on the screen? What makes this happen? It occurs because the ceramic capacitors in the oscilloscope’s acquisition system act as a piezoelectric material. Whether you are using a cheap oscilloscope or a high end oscilloscope, the piezoelectric effect is something to be aware of.

 

How does piezoelectricity work?

Piezoelectric materials are crystalline substances that produce an electric potential when subjected to mechanical stress.  Think about a crystal lattice. In general, a material’s molecules form into crystals because that is its most stable state. The molecular charges are arranged in an electrically neutral arrangement. Essentially, the positive and negative charges are all at a happy equilibrium. But as soon as an external physical pressure distorts the crystal structure, there will be an imbalance of charge. Take Fig 01 (GIF) for example. In a normal, non-compressed state the 2D lattice is at equilibrium. But as it’s compressed, the positive and negative charges “squish” out to opposite ends and create a potential across the structure. Basically, the lattice stops being an electrically neutral structure and has a charge distribution.

 

 

 Alternatively, you can apply a voltage to a crystalline structure and it will physically change the shape of the crystal – the “reverse piezoelectric effect.” This is especially useful if you want to generate or sense physical time-varying waves.

 

The piezoelectric effect and oscilloscopes

What does the piezoelectric effect have to do with oscilloscopes? Try this and see for yourself:

 

  1. Grab a standard 10:1 passive probe and connect it to your oscilloscope
  2. Zoom in vertically on your signal to a small voltage per division setting
  3. Set your trigger level slightly above your baseline signal
  4. Remove the probe’s grabber hat & tap the exposed probe tip on a hard surface
  5. Don’t panic and always carry a towel

 

You should then see a signal show up on your screen. Remember, you may have to put your oscilloscope into “Normal” trigger mode to keep the signal onscreen. Alternatively, you may be able to forgo the probe all together and simply tap on a bare BNC or even the top of the chassis (like in Figure 2). Now, don’t panic, this is a behavior that every scope in existence exhibits. It’s worth noting that I had to smack the oscilloscope pretty stinking hard to get this strong of an effect.

 

Scope Slap

A hand-numbingly hard slap demonstrates the piezoelectric effect on the Keysight InfiniiVision 1000 X-Series

 

A signal is showing up on the oscilloscope because designers use ceramic capacitors in both probes and in oscilloscope acquisition boards. Ceramic is a piezoelectric material, and the vibrations caused by physical force you exert on your probe and/or scope cause the capacitors to physically expand and contract slightly. This expansion and contraction creates an electric potential in the capacitors. Because these capacitors are part of the oscilloscopes acquisition system, that potential shows up on the oscilloscope screen. “So…” you ask me once you’re done hyperventilating, “have all of my measurements been bogus up to this point? Can I minimize this effect? Is this something I should worry about?”

 

No, yes, and probably not.

 

Unless you are working in the middle of a city-destroying earthquake or on the back of a kangaroo, you probably don’t have anything to worry about. Keysight oscilloscopes all go through extensive environmental and stress testing, including drop tests (up to 30 g’s of force!) and time on a vibration table (Fig 3). So, for Keysight oscilloscopes you can be confident that every-day vibrations won’t affect your measurements (but I can’t speak for other manufacturer’s testing procedures). If you are extra concerned about this or work on the back of a kangaroo, try using an equipment cart or table that has built-in suspension.

 

Drop Table

An Infiniivision 1000 X-Series oscilloscope being drop-tested. Just because it’s an inexpensive oscilloscope doesn’t mean it’s not rugged!

 

Wrapping up

Clearly, under the right circumstances, you can visibly observe the piezoelectric effect on your oscilloscopes. However, in my years at Keysight, I have not seen a single instance of this ever effecting an engineer’s measurements. To borrow the words of Mike from Mike’sElectricStuff:

Patient to Doctor: “Doc, it hurts when I do this!”

Doctor to Patient: “Don’t do that!”

The latest Infiniium software release (for Infiniium oscilloscopes and Infiniium Offline on your PC) includes a handful of new and improved tools to help you make more efficient measurements and documentation.

These updates include:

  • MIPI SPMI Protocol Decode
  • Generic Raw Decode for PAM-4 and NRZ
  • Symbolic Decode added to ARINC 429 and MIL-STD-1553 protocol decode 
  • Segmented Memory improvements
  • Measurement Reports
  • S-Parameter Viewer
  • Windows 10 Support

 

MIPI SPMI Protocol Trigger and Decode – N8845A

If you’re designing mobile devices, our new SPMI (System Power Management Interface) protocol decode license might interest you. SPMI is used to communicate from power controllers to one or multiple power management chips with up to 4 masters and 16 slaves on one bus. SPMI allows you to reduce the number of pins on your power controllers, reducing the size of your mobile designs. With Keysight’s SPMI protocol decode option you can decode and debug these designs. Like I always say when it comes to protocol decode software, have the oscilloscope decode for you so you can get right to the fun part - analyzing and debugging.

Figure 1 - MIPI SPMI protocol decode

 

Generic Raw Decode

You may have a proprietary bus or customized protocol that others may not have defined a specific protocol decode for. This means you don’t have the option of buying a convenient protocol decode license that will trigger and decode your serial bus, group bits, label packet types, and flag errors for you. However, you can still get the binary data extracted from your analog waveform. With Generic Raw, you can decode your NRZ and PAM-4 signals so you can process and analyze the data yourself. This software extracts the raw bit data from the analog signal based on the clock recovery and thresholds that you set. Generic Raw has one mode for NRZ* signals and another for PAM-4** signals.

Figure 2 - Generic Raw PAM-4 decode

 

Symbolic Decode – N8842A

If you’re working with ARINC 429 and MIL-STD-1553 protocols, this update is for you. How many of us sit with our protocol binders in our lap to translate the Hex values to meaningful English? It’s time to toss that binder in a desk drawer. Load your .xml file into the oscilloscope to view your protocol decode in ASCII instead of Hex. Look at the examples below to see the two versions side by side.

ARINC 429 protocol decode:

Figure 3 - ARINC 429 decode in Hex

 

ARINC 429 protocol decode with symbolic decode:

 

 Figure 4 - ARINC 429 decode in ASCII

 

 

Segmented Memory Updates

Segmented memory is a great way to make the most of your oscilloscope’s memory, especially if you have specific events of interest separated by long amounts of time that you don’t really care about. An example of this is a serial bus. You’ll be looking at a waveform with packets of data separated by dead time. You can acquire waveform data just around the trigger conditions you set – for example, a specific packet type or an error. Then you can view this same packet type as it changes over time, comparing the packets captured in each acquisition. Now segmented memory has been improved to make your life even easier with the following:

  • Auto Play - automatically play through all the segments after acquisition is completed
  • Time between segment playback is reduced to zero – optimized performance saving you time
  • Persistent data is preserved – you can view all your segments laid on top of each other to see how your signal changes between packets in one view
  • Measurement Log – track the changes in measurements over the number of acquisitions you specify

 

 

Measurement Reports

 

Have you ever had to compile measurement reports to keep records of exact test conditions, equipment settings, and measurement results? Now, the oscilloscope can do it all for you in a couple clicks. No more copying and pasting screenshots, pulling together separate setup files and recording the software versions, and organizing them into your favorite text editor. The 6.0 Infiniium software now provides a way to generate hassle free reports that include all of the information you’d want to record and keep in your archives for proof of your test results.

 

Measurement reports provide, in a single file, measurement results and screenshots, plus all the information about your oscilloscope setup including:

 

  • Oscilloscope configuration with model number and software version
  • Calibration status of the frame and individual channels
  • Acquisition settings
  • Horizontal settings
  • Bandwidth limits and filter type
  • Vertical and channel settings
  • Trigger setup

 

You can save your report as PDF or as MHTML (*.mht) format files. MHTML is a webpage archive format that includes images and html all in one file so you don’t have to save images and text based content separately.

 

S-Parameter Viewer

If you are using InfiniiSim to apply transfer functions to your waveform – useful when you want to model the effects of a probe or RC circuit on your design – there is now an option to view the s-parameters. Being able to view the s-parameters before applying it to your waveform can be a nice sanity check before you end up spending hours trying to understand why your circuit behaves so unexpectedly because you accidentally uploaded the wrong file (it’s happened to the best of us).

 

Figure 5 - S-Parameter Viewer

 

Windows 10

Infiniium software now supports Windows 10. If you are running Windows 10 on your PC, Infiniium Offline is now compatible! For a free trial of the Infiniium software, click here.

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*Standard with SDA option

**Requires PAM-4 Compliance App and SDA option