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Recently I was visiting the SIPI lab (Signal Integrity and Power Integrity) of a large corporation showing them how to make some solid power integrity measurements that they could trust (check out this application note if you’d like some tips). We had just finished making some ripple and noise measurements on one of the key supplies inside their system (using our power integrity analyzer) when the PI Lab manager shared a thought with us. He said “You know, I am a bit of a perfectionist. There is always more work we could do to clean up the supplies but I have limited resources, so I wish there was a button I could push that would tell me if it is worth it or not. What can I gain if I clean up the DC supplies.” I think many folks working on power integrity (PI) have this same thought. “Before I model, simulate, redesign, re-layout, fabricate, load and test a new revision, I’d like to know if there is a lot or little to be gained by cleaning up the DC supplies.


The issue that the above group was struggling with was power supply induced jitter (PSIJ). PSIJ is frequently the biggest source of clock/data jitter. The delay through a device varies as a function of the voltage applied to that device. Therefore, a system with very little DC supply noise will have very little PSIJ and conversely for a system with a lot of supply noise. The difficult part is getting an idea how much PSIJ your supplies are causing, because it varies from supply to supply, from device to device and target to target. To illustrate PSIJ, consider the example in Figure 1 where we are probing the 1.1V supply to an FPGA and one of the data lines from the FPGA. Initially, the supply has about ±5% Vpp ripple, noise and transients on the supply. We built an eye diagram of the transitions on the data line and could see that the eye width was around 70ps. Next, we did the heavy lifting and cleaned up the supply so that it is rock solid with <1% Vpp ripple, noise and transients. Again, we built up an eye diagram on the FPGA data line and found the eye width increase to about 115ps or about a 55% increase. The only thing that had changed was the amount of noise on the power rail.


Figure 1: The effects of power supply noise on the data lines of an FPGA


Don’t be fooled into thinking that this problem is reserved for those working on very high-speed designs. I have seen power supply induced data corruption on a little IoT device that is only clocked at a few MHz’s If you’d like to see this example, check out this video.


Understanding the impact of power supply noise on data lines, sensors, clocks, displays, cameras, et cetera can be difficult. Some users who have been doing PI for a long time may have a ‘gut feel’ that they trust but even these folks would find comfort in some ‘hard data’ to back up their intuition. Traditionally, the way to find answers to this question has come from doing extensive modeling and simulation—power-aware, signal integrity simulations. This approach is usually reserved for the few who work for institutions that can afford the simulation tools and the dedicated staffing to operate these tools.


Worry not, there is an answer for the rest of us. It is even as simple as pushing a button like our friend the SIPI lab manager wished for (okay, truth be told, I think it is about 3 or 4 mouse clicks not one but close enough).  The new Keysight N8846A Power Integrity Analysis application. To give you an idea of how the PI Analysis application can be helpful let’s return to our FPGA example. The setup is the same as our previous example except for this time we use the N8846A PI Analysis application to estimate what the eye width would be without the negative effects of power rail noise (we didn’t do anything to clean up the supply this time). Figure 2 below shows the results from the N8846A. You’ll notice the applications estimate for eye width with a clean supply matches what it really was when we cleaned up the supply.


Figure 2: The results from the N8846A Power Integrity Analysis Application. Note that the prediction from the application matches the results from Figure 1 where the supply was physically changed.


The N8846A PI analysis application lets users define a dc supply as either a victim of or an aggressor to, one other periodic transitioning signal and predicts the amount of adverse interaction involved. In this way, users can see what their dc supply and/or toggling signals would look like if they were immune to the negative effects of each other. With this insight, users can make informed decisions about what, if any, next steps they would take to clean up the dc supplies. This is the “button” that our friend at the SIPI lab was asking for.

Written by Rick Eads

The Need for a 16GT/s PCIe Interconnect

PCI Express represents one of the most successful computer interconnects yet devised and helped to enable high-speed connections between external devices such as displays and storage adapters to the internal CPU of the computer.  As networking speeds have increased (datacenter), as display resolution has increased (4K streaming) and as disk drive capacity and speed has increased (cloud computing), the need for an improvement in the speed of a host adapter to the CPU has also driven the development of the next generation of the PCI Express 4.0 Standard.  PCie 4.0 technology doubles the bandwidth of the previous generation of PCie 3.0 devices as PCIe 4.0 is able to achieve throughput of nearly 32 GBytes/s for 16 lanes.


PCie 4.0 technology doubles the bandwidth of the previous generation of PCie 3.0 devices as PCIe 4.0 is able to achieve throughput of nearly 32 GBytes/s for 16 lanes.

What’s New about the PCIe 4.0 standard versus PCie 3.0?

The biggest improvement brought by the PCIe 4.0 standard is a doubling of the speed to 16GT/s per lane.  Nevertheless, there are additional changes to the PCIe 4.0 specification versus the Gen3 Spec that is worth noting.  Due to the higher data rate, the maximum channel loss accommodated by the PCIe 4.0 standard is approximately -28dB, which implies a maximum channel length of about 12” (or 25cm) with a single CEM class connector.  To accommodate channels longer than 12”, the PCie 4.0 specification provides protocol and electrical requirements for a retimer device which can be used to extend the PCIe 4.0 channel and can help to accommodate multiple connector topologies.  Another new feature of the PCIe 4.0 specification has been the addition of a lane margining capability.  Lane margining allows for in-band (L0) based adjustment of the receiver sampling point which allows for an estimation of eye width, and an optional mode also allows for voltage margining which may provide information regarding eye height.  This feature is intended to help system integrators determine product readiness for shipment.


Testing PCIe 4.0 Card Electromechanical (CEM) Devices

While the PCI Express 4.0 BASE specification is nearing 1.0 status, it primarily is written to accommodate new silicon or ASIC devices operating at 16GT/s.  PCI Express also accommodates a system specification referred to as the Card Electromechanical (CEM) specification.  This is the specification that is used at PCISIG compliance workshops to determine if motherboards and add-in cards are compliant to the PCIe 4.0 application.  To accomplish this, the PCISIG has commissioned the development of both new CEM test fixtures and new test software.  As of this writing, both are still in development but are being used in US-based compliance workshops since April 2017.  One new aspect of testing CEM devices under the PCIe 4.0 specification is the addition of an external physical ISI channel that is chosen/calibrated to ensure the maximum CEM channel loss is achieved.



Fig 1: Prototype CEM Test Fixtures (CBB4 and CLB4)

Oscilloscope bandwidth requirement

The minimum oscilloscope bandwidth required for PCI Express 4.0 is 25GHz.  As the minimum eye height is 15mV, it is important to utilize real-time oscilloscopes with the lowest noise floor to minimize error and to maximize the measured margins.


The minimum oscilloscope bandwidth required for PCI Express 4.0 is 25GHz.  As the minimum eye height is 15mV, it is important to utilize real-time oscilloscopes with the lowest noise floor to minimize error and to maximize the measured margins.

Making PCI Express BASE spec transmitter measurements

Keysight’s new N5393F PCI Express transmitter test application provides step-by-step instructions to guide you through the process of configuring the test setup, selecting the tests and connecting the signals to the oscilloscope.   The N5393F supports PCI Express 4.0 testing at 16GT/s for BASE spec tests and also supports legacy testing under the PCIe 3.0, 2.0, and 1.0a/1.1 standards.  The N5393F test provides visual connection aids to facilitate the connection of your DUT to the oscilloscope and with optional software also integrates the ability to de-embed test fixtures, provide DUT automation for test mode selection (2.5G, 5G, 8G).



Figure 2: PCI Express 4.0 BASE spec connection diagram including replica channel for de-embedding



Figure 3: Keysight N5393F PCI Express automated test application for the oscilloscope showing the PCIe 4.0 BASE specification tests for 16GT/s validation


Keysight has been a consistent and valued contributor to the development and authorship of the PCI Express specification since the initial 1.0a draft and continues to provide valuable guidance to the PCISIG for transmitter testing, receiver testing, and channel testing and characterization.  Other optional tools such as the N5465A InfiniiSim Waveform Transformation toolset and N5461A equalization application can provide deep analysis and debug capability. In addition, Keysight has other comprehensive test solutions from design simulation to physical layer testing that includes transmitter, receiver and channel for the PCI Express 4.0 standard as well as previous generations of PCIe.

Written by Doug Beck

What is mask testing?

A fundamental aspect of electrical engineering is finding problems with signal integrity. In the real world, systems don’t always work as simulation says they are supposed to. There is the concept of a “golden trace” where the waveforms are precisely where they need to be. But there are a great number of things that can cause a waveform to vary from the “golden trace.” Unfortunately, these problems can also be quite rare, which makes them difficult to find.


The idea of a mask on an oscilloscope is to provide a region around the golden trace where the waveform can vary without causing any problems. An example is shown in Figure 1. The gray area is the mask. This can be thought of as “out of bounds” because if the waveform strays into the gray areas, it is considered an error. However, if the waveform remains within the black areas, the waveform is considered fine.


Figure 1. An example of a mask on a waveform


The most common alternative to using a mask is to use triggers such as glitch, run, slow edge, fast edge, etc. However, a mask combines all of these triggers into a single test. In general, oscilloscope hardware does not allow all of these errors to be searched for at the same time. The bottom line is that using a mask test can save quite a bit of time.


Standards-based mask testing

Because mask tests can be so effective, it is very common for masks to be provided for common standards. Examples include Ethernet and FlexRay. The good news is that if your signal happens to be one where an industry standard mask is available, the job is very easy. Just load the mask into an oscilloscope that supports mask testing and you will quickly find errors.


However, it is very common for industry standard masks to be unavailable. An obvious case is proprietary buses. In my experience, not having an industry standard mask is the rule rather than the exception. Fortunately, there are alternatives which allow users to create a mask on their own.


Creating your own mask with Auto Mask

A very simple method to create a mask is known as Auto Mask. The idea behind an Auto Mask is to take a single waveform and add a tolerance of the desired size around it both vertically and horizontally. An example of an Auto Mask setup dialog from a Keysight Infiniium oscilloscope is shown in Figure 2. It should be noted that the selection of the source channel is made in the main Mask Test dialog.


Figure 2. Auto-mask setup dialog


In one single step, a mask is created, such as the one shown previously in Figure 1. It is easy to think that this solves the problem completely. Regrettably, it does not work for most situations where masks are needed.


Auto Mask only works where the waveform only follows a specific path and only that path. This means if there is an entire unit interval (or more) on screen, the mask will continually fail. Consider the example in Figure 3. We have what seems to be a perfectly good auto mask drawn on the screen. However, in this case, we only have a single run’s worth of waveforms.



Figure 3. An auto mask of multiple unit intervals


Figure 4 shows what happens when we do multiple runs. Suddenly, we get a whole bunch of errors. What happened? The key point is: unless we are zoomed in on a very small part of the waveform, there are many points where either a rising edge or falling edge are valid. This means that the Auto Mask finds a great number of errors when there is actually none at all, in this case.


Figure 4. Showing the same auto mask as Figure 3 with multiple runs


This problem is just as bad with a single unit interval, which is known as an eye diagram. Eye diagrams are a very powerful method to determine signal integrity but auto mask will never work. An example of an eye diagram is shown in Figure 5. The middle opening is known as the “eye”. The size of this opening is one key measure of signal integrity.


Figure 5. Eye diagrams show a single unit interval


Creating your own mask with Draw Mask

For a long time, users who needed to create their own mask needed to use Excel to create a mask file. This is very error prone and tedious. However, starting in Version 6.0, Keysight Infiniium oscilloscopes provide a Draw Mask dialog. An example is shown in Figure 6. There is the option for manual creation of polygons which are used to indicate the areas where the waveform should not go. Much of the focus is typically on the eye area itself, although users are free to put the polygons anywhere.



Figure 6. Manually creating a mask using the Draw Mask dialog


Up to eight polygons are allowed, and each polygon can have up to fifteen points. Each point can be editable simply by clicking it and moving it to a new location. Moving the entire polygon at once is done by clicking anywhere inside it and dragging it to a new location. The bottom line is that users can create a new mask visually which is far less error-prone because they can see the existing waveforms in the editor. This is a massive improvement over creating mask files by hand in Excel!


The readout at the bottom shows the numeric values for each of the points in the selected polygon. These can be manually edited as shown in Figure 7. This is useful because sometimes users have specific values in mind to match a specification.


Figure 7. Entering a specific value for a point in a polygon


Each polygon can consist of as few as three points or as many as fifteen points. The reason to allow this flexibility is to give users the option to be as accurate as they desire. Fifteen points are the most accurate, but it also takes the most time to create.


An example of a complex multi-polygon mask is shown Figure 8. Notice the use of a different numbers of points. Creating this example took me only a couple of minutes. With Excel, it could have taken hours to get right.


Figure 8. Creating a complex mask in the Draw Mask dialog


The final result in the oscilloscope is shown in Figure 9. In this case, the waveform is well behaved and we have no errors.

Figure 9a. An eye diagram with a complex mask


Figure 10 shows the same mask with some errors. Notice the edges of the mask are now in a bright red. These are waveforms which violate the mask. Optionally, users can stop as soon as a failure occurs to allow them to see what led up to the problem.


Figure 9b. An eye diagram with a complex mask


It gets better: Automatic mask creation

While creating polygons is pretty fast and definitely a big improvement over manually creating mask files, users asked for even more efficiency. Can’t the oscilloscope automatically create shapes based upon an eye diagram? An example of doing this is shown in Figure 10. To use “Auto Eye,” users need to specify a tolerance and the maximum number of points and then click in the region where they want the shape created.  These shapes are still editable so that users can tweak them if desired. But most of the time the points are fairly close to the desired location so the amount of editing isn’t large. The key point of Auto Eye is not that it always gets the points exactly right but rather that it reduces the amount of editing by putting the points in approximately the correct location. That way, users might only have to adjust a few points of the polygon instead of moving all of them.


Figure 10. Automatic creation of polygons for an eye diagram


Mask testing transformed

Mask testing has come a long way! Because of enhancements such as Keysight’s Draw Mask dialog, it is now an analysis that can be used on any waveform. This means mask testing has moved from a relatively small niche to a fast and effective tool that should be used early in the testing process to quickly detect errors. Debugging electronics is challenging and can often be time-consuming, but mask testing is now a huge asset to oscilloscope users.



About the Author

Doug Beck is an Expert Usability Engineer with Keysight Technologies focused on oscilloscopes. He holds a PhD in Industrial & Operations Engineering from the University of Michigan and has 12 patents.

Pushing the limits of Ethernet speeds and NRZ

The increased demand for more data from consumers and businesses has required faster and faster Ethernet technologies. To that end, a shift from NRZ(Non-Return to Zero or PAM2) to PAM4 (Pulse Amplitude Modulation 4) has now become the answer to increasing data throughput. PAM has never been used at the high speeds we are seeing today and this is where the challenge begins. Margins have shrunk, edges are more critical and error rates increase when working with PAM4.


PAM has never been used at the high speeds we are seeing today and this is where the challenge begins. 


Pulse Amplitude Modulation (PAM4) vs. Non-Return to Zero (NRZ/PAM2)

First let’s look at the differences between NRZ (PAM2) and PAM4. NRZ(PAM2) (Fig 1) has two amplitude levels with 1 bit of information per symbol. The real-time eye shows one distinct eye opening with one distinct rise and fall time.



Figure 1: NRZ signal and Eye Diagram


The PAM4 (Fig 2) by contrast has four distinct amplitude levels and two bits of information per symbol. The real-time eye shows three distinct eye openings with six rise and fall times.



Figure 2: PAM4 signal and Eye Diagram


Data Throughput

PAM4 has four amplitude levels with two bits of information per symbol, while NRZ(PAM2) has one bit of information per symbol. Symbols are expressed in terms of baud (Bd). So PAM4 has twice the throughput for the same baud rate of NRZ.



Figure 3: Same data expressed as NRZ vs. PAM4


With standard (linear) PAM4 we have the potential for two transitions at the same time. These transitions can cause two bit errors per symbol. If we convert standard PAM4 to gray code, we can cut our bit error down to one bit error per symbol. This reduces our overall bit error in half.



Figure 4: Standard (Linear) PAM4 converted to Gray code


Clock Skew and Eye Vertical alignment

Clock skew can have a significant impact on the vertical alignment of PAM4 eyes. When the upper and lower eyes are skewed to the left relative to the middle eye as shown below, this indicates that the most significant bit (MSB) is early with respect to the least significant bit (LSB). We can imagine a mask (shown in green in Fig 5) that sets a margin for what skew is acceptable. In this case, a quarter unit interval (UI) mask might be a good starting point. As the eyes drift, further past center alignment of the middle eye, symbol errors (SER) will increase and data recovery suffers.




Figure 5: Eye Skew – skew between top, bottom eyes relative to the middle eye


Non-linearity and Amplitude compression

Non-linearity and amplitude compression are also an issue when rise/fall times differ between the upper (MSB) and lower (LSB) eyes or voltage amplitude for various levels are too high or low. In Fig 6 the lower eye is compressed with the upper eye dilated.



Figure 6: Non-linearity and amplitude compression can also effect SER.


To measure the transmitter linearity of the PAM4 signal, we measure the mean signal level transmitted for each PAM4 symbol. Symbol levels are derived from the voltage levels V0, V1, V2 and V3 as shown in Fig 7. Vmid is the halfway point between V0 and V3. To determine how far off we are from the ideal symbol level, we can calculate the effective symbol level (ES) as shown below. Ideally, we would like both ES1 and ES2 to be 1/3 so that the eyes are perfectly symmetrical and our voltage levels are aligned. This all brings us to level separation mismatch ratio (RLM). Ideally, we would want our level separation mismatch ratio to equal 1. This would imply that all our PAM4 eyes are symmetrical and open.


Figure 7: Transmitter linearity measurement and level separation mismatch ratio RLM


Forward Error Correction (FEC)

 With all that can go wrong with the signal from the transmitter though the channel and to the receiver, how can we correct for errors along the way? This is where error correction can help to correct at least some of the errors. With error correction, we have the advantage that we don’t need to retransmit the data again. The Reed-Solomon error correction scheme has properties that are well suited for PAM4, as it can correct for burst errors shown below. Reed Solomon error correction treats symbols the same no matter how many bits are contained in the symbol. So, with PAM4 having two bits per symbol compared to NRZ only having one bit per symbol doesn’t penalize the efficiency of the Reed Solomon correction scheme. Of course, with any error correction scheme, parity symbols are sent with the data and this will add to the overhead in our data stream.



Figure 8: Burst Errors showing corrupted bits



What PAM4 brings to the table is an opportunity to overcome NRZ (PAM2) speed challenges while doubling the data throughput at the same baud rate. Keeping PAM4 eyes open, symmetrical and un-skewed when transmitting data brings new challenges to designers. Understanding how these characteristics play together is important for successful implementation of PAM4 solutions. This introduction to PAM4 shows just a taste for what PAM4 has to offer and some of the challenges that must be overcome.


Understanding how these characteristics play together is important for successful implementation of PAM4 solutions. This introduction to PAM4 shows just a taste for what PAM4 has to offer and some of the challenges that must be overcome.


Keysight has been a key contributor to IEEE802.3bs and other Ethernet standards that use PAM4 and understands the test requirements. New test challenges with PAM4 can be overcome, and Keysight has the solutions to overcome these challenges. Keysight has other comprehensive test solutions from design simulation to physical layer testing that includes transmitter, receiver and channel for PAM4.


For more information go to


Request Free Master 400G poster


PAM4 Applications

N8827A PAM-4 Analysis Software for Infiniium Real-time Oscilloscopes

N8836A PAM-4 Measurement Application for Ethernet and OIF-CEI for Real-Time Oscilloscopes

86100D-9FP PAM-N Analysis Software for 86100D DCA-X Oscilloscopes

N1085A PAM-4 Measurement Application for Ethernet and OIF-CEI

Hacking the specs

Everyone loves a bargain. And who doesn’t love a hacked oscilloscope? Well, it would be pretty odd for an oscilloscope company to teach you how to hack your own hardware. Besides, that’s already been done (Fig. 1). So, I’m coining a new term: “spec hack.”


Webster’s dictionary will one day define it like this:


Spec hack  (/spek’hak/), n. When an engineer uses expert-level knowledge of their test equipment to achieve performance above and beyond typical expectations of said equipment. <Thanks to a spec hack of the flux capacitor, Doc Brown discovered he only needed to go 77 miles per hour to travel through time.>


Today’s spec hack will look at the built-in frequency counter on an InfiniiVision 1000 X-Series oscilloscope.


You may think that a 100 MHz oscilloscope will only let you see signals up to 100 MHz – but that’s not actually true. Why? Oscilloscope bandwidth isn’t as straightforward as the labeled spec.


Figure 1: just a few days after its release, the EEVBlog YouTube channel posted an oscilloscope hack to double the bandwidth of the 

1000 X-Series.


You may think that a 100 MHz oscilloscope will only let you see signals up to 100 MHz – but that’s not actually true. Why? Oscilloscope bandwidth isn’t as straightforward as the labeled spec.


Oscilloscope Bandwidth brush up

To fully understand how far you can push your frequency counter, you must first understand how your oscilloscope’s bandwidth works. If you are confident that you know all about bandwidth, feel free to skip this next little section. If not, get ready to have your mind blown (or at least maybe learn something new).



Essentially, if you have a 100 MHz oscilloscope bandwidth, it means you can view a sine wave (or frequency components of a non-sine wave) of 100 MHz with ≤ 3 dB of attenuation.


But, here’s the main take away – bandwidth is all about signal attenuation, not just about the frequencies you can or can’t see (Fig. 2).


Figure 2: A Keysight 6000 X-Series Oscilloscope demonstrates what a 2.5 Gbps waveform looks like at varying bandwidths.

Even at 200 MHz, there is still a visible signal.


Usually this won’t matter for your day-to-day oscilloscope usage. You may see round corners on what should be a crisp square wave, but it probably won’t change how you use your scope. But, when you’re using a built-in frequency counter, it can be used to your advantage.


But, when you’re using a built-in frequency counter, it can be used to your advantage.


How a frequency counter works

To understand why this effect can be advantageous, you need to understand how a frequency counter works. It’s called a frequency counter because it literally counts. It counts the number of edges found over a specific amount of time, called the gate time. The frequency is calculated like this:


Frequency = Number of pulses/Gate time


From a circuitry perspective, the counter is simply a comparator (to identify signal edges) and a microcontroller to count the output and display the results (Fig. 3) As it turns out, oscilloscopes already have this infrastructure inside their trigger systems.


Figure 3: An old-school HP frequency counter’s nixie tube display


Why oscilloscopes make great frequency counters

As luck would have it, the trigger circuitry of a scope often has comparators built into the signal path (think “edge trigger”). With some planning, it’s not difficult for oscilloscope designers to include a frequency counter built into the oscilloscope. It may sometimes require extra hardware, but the essentials already exist.


The most important specification of a counter is accuracy - the higher the precision of the time base, the more accurate the counter. Oscilloscopes also have to have a highly accurate time base, so a built in counter can just use the scope’s clock.


Finally, an oscilloscope’s trigger circuitry typically has its own special signal path designed specifically to extract the core signal and block out noise and unwanted frequency components. Unlike the oscilloscope’s acquisition circuitry, the trigger circuitry doesn’t need to recreate the signal with high accuracy, it only needs to do a fantastic job of finding edges. So, a frequency counter can use a scope’s trigger signal path instead of the acquisition signal path and get a higher fidelity edge.


The Spec Hack

Let’s put it all together. So far we’ve learned a few things:


  1. You can see signals (or signal edges) higher than the bandwidth of your oscilloscope, but it may have an attenuated amplitude.
  2. Frequency counters just need to count edges; they don’t care very much about the amplitude of the signal.
  3. Oscilloscopes have a dedicated, specially conditioned signal path dedicated to identifying edges.


So, a frequency counter built into an oscilloscope can measure frequencies higher than the bandwidth of the oscilloscope. The question is, how much higher?


So, a frequency counter built into an oscilloscope can measure frequencies higher than the bandwidth of the oscilloscope. The question is, how much higher?


One of the perks of working at Keysight is that that’s an easy question to answer. I pulled out my 100 MHz Keysight 1000 X-Series low-cost oscilloscope and a grossly unnecessary Keysight 67 GHz PSG (because hey, why not?) (Fig. 4) and ran a frequency sweep to see just how fast of a signal the oscilloscope’s frequency counter could measure.


Figure 4: a PSG producing a 529 MHz sine wave


The results blew me away!


The 100 MHz oscilloscope’s counter was able to measure up to 529 MHz! That’s over 5x the bandwidth of the oscilloscope (Fig. 5).


Figure 5: A screenshot of the frequency counter measuring 529 MHz. 


The lesson? Know your equipment!

It’s always fun to find a little, hidden gem in your test equipment. Sometimes it’s an Easter egg mini game hidden away in a secret menu; sometimes it’s a measurement you had no idea you could make. Having a good understanding of the fundamentals of the equipment you use will not only help you make better, more accurate measurements but also help you avoid any traps that might lead you down the wrong test path.


Having a good understanding of the fundamentals of the equipment you use will not only help you make better, more accurate measurements but also help you avoid any traps that might lead you down the wrong test path.


Are there any spec hacks that you’ve found? Be sure to let me know in the comments below or on Twitter (@Keysight_Daniel). Happy testing!

I’ll bet you have quite the stock of random probes on your bench. And, you’ve inevitably reached for a probe and made a measurement without even knowing which probe you were using. I’m guilty, too. Probe selection can seem awfully confusing, so we often resort to this “random selection.”


The fact is, probe selection can significantly affect your measurement, so let’s break this down and walk through the one simple thing every engineer should know about oscilloscope probes: what the difference is between passive and active. Use this guide as the first step to better, more accurate measurements.


The Basics

Passive probes

Passive probes (Fig 1) are the most widely used type of oscilloscope probe. They are rugged and economical and they are what you’ll typically find shipping with your oscilloscope. They’re what you’re bound to find on every engineer’s test bench. These high impedance probes are what we consider “general purpose.”


Figure 1: A typical passive oscilloscope probe

Active Probes

Active probes (Fig 2) are a level above passive probes in terms of performance, complexity, and typically cost. You’ll generally purchase these probes separately from an oscilloscope, for a specific measurement application. Compared to passive probes, they need to be handled carefully due to the active circuitry inside the probe head.


Figure 2: An active probe with headlights for increased visibility


The Big Differentiator: Power

Passive probes contain no active circuitry such as transistors or amplifiers and therefore require no power. A typical 10:1 passive probe and oscilloscope combined circuit is represented in Fig 3.


Reading from left to right, you’ll see a 9 Mega Ohm resistor that is one of two parts that make up the 10:1 divider of the probe. Continuing to the right, there’s an adjustable compensation capacitor which you can mechanically adjust to “match” the oscilloscope capacitance. This helps to make sure the probe and oscilloscope are equally compensated and thereby setting you up for an accurate measurement.


The oscilloscope makes up the second half of this combined circuit. The 1 Mega Ohm resistor completes the 10:1 divider ratio. In other words, the 9 Mega Ohm and 1 Mega Ohm resistors “divide down” the signal by a factor of 10. The input capacitance is the standard capacitance of your scope, typically printed on the front panel.


Figure 3: Oscilloscope-Passive probe combined circuit layout


Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier. You can see the typical active probe configuration in Fig 4.


Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier.


Figure 4: Typical active probe circuit layout


The physical look of an active probe is also much different from a passive probe. The head of the probe contains the active circuitry, where the filtering and conditioning are done. The pod interface that plugs into the scope tells the scope which probe has been plugged in, what we call “auto-sensing.” After sensing which probe is plugged in, there is auto-configuration that happens (i.e. settings are auto-adjusted based on the probe). This isn’t to say this isn’t possible with passive probes, it is just not as common.


Performance Difference: How to Select a Probe

Passive probes are great for making general purpose measurements. They have a wide dynamic range and bandwidth as high as 500 MHz when connected to the 1MOhm input of the oscilloscope. They work well if you’re working in the DC and low-frequency range. They can also be sufficient for making quick quantitative measurements, such as if a clock is running or if the source is on- simple “yes or no” questions where a high degree of accuracy isn’t required.


Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances.


Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances. The real driving factor is when you need a high degree of signal integrity, you should use an active probe. But why? It really has everything to do with probe loading.


Signal Integrity: What Probe Loading Has to Do With It

When you touch a probe to your DUT, the probe loads the signal and this is called probe loading. To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.


To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.


The probe’s input impedance is a function of frequency. It stays pretty flat from DC up to a certain frequency but as frequency continues to go up, the probe’s input impedance goes down, as the capacitance of the probe starts to come into play. The more capacitance, the lower the impedance. As the frequency goes up above the crossing point of ~10kHz, this is where we can really see a difference in performance between an active and passive probe. The active probe has a low capacitance value, leading to higher impedance and less loading. You can see this effect plotted in Figure 5.


This effect trips up a lot of engineers.


This effect trips up a lot of engineers. You might think that a passive probe is just fine because it worked well at low frequencies, but as soon as you try to use it to measure a higher frequency signal (past the crossover point of 70 MHz in the example in Figure 5), there’s a significant performance degradation where you’d be better off using an active probe.


Figure 5: Impedance vs Frequency comparison of active and passive probes


Key Takeaway

Probe selection does affect measurement accuracy and signal integrity. Next time you go to make a measurement, consider your signal’s speed and the type of measurement you’re trying to make (quantitative vs qualitative) before deciding between a passive and active probe. Probe “random selection” will be a thing of the past.


Check out the video version of this blog -> Active vs. Passive Probes- Take the Mystery Out of Probing - YouTube  

Access free probing resources ->  Take the Mystery Out of Probing | Download Probe Training Kit  

If you’re an Infiniium user, you’ll definitely want to keep reading. The 6.1 software update just launched and it’s packed with new features that will improve your testing efficiency. Use this as a guide to make use of the new tools and enhancements that you’ll find in this software update.


These updates include:

  • New Jitter Decision Feedback Equalization
  • Improved Clock Recovery and Mask Testing for PAM4 and NRZ
  • New Software Update Analysis Tool
  • New Impedance Warning Function
  • New Power Integrity Analysis Application


Jitter Decision Feedback Equalization

Technological advances towards achieving greater Ethernet speeds presents two design possibilities, NRZ and PAM4, and each comes with a unique set of challenges. NRZ (Non-Return-to-Zero) has evolved over 50 years to 100G (25/28G, 4 lanes) and 400G (56G, 8 lanes). From a time domain perspective, NRZ consists of 1’s and 0’s and can be referred to as PAM2 (pulse amplitude modulation, 2-level) with two amplitude levels that contain 1 bit of information in every symbol. The NRZ eye diagram provides timing and voltage used to measure link performance and contains a single eye. But this single eye technology requires advanced technology in order to achieve the higher 400 Gb/s data rate.


The current 400 Gb/s challenges include totally closed eyes, shorter unit intervals (UI), tighter jitter requirements, and the mandatory use of forward error correction (FEC). These closed eye issues require enhanced receiver equalization such as continuous-time-linear equalization (CTLE) and decision feedback equalization (DFE) to correct. Moreover, new communication standards are requiring increased receiver sensitivity (down to 50 mV) and jitter budgets are even tighter for 400G at 17ps.


This software update adds a Jitter Decision Feedback Equalization function (DFE) to meet these increasing demands.


Improved Clock Recovery and Mask Testing for PAM4 and NRZ

If you’re working in 400G, you’ll be glad to hear of some major enhancements to the existing PAM4 solution. In the new software update, an improved PAM4 clock recovery algorithm was added along with jitter measurements on Decision Feedback Equalization (DFE) for NRZ signals.


A “Draw Mask” feature was also added to allow users to draw their own polygon mask specific to their eye pattern, enabling exact mask tests to customer specified limits.


New Software Update Analysis Tool

If you’ve ever had difficulty determining if an application or protocol is compatible with your scope’s software, you’ll be interested in the new Software Update Analysis tool rolling out in this software update. This tool will help you quickly determine if all applications and protocols on an oscilloscope are compatible with the latest software. This feature also allows software updates to oscilloscopes in secured laboratories that cannot have access to internet-supplied software.


New Impedance Warning Function

Safety is always a top concern. We are always working to improve product quality and reliability, and with this comes a new Impedance Warning function. With the new software installed, you will be notified when you have selected a lower voltage 50 Ohm input, and you'll see the max input level allowed on screen. This will help to reduce the chances of user-damage to the oscilloscope input.


New Power Integrity Analysis Application- N8846A

The new Power Integrity Analysis Application-N8846A was added to Keysight’s already industry-leading set of N8833A/B cross talk applications. This application was specifically designed to target power supply-induced cross talk.


Download Now > Infiniium 6.10 System Software for Windows 7 | Keysight   

Knowing the quality of the scope’s measurement system is paramount when you need to have accurate measurement results.  While banner specs like bandwidth, sample rate, and memory depth provide a basis of comparison, these specifications alone don’t adequately describe oscilloscope measurement quality.  

Figure 1: Keysight Infiniium S-Series Oscilloscope


Seasoned scope users will also compare a scope’s update rate, intrinsic jitter, and noise floor, all of which enable better measurements.  For scopes with bandwidths in the GHz range, another quality metric involves characterizing a scope’s ENOB. 


What is ENOB in the first place?  It stands for Effective Number of Bits and is really the measure of how well your oscilloscope accurately represents the captured waveform. 


The higher the ENOB, the better the oscilloscope sees the signal the way the components in your design see the signal.


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, 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. I am often asked, “what is the ENOB of a specific Keysight oscilloscope?”  Many vendors simply state a specific single number for ENOB, for example, an ENOB of 5.5.  The reality of the situation is this is just not how effective number of bits work. They are frequency dependent. So, it may be 5.5 at one specific frequency setting but is probably not 5.5 across the entire bandwidth of the oscilloscope.


ENOB was established by IEEE in 1993 as a measurement of an oscilloscope’s signal integrity and measurement accuracy. 



It directly correlates to an oscilloscope’s signal to noise ratio.  A higher ENOB will provide better oscilloscope measurements for Jitter, eye height and width, and amplitude.  ENOB is a metric, and does not indicate what is causing signal integrity issues.


Effective number of bits is directly related to the ADC within an oscilloscope.  In general, the bits of resolution within the ADC determines the quantizing levels for your oscilloscope as shown in Figure 1.  



Bits of Resolution

Quantizing Levels

At 1 Volt, Full Scale

 1 LSB =

At 16 mV, Full Scale

1 LSB =



3900 uV

62.5 uV



976 uV

15.6 uV



244 uV

3.9 uV



61 uV

1.0 uV

 Figure 2: Oscilloscope specification comparison


Increasing the number of ADC bits makes each quantizing step size smaller, so the maximum error is minimized.


ENOB is measured as a fixed amplitude sine wave at varying frequencies.  Each curve is created at a specific vertical setting while frequency is varied. ENOB calculations are easy to make. 


  1. First, input a perfect sine wave, capture it on a scope and measure the deviation from the result vs the input.                                                                                                                                                                          For example, input a sine wave from a PSG at 1 GHz into the scope and measure the 1 GHz sine wave. 
  2. Then fit it against a perfect 1 GHz sine wave. 


The difference between the data record and best fit sine wave is assumed to be signal error.  ENOB considers noise, ADC non-linearities, interleaving errors, and other error sources. 


What erodes the bits of resolution?

ENOB is primarily impacted by noise and distortion.  Noise of course effects your signal-to-noise ratio and distortion impacts the total harmonic distortion.   If the base noise of an oscilloscope is greater than the quantizing levels of the ADC, then there is no way for the scope to accurately represent the digital signal level to the least significant bit. 


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. ENOB doesn’t consider offset errors or phase distortion that the scope may inject.  So, it is also important to look at the base noise of an oscilloscope as well as its frequency response (amplitude flatness), phase linearity and gain accuracy to get a complete picture of the accuracy of an oscilloscope.  


In general, by choosing an oscilloscope with superior ENOB, you are choosing a scope with better signal integrity.


You not only impress your colleagues but you also get more accurate waveform shapes, more accurate and repeatable measurements, wider eye diagrams and less jitter.


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


For more information on determining measurement quality, check out the Scopes University S1E4 video, Determining Oscilloscope Measurement Quality.

Eye diagrams are extremely helpful in testing the physical layer fidelity of clock or serial data, but many engineers don’t know:

  • What they are
  • Why they should use one
  • How to easily set one up


They actually aren’t that complex when broken down.

Eye diagrams can quickly give you insight into your signal, along with any jitter or anomalies that may be present that you might know exist.


What they are

What is an eye diagram? Eye diagrams are a layered view of every bit transition combination. There are eight of these in total. You can see in Figure 1 how each of these are layered to make up the eye. This provides a composite picture of the overall quality of a system’s physical layer characteristics like amplitude variation, timing uncertainties, or infrequent glitches.


Figure 1: Bit transition combinations


Why you should care

An eye diagram is used to detect jitter, but what is jitter and why is jitter bad?


Jitter can cause errors in the data that you are trying to transmit. If there is too much of it riding on your signal, the data that is sent will be interpreted incorrectly by the receiving end because the edge crossings aren’t occurring when they should be.


Figure 2: Jitter causes errors in the interpreted waveform.


How to create an eye diagram

One of the first things many people think when they see an eye diagram on an oscilloscope is “how do you get it to look like that?”


This layered view of bit transitions is not something that a normal trigger would be capable of displaying. The answer is, the oscilloscope utilizes a built-in clock recovery system. Clock recovery is actually pretty straightforward. Some signals have an explicit clock signal, and some have an embedded clock. An explicit clock can be driven right into one of the oscilloscope channels, but embedded clocks have to somehow be de-embedded, or recovered, hence “clock recovery.”


Figure 3: Clock recovery dialog


There are three different ways to utilize the clock recovery system, which all depends on how well you know the bitrate of your signal (the width of each bit):

  1. Fully automatic
    1. The oscilloscope will calculate the ideal bitrate (or nominal data rate) of your signal.
    2. This should be used when you have no idea what the bitrate is and you need the oscilloscope to figure it out.
    3. However, this method is only about 80% accurate.
  2. Semi-automatic
    1. In most situations, you should have a rough idea of what the bitrate should be. You can very easily make a bitrate measurement on the oscilloscope to find this estimation. This method allows you to enter your rough measurement and then use that information as a seed as it calculates the exact ideal bitrate.
    2. This method is significantly more accurate.
  3. Manual
    1. This method should be used if you know the exact ideal bitrate of your signal.
    2. This is the most accurate method of clock recovery.


There are a few other settings within the clock recovery menu. To learn more about these, make sure you check out episode 3, How to set up an Eye Diagram, and episode 5, How to Measure Jitter, of Scopes University video series.


Once you have your clock recovery system set up, all you have to do to set up the eye is press “auto setup”. You will see in just a few seconds that the eye diagram has begun to form. Over time, you will be able to see if there is any jitter or anomalies in your signal. Generally, you will want to let the test run for a longer period of time.


The longer you let it run, the more data is collected, and the more jitter, anomalies, or any infrequent events you can see.

Figure 4: Eye diagram with jitter


From here, you can analyze the eye diagram further by using the color grading key. This allows you to visually analyze the frequency of each edge crossing. You can also very easily turn on a histogram on the eye to determine whether the jitter is deterministic or random. This will help you decide if this is something you can fix with a phase locked loop filter or if you have to redesign the component. Perhaps this is a topic for the next blog, though!

Figure 5: Histogram of the eye diagram


To learn more about what I talk about here, check out these Scopes University videos.

Episode 3: How to set up an Eye Diagram

Episode 5: How to Measure Jitter

IEEE Standards for 400G optical Ethernet links have been in development for many months, and the specifications are finally stabilizing as of mid-2017. The result will be the first optical standards to employ PAM4 modulation. This requires a new set of measurements, with the TDECQ (Transmitter and Dispersion Eye Closure Quaternary) measurement getting the most attention. Increasingly more cutting edge companies are ready to evaluate their 400G products and need the TDECQ capability now.


TDECQ is a new and significantly easier method of calculating the penalty for transmitters that have unequal sub-eyes. This software calculation requires only an oscilloscope and are achieved by a direct measurement of the transmitter eye diagram.


It is simpler, faster and less expensive than older TDP (transmitter dispersion penalty) measurements, which would need a reference transmitter or an optical enabled BERT.


In its simplest definition, the TDECQ measurement creates two vertical histograms measured on an eye diagram like Figure 1 below. The histograms are centered at 0.45 and 0.55 unit intervals and each spans all modulation levels of the PAM4 eye diagram.


Figure 1: Illustration of the TDECQ Measurement


The amount of noise captured in the histogram is compared to an ideal receiver, and the dB difference in noise levels represents the power penalty for the transmitter under test.


One common mistake is to confuse the TDECQ result in dB with BER measurements.


We must keep in mind that TDECQ is a measurement of transmitter eye opening quality relative to an ideal transmitter and not a bit error rate.


Keysight engaged early in the development of a TDECQ solution with the IEEE Standards Association, sharing many of our hardware evaluations with the 400G committee. As a result, our TDECQ solution is easy to set up and creates fast measurements for use in both R&D and manufacturing environments. Our competition has also developed TDECQ capability, but their solution is separated from the scope and runs much slower.


The Keysight solution was designed for easy integration into a manufacturing system and can be quickly updated at the customer site if any changes are made to the IEEE 400G Standard.


Due to its low noise and fast sampling, the primary Keysight TDECQ solution is the low-cost N1092 DCA-M sampling scope module. It has predefined TDECQ reference receivers for both 26 GBaud and 53 GBaud transmitters. In addition, the Keysight 86105D-281 and 86116C-025 plug-in modules can also be used with the 86100 mainframes with the TDECQ option for the same result. 


View more information & order your solution today >>

N1092A DCA-M Sampling Oscilloscope

86105D 34 GHz Optical, 50 GHz Electrical Module

86116C 40 to 65 GHz Optical and 80 GHz Electrical Plug-in Modules

86100D Infiniium DCA-X Wide-Bandwidth Oscilloscope Mainframe


Get a free 400G poster!

By now, you’ve probably heard about Keysight’s new 1000 X-Series oscilloscopes. We talked about them a lot during Scope Month, and you might’ve seen some of the blogger reviews from Hackaday, ElectroBOOM, and EEVblog.


The 1000 X-Series features ultra-low cost oscilloscopes from 50 to 100 MHz bandwidths. They have a starting price of $449 (USD) and let you capture more of your signal and see more signal detail than similarly-priced scopes with a sampling rate of up to 2 GSa/s and update rate of 50,000 waveforms per second.


Fig 1: Balancing cost and quality


But how did Keysight manage to create such a high-quality oscilloscopes for so cheap?

Back when the 1000 X-Series oscilloscope project was started, a large group of Keysight oscilloscope R&D, supply chain, order fulfillment, and other teams got together to discuss what was needed to create a low-cost oscilloscope that not only met engineers’ expectations but exceeded them.


Creating a low-cost scope could be an easy task, but making one that was also high-quality and feature-rich was a much more time-intensive one.


For the R&D teams, leveraging Keysight’s existing technology when designing the 1000 X-Series was a great value. Keysight Technologies used to be Agilent Technologies, which used to be Hewlett Packard, and this company has been creating oscilloscopes for >60 years. This history of technology advancements and innovation not only helped lower the price of designing the 1000 X-Series but also made it possible to include functionality like the integrated WaveGen function generator, segmented memory, and frequency response analysis capability. Because Keysight already has these capabilities in high-performance oscilloscopes, it was easier to find ways to cheaply integrate them into the new 1000 X-Series.


For the Order Fulfillment and Supply Chain teams, creating a cheap oscilloscope meant reassessing everything that makes up these oscilloscopes (from the oscilloscopes themselves to the packaging that ships with the products) to look for ways to reduce costs without impacting product quality. Nothing was non-negotiable as these teams worked together for months, assessing and reassessing hundreds of decisions that would save $.15 cents here, a fraction of a penny there – all adding up to a lower-cost oscilloscope for engineers.


Examples of real conversations these teams had:

  • “If we reduce the number of layers in our board, we can lower the overall cost of the board. This will make it more challenging for our engineers to route the boards, but let’s see if we can make it work.”
  • “It’s pretty expensive to print this document that ships with every product. Let’s redesign it to use less paper and a different paper type to save $3 per printing.”
  • “Black ink costs $.10 cents less, so let’s go with that in this area instead of red.”
  • “Is it necessary to have two printed circuit boards? Let’s try to design these oscilloscopes to have only one PCB to keep costs down.”
  • “Do we really need to ship a CD with every oscilloscope? Do customers even need a CD when the majority of our laptops don’t even have a CD player anymore?”
  • “Is it necessary for us to use an envelope with a clear window? Why not use a cheaper one with no window?”


On top of these considerations, Keysight also leveraged its position with vendors to reduce costs. Because some of the 1000 X-Series parts are the same as other Keysight oscilloscopes’ parts, Keysight can buy these items in bulk to negotiate better prices and pass these savings on to customers.


Another result of these efforts is that Keysight can leverage a lot of these same cost-saving ideas when developing future products to help keep prices down for engineers.


While these Keysight teams were looking for ways to reduce the 1000 X-Series’ cost, they also kept quality at the forefront of these conversations. During every stage of the 1000 X-Series’ design and development process, the oscilloscopes underwent the same rigorous level of quality testing that Keysight’s half-a-million dollar oscilloscopes undergo. So, while hundreds of decisions went into reducing costs, you know you’re still going to get a professional-level oscilloscope you can trust. To prove it, Keysight offers a 3-year warranty on the 1000 X-Series that is upgradable to 5 years.


We’re often wary of cheaper products because we think low cost equals low quality, but this isn’t always the case!


Sometimes it just takes a room full of people dedicating their time and resources to a common goal like creating a low-price oscilloscope that doesn’t sacrifice quality.


Click here to learn more about how Keysight created the ASIC for the 1000 X-Series oscilloscopes.

Click here to learn more about 1000 X-Series oscilloscopes.

Written by Min-Jie Chong


The Need for New SAS-4 Storage Standard

The increase of data traffic due to the advent of internet of things has driven the need for faster backbone and storage transmission to meet this need. The Serial Attached SCSI - 4 (SAS-4) is a new enterprise storage standard that is being created to meet this need. It supports data rate of 22.5 Gb/s which doubles the data throughput of previous generation SAS-3 standard.


What is SAS-4 standard?

The SAS-4 working committee decided to leverage the OIF-CEI 3.1 specification to speed up the development of SAS-4 specification. The consequence is the test methodology in SAS-4 will deviate from the previous SAS generations. In the previous generation, reference transmitter and receiver are defined, which describe how a “perfect” design would handle the outgoing and incoming signal. However, the OIF-CEI specification does not provide any reference designs, which changes how the SAS-4 designs are tested.


First thing first, accessing SAS-4 signals

SAS-4 specification has a new recommendation of the insertion loss profile of the test fixture being used for testing. The intents are to more accurately test the 22.5 Gb/s signal without the effect of test fixtures so the industry can get more consistent results and avoid marginal design from passing using better test fixtures, but not meet the actual performance in real world. Keysight has found the Wilder Technologies SAS-3 test fixtures to be suitable for SAS-4. They perform better than what the specification recommends. This is a good outcome because it is easier to supplement loss using the embedding methodology, using Keysight’s N5465A InfiniiSim software toolset.


Oscilloscope bandwidth requirement

The SAS-4 specification does recommend a minimum of 33 GHz oscilloscope bandwidth for transmitter test. Keysight’s Z and V Series oscilloscope models (i.e. DSAV334A, DSAZ334A and DSAZ504A) have bandwidth that meet this requirement.


Making SAS-4 transmitter measurements

Keysight’s new N5412E SAS-4 transmitter test application provides step-by-step instructions to guide an engineer through the process of configuring the test setup, selecting the tests and connecting the signals to the oscilloscope. After everything is setup correctly, the application will then make the necessary measurements and analysis, and then presenting a pass or fail status of the signal under test. A test report will also be automatically generated at the end of this process, documenting the test results and measurement screenshots. This can really remove the complexity of learning the specifications, which can save engineers a lot of time and effort.


 transmitter measurements

Figure 1: Keysight N5412E SAS-4 automated test application for the oscilloscope, which covers all the required transmitter test requirements.


The application includes all the transmitter requirements listed below.

  1. Spread spectrum clocking (SSC)
  2. Transmitter signal quality (TSG)
  3. Transmitter equalization (TXEQ) coefficient request and circuit response
  4. Out-of-band (OOB) signaling


SAS-4 is highly susceptible to crosstalk

SAS-4 interface packs a lot of high speed lanes densely in a connector, which makes it highly susceptible to crosstalk effect. It is important for oscilloscope jitter separation algorithm to be able to handle presence of crosstalk. The earlier, more common jitter separation with the “spectral” method is not capable of separating crosstalk from random jitter. Keysight oscilloscope uses a newer, more advanced “tail fit” method that can correctly separate the effect of crosstalk from the random jitter.  


After determining the presence of crosstalk, the Keysight N8833A crosstalk analysis tool can provide deep analysis and debug capabilities. The tool is able to identify which potential aggressor is aggressing at the victim, quantify the amount of crosstalk the aggressor is coupling into the victim and then removing the crosstalk from the victim signal. We can check if the design can pass the specification and how much margins can be recovered without the crosstalk. This can assist in making important design decisions such as whether improving the crosstalk can make our design passes the specification, and which part of the design needs to be fixed.


eye diagram

Figure 2: Keysight N8833A crosstalk analysis application showing the eye diagram before (top) and after (bottom) removing crosstalk from the signal. The eye height and width can be measured to see the improvements of the signal without crosstalk.


Vendor specific SAS-4 receiver equalization implementation

While SAS-3 mandates 5 tap of decision feedback equalizer (DFE) implementation to recover a closed eye at the receiver, the SAS-4 does not mandate any specific number of taps. It is left to the vendor specific implementation how many taps will be sufficient to open up the eye. Keysight’s N5461A equalization tool with DFE allows engineers to recover the eye with up to 40 taps. Engineers can specify the value for each tap and check the effect on the eye opening or use the tap optimization feature that will compute the values based on the constraints given by the engineers. This feature is very useful to reproduce the eye opening that the receiver sees after the DFE process.


equalization tool

Figure 3: Keysight’s N5461A equalization tool is used to open up the closed eyes at various SAS data rates, and what the SAS receiver would see after DFE is performed.



Keysight has been a key contributor to SAS-4 and previous standards, and understand the test requirements. New test and interoperability challenges exist at 22.5 Gb/s and Keysight has the solutions to overcome these challenges. The automated N5412E SAS-4 test application covers the complete transmitter test requirements. Other tools such as the N8833A crosstalk analysis and N5461A equalization application can provide deep analysis and debug capability. In addition, Keysight has other comprehensive test solutions from design simulation to physical layer testing that includes transmitter, receiver and channel for SAS-4 standard.

Understanding the effects of crosstalk in high speed communication circuitry and pin pointing the root causes have been extremely hard for designers, until now. Keysight’s industry leading N8833A and N8833B software not only identifies the crosstalk but also allows the user to determine the sources. In addition, it can remove the effects of the crosstalk and determine the recovered margin.

First let’s discuss the many types of crosstalk and their origins.  Crosstalk has become an important issue as data rates have increased, and more and more lanes are being packed into smaller and smaller spaces. The amplitude interference of crosstalk impacts the signal fidelity of a communication eye diagram, essentially causing the eye to become more closed. The majority of crosstalk is caused by capacitive or inductive coupling between multiple transmission lines and/or power delivery networks. Prominent sources are near end crosstalk, far end crosstalk, power supply induced jitter, voltage dependent amplitude noise and simultaneous switching noise. Transmission line crosstalk is the result of electromagnetic interference between electrical components and is mainly caused by capacitive or inductive coupling. Forward traveling or far end crosstalk travels the same direction as the aggressor signal and its energy grows and it travels down the transmission line resulting in amplitude bulge in one area of the eye pattern. Reverse traveling or near end crosstalk is constantly moving away from the aggressor edge and is spread somewhat evenly over the transmission line resulting in a smearing of the entire eye pattern. Power supply aggressor crosstalk is created by noise on the power rail supply and caused phase noise changes or jitter. Voltage dependent amplitude noise crosstalk adds noise to the voltage and ground bus and causes non-linear effects on each logic level. A power supply can also be a victim of crosstalk due to simultaneous switch noise on serial lines and this is caused parasitic inductances lying between board and system ground and is also known as ground bounce. The above crosstalk origins, effects and sources may seem overwhelming at first but Keysight’s N8833 application greatly simplifies both the user knowledge and effort needed to get to root cause.


Legacy methods of determining crosstalk digital communications systems has relied on the process of selectively disabling some channels while enabling others. This process usually took significant time and effort. Power supply noise adds yet another analysis hurdle creating a non-linear transfer on the serial data timing called the Time Interval Error and has been difficult to solve and correlate. Past troubleshooting methods also required special design and test modes to analyze the crosstalk. Another challenge is that many times crosstalk aggressor signals are created within a package or in a system that is not accessible to probing.


The new Keysight crosstalk analysis application meets all of the above challenges by:

1) identifying the sources of crosstalk affecting the victim,

2) quantifying how much each aggressor is disrupting the victim,

3) removing the effect of crosstalk from the victim signal for analysis and

4) checking how much design margin is recovered when crosstalk is removed from victim.


Other features of the Keysight software are:

1) analyzes up to four signals (aggressors or victims) at once,

2) requires no crosstalk simulation or model,

3) identifies and reports the amount of crosstalk present on victims from each aggressors,

4) plots waveforms without crosstalk,

5) compares them with the original waveforms using scope tools such as eye diagram and jitter separation to see how much margins can be recovered.


The key types of crosstalk that can be analyzed are:

1) transmission line aggressors: Near-End Crosstalk (NEXT) and Far-End Crosstalk (FEXT),

2) power supply aggressors: Power Supply Induced Jitter (PSIJ) and voltage-dependent amplitude noise,

3) power supply victim: Simultaneous Switching Network (SSN).


Finally the software can report the results in different ways:

1) Inter-Symbol Interference (ISI) magnitude of the victim on itself,

2) crosstalk magnitude of the transmission line aggressors on the serial data victim,

3) crosstalk magnitude and jitter of the power supply aggressors on the serial data victim,

4) crosstalk magnitude of the transmission line aggressors on the power supply victim.


The Keysight crosstalk application N8833A/B is the most comprehensive solution in the market enabling a closed loop design cycle, saving designers both time and money. This application effectively solves the challenges mentioned above by enabling designers and engineers to: 1) identify which signals are coupling into your victim signal, 2) quantifying how much error each aggressor signal adds to your victim signal, 3) see what the victim signal would look like without the crosstalk and how much eye margin can be recovered without the crosstalk, 4) determine if the existing crosstalk justifies a design change and where to improve the circuit or system design.

Written by Sheri Detomasi


There are many similarities and differences between oscilloscopes and wideband digitizers.  How do you know which is the right tool for your measurement need? 


Oscilloscopes use wideband data converters and typically provide a broad range of functionality.  They provide probing and visualization of time variant waveforms.  When debugging or troubleshooting a project, it’s important to see as much signal detail as possible.  Oscilloscopes typically provide waveform reconstruction filters for improved signal visualization. If the ADC waveform data is displayed with no waveform reconstruction, you would see a confusing cluster of points as shown in (a) below.  Whereas (b) shows with the waveform reconstructions.  Same with fast rise-times in (c) and (d)

 Waveform reconstructions


For visualization purposes, an oscilloscope also has continuous waveform acquisitions in display memory.  An oscilloscope can produce an extremely high waveform update rate > 1,000,000 waveforms per second.   Shown below, with an oscilloscope’s high speed waveform update rate and its ability to pick up glitches or unexpected events.   


Different measurement capabilities




Many oscilloscopes have a wide range of automatic measurement capabilities like rise/fall time, delay, peak to peak, zone triggering, etc. In addition, with the wideband acquisition, oscilloscopes are also ideal for high speed digital test, emerging serial protocols, and advanced communications.  With the wide bandwidth, vast measurement capabilities and robust user friendly interface, weather in bench or modular form factor, an oscilloscope is a general purpose tool that can be used for many applications. Keysight’s modular oscilloscopes, the M924xA series, range from 200 MHz to 1 GHz and feature the same functionality you would find on a benchtop oscilloscope.



Digitizers are more purpose built.  Their main goal is to capture many channels of data with high resolution to achieve the best measurement fidelity.  While oscilloscopes typically have 8- to 10-bit ADCs, a digitizer is usually 10- to 16-bits.  This doesn’t tell the whole story though.  There are other noise factors to consider such as ADC differential and integral nonlinearities, thermal and shot noise, input signal distortion, as well as sample aperture jitter and ADC sample clock noise.  Therefore, a better measure of the resolution is the ENOB, or Effective Number of Bits.  One technique digitizers use to get even more ENOBs is digital down conversion (DDC).  DDC is extremely valuable when analyzing a small slice of spectrum within a wideband acquisition, allowing the user to reduce the bandwidth and ‘tune and zoom’ into a specific part of the signal.  Here is the digitizer DDC block diagram.

 ACD memory



It’s common for oscilloscopes to provide extremely wide bandwidth while digitizers provide higher ENOBs over smaller bandwidths. 


In normal use a digitizer will acquire many channels of data over longer time periods, producing lots and lots of data.  The data is either analyzed onboard or sent to a PC or storage device for post processing. For this reason, digitizers typically have deep memory buffers behind each ADC and very high data transfer rates.   For on-board processing, it’s useful to access the digitizers internal FPGA to do some real-time signal processing.  This allows the data processing and manipulation routines to reside in the hardware at GS/s processing rates and is useful for embedding algorithms to implement onboard custom filtering, correction routines, data reduction schemes as well as application specific routines.  This provides very specific application needs at very high speeds.  Here you can see a process for acquiring the data, processing, extraction, analysis and playback.



A digitizer is always connected to a PC and is controlled through a computer soft front panel or an automated program.  With this, there is less of a need for high-speed waveform update rate to the computer display.  The purpose-built nature of a digitizer makes it more of a dedicated tool for specific applications.





Ideal use

Interactive test and analysis with high performance user interface

Data capture with deep dive software analysis

Resolution and dynamic range



Measurement and analysis

Better automatic measures

Better data collection for post processing




Acquisition memory (record time)


Good, extendable to external storage

Number of channels


Good, expandable

Waveform update rate

Better visual (display) update rate


Data streaming


Better, high data throughput




FPGA access

Not typically








A word of caution: some test equipment vendors will promote a digitizer as an oscilloscope or an oscilloscope as  a digitizer.  This may cause some confusion and the wrong choice can cause headaches down the road. Ensure you understand your needs and select the right instrument for your test application.  To find out more, check out these resources as well as a video of Keysight experts talking about the blog:



App Note: Understanding the Differences Between Oscilloscopes and Digitizers for Wideband Signal Acquisitions  

Webcast: "Oscilloscope or Digitizer for Wideband analysis - Why care?" .

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