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Oscilloscopes Blog

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Written by David Liu

Routine electrical tests are time-consuming

Routine electrical tests can be mundane and quite time-consuming, especially when you’re required to repeat the same test procedure with just one or two parameter changes. Take the testing of a DC-DC converter, for example. In the diagram below, you’ll notice that setup requires a few instruments to work together, including a DC power supply, digital multimeters, a DC electronic load and an oscilloscope.

 

Typical schematic of a DC-DC converter test setup

 

Figure 1. Typical schematic of a DC-DC converter test setup

 

DC-DC converter testing

A typical DC-DC converter test procedure requires the recording of voltage, current and power values under different load conditions. The figure below simulates a table of results that you’d likely need to fill up as you complete the characterization of a DC-DC converter under test. Imagine the time that’s needed for you to manually change each instrument setting, measure the values at each test point and record them in the table. Not to mention, all those steps on repeat!

DC-DC Converter Test Table

Figure 2. Typical table for manual recording of DC-DC converter characterization results

 

Try automating routine electrical tests

The steps involved in a routine electrical test, such as the one described above, could be drastically simplified by programming the whole procedure and automating the test. But first, you need someone to write the program, and for efficiency, the program should cover steps from configuring instruments, to measuring outputs or inputs, and tabulating results.

 

 

BenchVue Test Flow saves on manual programming and testing

So, you could write your own program from scratch, or you could try Keysight BenchVue. With Keysight BenchVue, you don’t have to be a programming wizard. BenchVue’s drag-and-drop interface and its Test Flow feature lets you create simple test procedures or sequences – quickly and easily. Something that could take days with writing traditional programming languages from scratch.

 

With BenchVue Test Flow, you can:

  • Create custom test sequences easily and quickly
  • Combine multiple instruments into a sequence seamlessly for a more complete DUT characterization
  • Drag-and-drop controls for rapid test prototyping
  • Code flexibly with the capability to:
    • Incorporate various utility blocks that simplify programming including statistics, math function, step controls and loops
    • Run SCPI commands, integrated Command Expert sequence blocks, or external programs

BenchVue Test Flow intuitive interface

Figure 3. BenchVue Test Flow’s intuitive interface

 

BenchVue supports multiple Keysight instruments

Figure 4. BenchVue’s seamless support for multiple Keysight instruments

 

Using Keysight BenchVue Test Flow, you can complete the DC-DC converter test above within 10 seconds. Not only will you be able to export test results in a perfectly-filled spreadsheet, you will also be able to capture waveform files automatically.

 

BenchVue Test Flow completes DC-DC converter test in 10 s

Figure 5. BenchVue Test Flow completes DC-DC converter test in 10 seconds and exports tabulated results in a spreadsheet for easy analysis

 

BenchVue Test Flow  eases data recording, export and analysis

BenchVue Test Flow incorporates features that are designed to speed up your data analysis, so you can focus on your next measurement tasks.

  • The “preview tool” helps you validate the sequence setup at a quick glance.
  • Customize how and what measurements you’d like to view on the X- and Y-axis
  • View data logs easily in tabular form
  • For reporting purposes, or for further analysis, export data easily to popular software applications including MATLAB, Microsoft Word and Microsoft Excel

BenchVue Test Flow features for quick data analysis

Figure 6. BenchVue Test Flow’s easy setup validation with the preview tool, customizable X-Y chart and exportability to popular software applications

 

If you are looking for a software app that can easily control your instruments and simplify automation, we recommend that you check out the latest updates and features on the Keysight BenchVue page.

mike1305

Keysight Crossword Fun!

Posted by mike1305 Employee Dec 6, 2017

Hey readers!

 

For today’s blog we decided to do something a bit different. I’m a big fan of the New York Times crossword puzzle (Monday record time 4m35s, let me know yours below) and thought this would be a fun way to share that with all of you. Grab a cup of coffee and give this puzzle a try! Let us know in the comments if you’d like to see more of these in the future, or if you need any extra hints to solve it.

 

Cheers!

-Mike

 

*Download the attachment and print for the best experience*

If you are dealing with more than a couple tens of amperes of AC current and want to make flexible current measurements, consider the Rogowski current probe.

 

A Rogowski coil is an electrical transducer used for measuring AC currents such as high-speed transients, pulsed currents of a power device, or power line sinusoidal currents at 50 or 60 Hz. The Rogowski coil has a flexible clip-around sensor coil that can easily be wrapped around the current-carrying conductor for measurement and can measure up to a couple thousand amperes of very large currents without an increase in transducer size.

 

How does a Rogowski coil work?

The theory of operation behind the Rogowski coil is based on Faraday’s Law which states that the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The key difference is that the Rogowski coil has an air core as opposed to the current transformer, which relies on a high-permeability steel core to magnetically couple with a secondary winding. The air core design has a lower insertion impedance, which enables a faster signal response and a very linear signal voltage.

 

An air-cored coil is placed around the current-carrying conductor in a toroidal fashion and the magnetic field produced by the AC current induces a voltage in the coil. The Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of the current enclosed by the coil-loop. The coil voltage is then integrated in order for the probe to provide an output voltage that is proportional to the input current signal.

 

 

Advantages

Rogowski coil current probes offer many advantages over different types of current transducers or sensing techniques.

  • Large current measurement without core saturation -- Rogowski coils have the capability to measure large currents (a very wide range from a few mA to more than a few hundred kA) without saturating the core because the probe employs non-magnetic “air” core. The upper range of the measurable current is limited by either the maximum input voltage of a measuring instrument or by the voltage breakdown limits of the coil or the integrator circuit elements. Unlike other current transducers, which get bulkier and heavier as the measurable current range grows, the Rogowski coil remains the same small size coil independent of the amplitude of current being measured. This makes the Rogowski coil the most effective measurement tool for making several hundreds or even thousands of amperes of large AC current measurements.

 

 

  • Very flexible to use -- The lightweight clip-around sensor coil is flexible and easy to wrap around a current-carrying conductor. It can easily be inserted into hard-to-reach components in the circuit. Most Rogowski coils are thin enough to fit between the legs of a T0-220 or TO-247 power semiconductor package without needing an additional loop of wire to connect the current probe. This also gives an advantage in achieving high signal integrity measurement.
  • Wide bandwidth up to >30 MHz -- This enables the Rogowski coil to measure the very rapidly changing current signal – e.g., several thousand A/usec. High bandwidth characteristic allows for analyzing high-order harmonics in systems operating at high switching frequencies, or accurately monitoring switching waveforms with rapid rise- or fall-times.
  • Non-intrusive or lossless measurement -- The Rogowski coil draws extremely little current from the DUT because of low insertion impedance. The impedance injected into the DUT due to the probe is only a few pico-Henries, which enables a faster signal response and very linear signal voltage.
  • Low cost Compared to a hall effect sensor/transformer current probe, the Rogowski coil typically comes in at lower price point.

 

Limitations

  • AC only -- Rogowski cannot handle DC current. It is AC only.
  • Sensitivity - Rogowski coil has a lower sensitivity compared to a current transformer due to the absence of a high permeability magnetic core.

 

Applications

Rogowski coil current probes have a large number of applications in broad power industries and power measurement applications. The following are some examples of Rogowski coil applications:

  • Flexible current measurement of power devices such as MOSFET or IGBT device as small as TO-220 or TO-247 package or around the terminals of large power modules
  • To measure power losses in power semiconductors
  • To monitor currents in small inductors, capacitors, snubber circuits, etc.
  • To measure small AC current on a conductor with high DC current or in the presence of a high DC magnetic field.
  • To measure high frequency sinusoidal, pulsed, or transient currents from power line frequency to RF applications
  • To measure current in motor drives and, in particular, power quality measurements in VSD, UPS or SMPS circuits
  • To evaluate switching performance of power semiconductor switches (double pulse tester).
  • Power distribution line monitoring or utilities pole probe monitoring
  • Smart grid applications
  • Plasma current measurement

 

Conclusion

There are a number of different ways of measuring electric current where each method has advantages and limitations.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. However, Rogowski coils have the capability to measure large currents (very wide range from a few mA to more than a few kA) without saturation because of its non-magnetic “air” core. The air core design also has a lower insertion impedance to enable a faster signal response and a very linear signal voltage and is very cost effective compared to its hall effect sensor/current transformer counterpart. This makes the Rogowski coil the most effective measurement tool to make several hundreds or thousands of amperes of large AC current measurement.

To measure current with an oscilloscope, you have to use a current probe. The problem is, there are so many types of current probes available now, and it’s hard to know when to use what. When you think about current probing, there are three main categories that most applications fit into:

  1. High current

  2. General purpose

  3. Low current

Each of the current probes fits into a specific category. Breaking it up like this makes it a little bit easier to identify which probe will be the best for your test.

 

Table 1: Current probing categories

 

High Current

When I say high current, I mean HIGH current, as in hundreds or thousands of amperes. You will run into large currents like this when measuring inrush currents or switching transients of switching power supplies, motors, and more. How do you think it would go clamping on a standard current probe to something like this? For this special type of measurement, you need a special type of probe: a Rogowski coil. The technology used in a Rogowski coil allows for measuring extremely high currents, something that no other probing technology can do. This is due to the fact that it uses an air core, where traditionally a metalcore would be used. Normally, we would worry about the metal core getting saturated when measuring too high of a current, but with air we don’t have to worry about that. Make sure you check back in a couple weeks for more details on the N7040A/41A/42A Rogowski coil probes!

 

Image 2: N7040A/41A/42A Rogowski coil probes

 

There are a few other reasons why you would want to use the N704xA probes rather than the other one you see listed in this category in Table 1, besides the fact that the Rogowski coil can measure much higher current. One reason is the other probe listed is a clamp-on style. One of the big problems you can run into with this is that you aren’t able to actually fit the larger devices you want to measure in the clamp itself. You may have to add in some additional wires and do some extra work to get the measurement you need, often causing changes in the characteristics of the DUT. The N704xA has been getting a lot of attention from engineers because it’s extremely easy to use and connect directly to your device under test. The probe head is just a flexible loop that you can wrap around anything you want to test, as you can see in image 2.

 

General Purpose

Typically for more general current measurements, the most commonly used current probe is the clamp-on style, or the magnetic core current probe. This refers to measurements such as current consumption of high frequency digital circuit, ICs, or power supplies. You can also make measurements on motor drives, power inverters, to test lighting fixtures, and the list goes on.

 

Clamp-on style probes are convenient because they are really easy to use.

 

To use this style probe, all you have to do is clamp it around a wire and you’re ready to start measuring, there are no accessories or extra components to worry about. You’re able to quickly and easily make accurate measurements with low loading and a very flat response. Most Keysight clamp-on current probes employ a hybrid AC/DC measurement technology, integrating both the Hall effect sensory element for measuring DC/low-frequency contents and the current transformer for measuring AC into a single probe. With clamp-on probes, you can have the ability to measure AC and DC current, so you are able to account for the DC offset in your signal rather than blocking it out and not knowing what is going on there.

 

There are some advantages to the new N7026A over the other clamp-on current probes used for general purpose measurements. Keysight’s clamp-on current probe with 0.1V/A (or 10:1) conversion factor can only go down to 10 mA/div of vertical scale setting, which isn’t good enough for most lower level current measurements. The new N7026A now allows for more accurate low-level measurements, down to 1 mA/div sensitivity with much lower noise and higher accuracy. The higher sensitivity means you will find about 5x lower noise with this probe, which makes a huge difference when measuring in the mA range. This is evident in images 3 and 4 below. The N7026A also introduces a wider range, going from the mA’s up to 30 Arms or 40 Apk, along with the highest bandwidth of any clamp-on current probe available, at 150 MHz.

 

Image 3: 10mApp, 100kHz sine wave showing dramatically improved noise and sensitivity (Ch1 in yellow = N7026A 1V/A, Ch2 in green = N2893A 0.1V/A)

 

Image 4: CAN bus current shows improved noise means higher accuracy with low-level measurements (Ch1 in yellow = N7026A 1V/A, Ch2 in green = N2893A 0.1V/A)

 

Low Current

As power consumption becomes more important with battery powered devices, it becomes necessary to measure extremely low-level currents accurately.

 

To maximize battery life, engineers must minimize the power consumption.

 

Low current here refers to measuring down from the µA to the A range, on components like charging devices, memory chips, battery-powered devices, and more. When you need to measure at these low levels, your probe must be more sensitive than ever before with much lower noise and have high enough dynamic range to cover the input signal range.

 

To have sensitivity this high, Keysight offers breakthrough technology in the N2820A/21A high sensitivity current probe that allows you to connect a sense resistor directly to your current path and make high sensitivity measurements. This will allow you to measure the voltage drop across the resistor, which is proportional to the current flow through the resistor. The N2820A/21A current probes are optimized for measuring current flow within the DUT to characterize sub-circuits, allowing the user to see both large signals and details on fast and wide-dynamic current waveforms.

Many times, when engineers are working on these devices that require low-level measurements, they also must be able to analyze larger currents at the same time, such as analyzing a phone’s idle state. The solution to this is the 2-channel version of the N2820A. One channel will act as the low gain side that allows you to see the entire waveform, or the “zoomed out” view. The other channel is the high gain amplifier that provides the “zoomed in” view of the extremely low-level currents. Now, with the N2820A/21A current probes, engineers are able to see the details and the big picture on dynamic current waveforms like never before.

 

Image 5: 2-Channel N2820A shows “zoomed in” and “zoomed out” view

 

Conclusion

There are so many probes available! Now you are an expert and know exactly which current probes to use for your measurements. Using the probe that aligns with your corresponding category will enable you to make the most accurate measurements possible, therefore creating the best product possible.

 

To further help you select the correct probe for your applications, check out this probes and accessories selection guide

To learn more about the high current and general current measurements we discussed, download this application note

To learn more about the N2820A for extremely low-level current measurements, check out this application note 

Comparing different manufacturers’ oscilloscopes and their various specifications and features can be time-consuming and confusing. Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Bandwidth

The primary oscilloscope specification is bandwidth, and it is critical that you select an oscilloscope that has sufficient bandwidth to accurately capture the highest frequency content of your signals. Most oscilloscopes with bandwidth below 8 GHz have a Gaussian, or single-pole low-pass filter frequency response. The oscilloscope’s bandwidth is the frequency at which the input signal is attenuated by 3 dB. Because of this, you cannot expect to make accurate measurements near your oscilloscope’s specified bandwidth frequency.

 

Rule of Thumb:

  • Analog applications: Choose a bandwidth at least 3x higher than the highest sine wave frequencies you will measure
  • Digital applications: Choose a bandwidth at least 5x the highest clock rate in your system.

 

This will allow you to capture the fifth harmonic with minimum signal attenuation. This fifth harmonic of the signal is critical in determining the overall shape of your digital signals. This 5-to-1 rule-of-thumb does not take into account lower clock-rate signals that may have relatively fast edge speeds. These clock signals may contain significant frequency components beyond the fifth harmonic and require even higher bandwidths.

 

Keysight’s S-Series oscilloscope has bandwidth options of 1, 2, 2.5, 4, 6, and 8 GHz  

 

 

Figure 1 – Keysight’s S-Series Oscilloscope from 500 MHz to 8 GHz

 

Sample Rate

A digital oscilloscope can spend a lot of time calculating between the trigger event, the signal displayed, and the next trigger. This can result in only a few captures of your signal each second. Your oscilloscope will fail to capture intermittent errors or faults in your signal without a high enough update date.  This happens because the oscilloscope is busy calculating the last acquisition captured instead of acquiring. The higher the update rate, the higher your chances are of capturing that rare event.

 

Memory Depth

Closely related to an oscilloscope’s maximum sample rate is its maximum available acquisition memory depth. You should select an oscilloscope that has sufficient acquisition memory to capture your most complex signals with high resolution. Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. Oscilloscopes sample at their fastest rates when the time base is set on one of the faster time ranges. But when the time base is set to slower ranges to capture longer time spans across the oscilloscope’s display, the scope automatically reduces the sample rate based on the available acquisition memory. Maintaining the oscilloscope’s fastest sample rate at the slower time base ranges requires that the scope have additional acquisition memory. Determining the amount of acquisition memory you require is based on the time span of your signal and your desired maximum sample rate:

 

Acquisition Memory = Time Span x Required Sample Rate

 

Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. 

 

To better understand the relationship between bandwidth, sampling rate, and memory depth, let’s look at a real-world example. Consider trying to capture one frame data that lasts 1 ms and has serial data transmitted at 12 Mbps. So let’s assume that we have to capture a 12 MHz square wave for 1 ms.

  • Bandwidth — to measure the 12 MHz signal, we need an absolute minimum of 12 MHz, however, this will give a very distorted signal. So a scope with at least 50 MHz bandwidth should be selected.
  • Sampling rate — to reconstruct the 12 MHz signal, we need around 5 points per waveform, so a minimum sampling rate of 60 MS/s is required.
  • Memory depth — to capture data at 60 MS/s for 1 ms requires a minimum memory depth of 60,000 samples.

 

Triggering

Triggering allows you to synchronize the oscilloscope’s acquisition and display particular parts of your signal under test. Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals. Some oscilloscopes have the ability to trigger on pulses that meet a particular timing qualification. For example, trigger only when a pulse is less than 20 ns wide. This type of triggering (qualified pulse-width) can be very useful for triggering on unsuspected glitches. Pattern triggering is also very common and allows you to set up the oscilloscope to trigger on a logical/Boolean combination of highs (or 1s) and lows (or 0s) across two or more input channels. More advanced oscilloscopes even provide triggering that can synchronize on signals that have parametric violations. In other words, trigger only if the input signal violates a particular parametric condition such as reduced pulse height (runt trigger), edge speed violation (rise/fall time), or perhaps a clock to data timing violation (setup and hold time trigger).

 

Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals.

 

Display Quality

The quality of your oscilloscope’s display can make a big difference in your ability to effectively troubleshoot your designs. You should select an oscilloscope that provides multiple levels of trace intensity gradation in order to display subtle waveform details like noise distribution, jitter, and other signal anomalies. For the highest oscilloscope display quality in the industry, go to Keysight.com. An example is shown in Figure 2 below.

 

 

Figure 2 – High levels of display quality are required when viewing complex modulated signals such as video

 

Serial Bus Applications

To help you debug your designs faster, select an oscilloscope that can trigger on and decode serial buses. Serial buses such as I2C, SPI, RS232/UART, CAN, USB, etc., are pervasive in many of today’s digital and mixed-signal designs. Verifying proper bus communication along with analog signal quality measurements requires an oscilloscope. Many of today’s oscilloscopes have optional built-in serial bus protocol decode and triggering capabilities. If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster.

 

If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster. 

 

Connectivity and Documentation

Selecting an oscilloscope that meets your hardware connectivity, test automation, and electronic documentation requirements are key. Automated testing requires that the oscilloscope’s ports be programmable. So make sure any measurement that can be performed using the oscilloscope’s front panel and menu controls can also be programmed remotely via LAN or USB connectivity.

 

Keysight’s InfiniiVision X-Series and Infiniium Series oscilloscopes are all fully programmable via SCPI commands as well as National Instruments IVI drivers. Saved images (screen-shots) and data (waveforms) can also be easily imported into various word processors, spreadsheets, and applications such as MATLAB. Keysight’s N8900A InfiniiView offline analysis software lets you easily capture waveforms on your oscilloscope, save them to a file, and recall the waveforms into the application. With Keysight’s > 50 standard automated measurements with statistics and 16 independent math functions, you’ll be able to analyze a wide variety of tests.

 

Probing

Your oscilloscope measurements can only be as good as the data your probe delivers to the oscilloscope’s BNC inputs. Always keep in mind that when probing your circuit, your oscilloscope and probe become part of your device-under-test. This means it can change the behavior of your device-under-test signals due to capacitive or inductive loading. Select an appropriate probe for your measurement to minimize these loading effects. This will prevent disturbance of the input signal and deliver a true representation of your signal as it existed in your circuit before the probe was attached.

 

Select an appropriate probe for your measurement to minimize these loading effects.

 

Ease of Use

Your oscilloscope should be user-friendly and intuitive. This can be just as important as specified performance characteristics.  Oscilloscopes have evolved over the years with many additional features and capabilities but ease of use should not be compromised. Although most oscilloscope vendors will claim that their oscilloscopes are the easiest to use, usability is not a specified parameter that you can compare against in a product’s data sheet. Ease-of-use is subjective, and you must evaluate it for yourself. However, there are a few things you can look for when evaluating ease of use:

  • Built-in help menus, which reduce the need to reference manuals
  • Large color displays, which allow views of waveforms and measurement data at the same time
  • Voice control, which allows for hands-free control

Always request a demo when picking out your oscilloscope with a reputable company providing field engineers to help demo the scope or go to trade shows that provide hands-on demos.

 

Learn more about these and other oscilloscope selection topics:

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

Summary

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

 

Conclusion

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 https://www.keysight.com/find/PAM4

 

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).

 

Bandwidth

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 =

8-bit

256

3900 uV

62.5 uV

10-bit

1024

976 uV

15.6 uV

12-bit

4,096

244 uV

3.9 uV

14-bit

16,384

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

 

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