A Serene Workspace for Board Evaluation and Writing

 Elecronics engineer, entrepreneur, and author Jack Ganssle recently sent us information about his Finksburg, MD, workspace:

I’m in a very rural area and I value the quietness and the view out of the window over my desk. However, there are more farmers than engineers here so there’s not much of a high-tech community! I work out of the house and share an office with my wife, who handles all of my travel and administrative matters. My corner is both lab space and desk. Some of the equipment changes fairly rapidly as vendors send in gear for reviews and evaluation.

ganssle-workspace

Ganssle’s desk is home to ever-changing equipment. His Agilent Technologies MSO-X-3054A mixed-signal oscilloscope is a mainstay.

The centerpiece, though, is my Agilent Technologies MSO-X-3054A mixed-signal oscilloscope. It’s 500 MHz, 4 GSps, and includes four analog channels and 16 digital channels, as well as a waveform generator and protocol analyzer. I capture a lot of oscilloscope traces for articles and talks, and the USB interface sure makes that easy. That’s pretty common on oscilloscopes, now, but being an old-timer I remember struggling with a Polaroid scope camera.

The oscilloscope’s waveform generator has somewhat slow (20-ns) rise time when making pulses, so the little circuit attached to it sharpens this to 700 ps, which is much more useful for my work. The photo shows a Siglent SDS1102CML oscilloscope on the bench that I’m currently evaluating. It’s amazing how much capability gets packed into these inexpensive instruments.

The place is actually packed with oscilloscopes and logic analyzers, but most are tucked away. I don’t know how many of those little USB oscilloscope/logic analyzers vendors have sent for reviews. I’m partial to bench instruments, but do like the fact that the USB instruments are typically quite cheap. Most have so-so analog performance but the digital sampling is generally great.

Only barely visible in the picture, under the bench there’s an oscilloscope from 1946 with a 2” CRT I got on eBay just for fun. It’s a piece of garbage with a very nonlinear timebase, but a lot of fun. The beam is aimed by moving a magnet around! Including the CRT there are only four tubes. Can you imagine making anything with just four transistors today?

The big signal generator is a Hewlett-Packward 8640B, one of the finest ever made with astonishing spectral purity and a 0.5-dB amplitude flatness across 0.5 MHz to 1 GHz. A couple of digital multimeters and a pair of power supplies are visible as well. The KORAD supply has a USB connection and a serviceable, if klunky, PC application that drives it. Sometimes an experiment needs a slowly changing voltage, which the KORAD manages pretty well.

They’re mostly packed away, but I have a ton of evaluation kits and development boards. A Xilinx MicroZed is shown on the bench. It’s is a very cool board that has a pair of Cortex-A9s plus FPGA fabric in a single chip.

I use IDEs and debuggers from, well, everyone: Microchip Technology, IAR Systems, Keil, Segger, you name it. These run on a variety of processors but, along with so many others, more and more I’m using Cortex-M series parts.

My usual lab work is either evaluating boards, products and instruments, or running experiments that turn into articles. It pains me to see so much engineering is done via superstition today. For example, people pick switch contact debounce times based on hearsay or smoke signals or something. Engineers need data, so I tested about 50 pairs of switches to determine what real bounce characteristics are. The results are on my website. Ditto for watchdog timers and other important issues embedded people deal with.

Ganssle notes that his other “bench” is his woodworking shop. To learn more about Ganssle, read our 2013 interview.

Pulse-Shaping Basics

Pulse shaping (i.e., base-band filtering) can vastly improve the behavior of wired or wireless communication links in an electrical system. With that in mind, Circuit Cellar columnist Robert Lacoste explains the advantages of filtering and examines Fourier transforms; random non-return-to-zero NRZ signaling; and low-pass, Gaussian, Nyquist, and raised-cosine filters.

Lacoste’s article, which appears in Circuit Cellar’s April 2014 issue, includes an abundance of graphic simulations created with Scilab Enterprises’s open-source software. The simulations will help readers grasp the details of pulse shaping, even if they aren’t math experts. (Note: You can download the Scilab source files Lacoste developed for his article from Circuit Cellar’s FTP site.)

Excerpts from Lacoste’s article below explain the importance of filtering and provide a closer look at low-pass filters:

WHY FILTERING?
I’ll begin with an example. Imagine you have a 1-Mbps continuous digital signal you need to transmit between two points. You don’t want to specifically encode these bits; you just want to transfer them one by one as they are.

Before transmission, you will need to transform the 1 and 0s into an actual analog signal any way you like. You can use a straightforward method. Simply define a pair of voltages (e.g., 0 and 5 V) and put 0 V on the line for a 0-level bit and put 5 V on the line for a 1-level bit.


This method is pedantically called non-return-to-zero (NRZ). This is exactly what a TTL UART is doing; there is nothing new here. This analog signal (i.e., the base-band signal) can then be sent through the transmission channel and received at the other end (see top image in Figure 1).


Note: In this article I am not considering any specific transmission channel. It could range from a simple pair of copper wires to elaborate wireless links using amplitude, frequency and/or phase modulation, power line modems, or even optical links. Everything I will discuss will basically be applicable to any kind of transmission as it is linked to the base-band signal encoding prior to any modulation.

Directly transmitting a raw digital signal, such as this 1-Mbps non-return-to-zero (NRZ) stream (at top), is a waste of bandwidth. b—Using a pulse-shaping filter (bottom) reduces the required bandwidth for the same bit rate, but with a risk of increased transmission errors.

Figure 1: Directly transmitting a raw digital signal, such as this 1-Mbps non-return-to-zero (NRZ) stream (top), is a waste of bandwidth. Using a pulse-shaping filter (bottom) reduces the required bandwidth for the same bit rate, but with a risk of increased transmission errors.


Now, what is the issue when using simple 0/5-V NRZ encoding? Bandwidth efficiency. You will use more megahertz than needed for your 1-Mbps signal transmission. This may not be an issue if the channel has plenty of extra capacity (e.g., if you are using a Category 6 1-Gbps-compliant shielded twisted pair cable to transmit these 1 Mbps over a couple of meters).


Unfortunately, in real life you will often need to optimize the bandwidth. This could be for cost reasons, for environmental concerns (e.g., EMC perturbations), for regulatory issues (e.g., RF channelization), or simply to increase the effective bit rate as much as possible for a given channel.


Therefore, a good engineering practice is to use just the required bandwidth through a pulse-shaping filter. This filter is fitted between your data source and the transmitter (see bottom of Figure 1).


The filter’s goal is to reduce as much as possible the occupied bandwidth of your base-band signal without affecting the system performance in terms of bit error rate. These may seem like contradictory requirements. How can you design such a filter? That’s what I will try to explain in this article….


LOW-PASS FILTERS

A base-band filter is needed between the binary signal source and the transmission media or modulator. But what characteristics should this filter include? It must attenuate as quickly as possible the unnecessary high frequencies. But it must also enable the receiver to decode the signal without errors, or more exactly without more errors than specified. You will need a low-pass filter to limit the high frequencies. As a first example, I used a classic Butterworth second-order filter with varying cut-off frequencies to make the simulation. Figure 2 shows the results. Let me explain the graphs.

Figure 2: This random non-return-to-zero (NRZ) signal (top row) was passed through a second-order Butterworth low-pass filter. When the cut-off frequency is low (310 kHz), the filtered signal (middle row) is distorted and the eye diagram is closed. With a higher cutoff (410 kHz, bottom row), the intersymbol interference (ISI) is lower but the frequency content is visible up to 2 MHz.

Figure 2: This random non-return-to-zero (NRZ) signal (top row) was passed through a second-order Butterworth low-pass filter. When the cut-off frequency is low (310 kHz), the filtered signal (middle row) is distorted and the eye diagram is closed. With a higher cutoff (410 kHz, bottom row), the intersymbol interference (ISI) is lower but the frequency content is visible up to 2 MHz.

The leftmost column shows the signal frequency spectrum after filtering with the filter frequency response in red as a reference. The middle column shows a couple of bits of the filtered signal (i.e., in the time domain), as if you were using an oscilloscope. Last, the rightmost column shows the received signal’s so-called “eye pattern.” This may seem impressive, but the concept is very simple.

Imagine you have an oscilloscope. Trigger it on any rising or falling front of the signal, scale the display to show one bit time in the middle of the screen, and accumulate plenty of random bits on the screen. You’ve got the eye diagram. It provides a visual representation of the difficulty the receiver will have to recover the bits. The more “open” the eye, the easier it is. Moreover, if the successive bits’ trajectories don’t superpose to each other, there is a kind of memory effect. The voltage for a given bit varies depending on the previously transmitted bits. This phenomenon is called intersymbol interference (ISI) and it makes life significantly more difficult for decoding.


Take another look at the Butterworth filter simulations. The first line is the unfiltered signal as a reference (see Figure 2, top row). The second line with a 3-dB, 310-kHz cut-off frequency shows a frequency spectrum significantly reduced after 1 MHz but with a high level of ISI. The eye diagram is nearly closed (see Figure 2, middle row). The third line shows the result with a 410-kHz Butterworth low-pass filter (see Figure 2, bottom row). Its ISI is significantly lower, even if it is still visible. (The successive spot trajectories don’t pass through the same single point.) Anyway, the frequency spectrum is far cleaner than the raw signal, at least from 2 MHz.

Lacoste’s article serves as solid introduction to the broad subject of pulse-shaping. And it concludes by re-emphasizing a few important points and additional resources for readers:

Transmitting a raw digital signal on any medium is a waste of bandwidth. A filter can drastically improve the performance. However, this filter must be well designed to minimize intersymbol interference.

The ideal solution, namely the Nyquist filter, enables you to restrict the used spectrum to half the transmitted bit rate. However, this filter is just a mathematician’s dream. Raised cosine filters and Gaussian filters are two classes of real-life filters that can provide an adequate complexity vs performance ratio.

At least you will no longer be surprised if you see references to such filters in electronic parts’ datasheets. As an example, see Figure 3, which is a block diagram of Analog Devices’s ADF7021 high-performance RF transceiver.

This is a block diagram of Analog Devices’s ADF7021 high-performance transceiver. On the bottom right there is a “Gaussian/raised cosine filter” block, which is a key factor in efficient RF bandwidth usage.

Figure 3: This is a block diagram of Analog Devices’s ADF7021 high-performance transceiver. On the bottom right there is a “Gaussian/raised cosine filter” block, which is a key factor in efficient RF bandwidth usage.

The subject is not easy and can be easily misunderstood. I hope this article will encourage you to learn more about the subject. Bernard Sklar’s book Digital Communications: Fundamentals and Applications is a good reference. Playing with simulations is also a good way to understand, so don’t hesitate to read and modify the Scilab examples I provided for you on Circuit Cellar’s FTP site.  

Lacoste’s full article is in the April issue, now available for membership download or single issue purchase. And for more information about improving the efficiency of wireless communication links, check out Lacoste’s 2011 article “Line-Coding Techniques,” Circuit Cellar 255, which tells you how you can encode your bits before transmission.

A Workspace for Microwave Imaging, Small Radar Systems, and More

Gregory L. Charvat stays very busy as an author, a visiting research scientist at the Massachusetts Institute of Technology (MIT) Media Lab, and the hardware team leader at the Butterfly Network, which brings together experts in computer science, physics, and electrical engineering to create new approaches to medical diagnostic imaging and treatment.

If that wasn’t enough, he also works as a start-up business consultant and pursues personal projects out of the basement-garage workspace of his Westbrook, CT, home (see Photo 1). Recently, he sent Circuit Cellar photos and a description of his lab layout and projects.

Photo 1

Photo 1: Charvat, seated at his workbench, keeps his equipment atop sturdy World War II-era surplus lab tables.

Charvat’s home setup not only provides his ideal working conditions, but also considers  frequent moves required by his work.

Key is lots of table space using WW II surplus lab tables (they built things better back then), lots of lighting, and good power distribution.

I’m involved in start-ups, so my wife and I move a lot. So, we rent houses. When renting, you cannot install the outlets and things needed for a lab like this. For this reason, I built my own line voltage distribution panel; it’s the big thing with red lights in the middle upper left of the photos of the lab space (see Photo 2).  It has 16 outlets, each with its own breaker, pilot lamp (not LED).  The entire thing has a volt and amp meter to monitor power consumption and all power is fed through a large EMI filter.

Photo 2: This is another view of the lab, where strong lighting and two oscilloscopes are the minimum requirements.

Photo 2: This is another view of the lab, where strong lighting and two oscilloscopes are the minimum requirements.

Projects in the basement-area workplace reflect Charvat’s passion for everything from microwave imaging systems and small radar sensor technology to working with vacuum tubes and restoring antique electronics.

My primary focus is the development of microwave imaging systems, including near-field phased array, quasi-optical, and synthetic-aperture radar (SAR). Additionally, I develop small radar sensors as part of these systems or in addition to. Furthermore, I build amateur radio transceivers from scratch. I developed the only all-tube home theater system (published in the May-June 2012 issues of audioXpress magazine) and like to restore antique radio gear, watches, and clocks.

Charvat says he finds efficient, albeit aging, gear for his “fully equipped microwave, analog, and digital lab—just two generations too late.”

We’re fortunate to have access to excellent test gear that is old. I procure all of this gear at ham fests, and maintain and repair it myself. I prefer analog oscilloscopes, analog everything. These instruments work extremely well in the modern era. The key is you have to think before you measure.

Adequate storage is also important in a lab housing many pieces for Charvat’s many interests.

I have over 700 small drawers full of new inventory.  All standard analog parts, transistors, resistors, capacitors of all types, logic, IF cans, various radio parts, RF power transistors, etc., etc.

And it is critical to keep an orderly workbench, so he can move quickly from one project to the next.

No, it cannot be a mess. It must be clean and organized. It can become a mess during a project, but between projects it must be cleaned up and reset. This is the way to go fast.  When you work full time and like to dabble in your “free time” you must have it together, you must be organized, efficient, and fast.

Photos 3–7 below show many of the radar and imaging systems Charvat says he is testing in his lab, including linear rail SAR imaging systems (X and X-band), a near-field S-band phased-array radar, a UWB impulse X-band imaging system, and his “quasi-optical imaging system (with the big parabolic dish).”

Photo 3: This shows impulse rail synthetic aperture radar (SAR) in action, one of many SAR imaging systems developed in Charvat’s basement-garage lab.

Photo 3: This photo shows the impulse rail synthetic aperture radar (SAR) in action, one of many SAR imaging systems developed in Charvat’s basement-garage lab.

Photo 4: Charvat built this S-band, range-gated frequency-modulated continuous-wave (FMCW) rail SAR imaging system

Photo 4: Charvat built this S-band, range-gated frequency-modulated continuous-wave (FMCW) rail SAR imaging system.

Photo 5: Charvat designed an S-band near-field phased-array imaging system that enables through-wall imaging.

Photo 5: Charvat designed an S-band near-field phased-array imaging system that enables through-wall imaging.

Photo 5: Charvat's X-band, range-gated UWB FMCW rail SAR system is shown imaging his bike.

Photo 6: Charvat’s X-band, range-gated UWB FMCW rail SAR system is shown imaging his bike.

Photo 7: Charvat’s quasi-optical imaging system includes a parabolic dish.

Photo 7: Charvat’s quasi-optical imaging system includes a parabolic dish.

To learn more about Charvat and his projects, read this interview published in audioXpress (October 2013). Also, Circuit Cellar recently featured Charvat’s essay examining the promising future of small radar technology. You can also visit Charvat’s project website or follow him on Twitter @MrVacuumTube.

Evaluating Oscilloscopes (Part 4)

In this final installment of my four-part mini-series about selecting an oscilloscope, I’ll look at triggering, waveform generators, and clock synchronization, and I’ll wrap up with a series summary.

My previous posts have included Part 1, which discusses probes and physical characteristics of stand-alone vs. PC-based oscilloscopes; Part 2, which examines core specifications such as bandwidth, sample rate, and ADC resolution; and Part 3, which focuses on software. My posts are more a “collection of notes” based on my own research rather than a completely thorough guide. But I hope they are useful and cover some points you might not have otherwise considered before choosing an oscilloscope.

This is a screenshot from Colin O'Flynn's YouTube video "Using PicoScope AWG for Testing Serial Data Limits."

This is a screenshot from Colin O’Flynn’s YouTube video “Using PicoScope AWG for Testing Serial Data Limits.”

Topic 1: Triggering Methods
Triggering your oscilloscope properly can make a huge difference in being able to capture useful waveforms. The most basic triggering method is just a “rising” or “falling” edge, which almost everyone is (or should be) familiar with.

Whether you need a more advanced trigger method will depend greatly on your usage scenario and a bit on other details of your oscilloscope. If you have a very long buffer length or ability to rapid-fire record a number of waveforms, you might be able to live with a simple trigger since you can easily throw away data that isn’t what you are looking for. If your oscilloscope has a more limited buffer length, you’ll need to trigger on the exact moment of interest.

Before I detail some of the other methods, I want to mention that you can sometimes use external instruments for triggering. For example, you might have a logic analyzer with an extremely advanced triggering mechanism.  If that logic analyzer has a “trigger out,” you can trigger the oscilloscope from your logic analyzer.

On to the trigger methods! There are a number of them related to finding “odd” pulses: for example, finding glitches shorter or wider than some length or finding a pulse that is lower than the regular height (called a “runt pulse”). By knowing your scope triggers and having a bit of creativity, you can perform some more advanced troubleshooting. For example, when troubleshooting an embedded microcontroller, you can have it toggle an I/O pin when a task runs. Using a trigger to detect a “pulse dropout,” you can trigger your oscilloscope when the system crashes—thus trying to see if the problem is a power supply glitch, for example.

If you are dealing with digital systems, be on the lookout for triggers that can function on serial protocols. For example, the Rigol Technologies stand-alone units have this ability, although you’ll also need an add-on to decode the protocols! In fact, most of the serious stand-alone oscilloscopes seem to have this ability (e.g., those from Agilent, Tektronix, and Teledyne LeCroy); you may just need to pay extra to enable it.

Topic 2: External Trigger Input
Most oscilloscopes also have an “external trigger input.”  This external input doesn’t display on the screen but can be used for triggering. Specifically, this means your trigger channel doesn’t count against your “ADC channels.” So if you need the full sample rate on one channel but want to trigger on another, you can use the “ext in” as the trigger.
Oscilloscopes that include this feature on the front panel make it slightly easier to use; otherwise, you’re reaching around behind the instrument to find the trigger input.

Topic 3: Arbitrary Waveform Generator
This isn’t strictly an oscilloscope-related function, but since enough oscilloscopes include some sort of function generator it’s worth mentioning. This may be a standard “signal generator,” which can generate waveforms such as sine, square, triangle, etc. A more advanced feature, called an arbitrary waveform generator (AWG), enables you to generate any waveform you want.

I previously had a (now very old) TiePie engineering HS801 that included an AWG function. The control software made it easy to generate sine, square, triangle, and a few other waveforms. But the only method of generating an arbitrary waveform was to load a file you created in another application, which meant I almost never used the “arbitrary” portion of the AWG. The lesson here is that if you are going to invest in an AWG, make sure the software is reasonable to use.

The AWG may have a few different specifications; look for the maximum analog bandwidth along with the sample rate. Be careful of outlandish claims: a 200 MS/s digital to analog converter (DAC) could hypothetically have a 100-MHz analog bandwidth, but the signal would be almost useless. You could only generate some sort of sine wave at that frequency, which would probably be full of harmonics. Even if you generated a lower-frequency sine wave (e.g., 10 MHz), it would likely contain a fair amount of harmonics since the DAC’s output filter has a roll-off at such a high frequency.

Better systems will have a low-pass analog filter to reduce harmonics, with the DAC’s sample rate being several times higher than the output filter roll-off. The Pico Technology PicoScope 6403D oscilloscope I’m using can generate a 20-MHz signal but has a 200 MS/s sample rate on the DAC. Similarly, the TiePie engineering HS5-530 has a 30-MHz signal bandwidth, and similarly uses a 240 MS/s sample rate. A sample rate of around five to 10 times the analog bandwidth seems about standard.

Having the AWG integrated into the oscilloscope opens up a few useful features. When implementing a serial protocol decoder, you may want to know what happens if the baud rate is slightly off from the expected rate. You can quickly perform this test by recording a serial data packet on the oscilloscope, copying it to the AWG, and adjusting the AWG sample rate to slightly raise or lower the baud rate. I illustrate this in the following video.


Topic 4: Clock Synchronization

One final issue of interest: In certain applications, you may need to synchronize the sample rate to an external device. Oscilloscopes will often have two features for doing this. One will output a clock from the oscilloscope, the other will allow you to feed an external clock into the oscilloscope.

The obvious application is synchronizing a capture between multiple oscilloscopes. You can, however, use this for any application where you wish to use a synchronous capture methodology. For example, if you wish to use the oscilloscope as part of a software-defined radio (SDR), you may want to ensure the sampling happens synchronous to a recovered clock.

The input frequency of this clock is typically 10 MHz, although some devices enable you to select between several allowed frequencies. If the source of this clock is anything besides another instrument, you may have to do some clock conditioning to convert it into one of the valid clock source ranges.

Summary and Closing Comments
That’s it! Over the past four weeks I’ve tried to raise a number of issues to consider when selecting an oscilloscope. As previously mentioned, the examples were often PicoScope-heavy simply because it is the oscilloscope I own. But all the topics have been relevant to any other oscilloscope you may have.

You can check out my YouTube playlist dealing with oscilloscope selection and review.  Some topics might suggest further questions to ask.

I’ve probably overlooked a few issues, but I can’t cover every possible oscilloscope and option. When selecting a device, my final piece of advice is to download the user manual and study it carefully, especially for features you find most important. Although the datasheet may gloss over some details, the user manual will typically address the limitations you’ll run into, such as FFT length or the memory depths you can configure.

Author’s note: Every reasonable effort has been made to ensure example specifications are accurate. There may, however, be errors or omissions in this article. Please confirm all referenced specifications with the device vendor.

Evaluating Oscilloscopes (Part 3)

In Part 3 of my series on selecting an oscilloscope, I look at the software running the oscilloscope and details such as remote control, fast Fourier transform (FFT) features, digital decoding, and buffer types.

Two weeks ago, I covered the differences between PC-based and stand-alone oscilloscopes and discussed the physical probe characteristics. Last week I discussed the “core” specifications: analog bandwidth, sample rate, and analog-to-digital converter (ADC) resolution. Next week, I will look into a few remaining features such as external trigger and clock synchronization, and I will summarize all the material I’ve covered.

Topic 1: Memory Depth
The digital oscilloscope works by sampling an ADC and then stores these samples somewhere. Thus an important consideration will be how many samples it can actually store. This especially becomes apparent at higher sample rates—at 5 gigasamples per second (GS/s), for example, even 1 million samples (i.e., 1 megasample or 1 MS) means 200 µs of data. If you are looking at very low-cost oscilloscopes, be aware that many of them have very small buffers. Searching on eBay, you can find an oscilloscope such as the Hantek DSO5202P, which has a 1 GS/s sample rate and costs only $400. The record length is only 24 kilosamples (KS) however, which would be 24 µs of data. You can find even smaller buffers:  the Tektronix TDS2000C series has only a 2,500-sample (2.5 KS) buffer length. If you only want to look around the trigger signal, you can live with a small buffer. Unfortunately, when it comes to troubleshooting you rarely have a perfect trigger, and you may need to do a fair amount of “exploration.”  A small buffer means the somewhat frustrating experience of trying to capture the signal of interest within your tiny window of opportunity.

Even if the buffer space is advertised as being huge, you may not be able to easily access the entire space. The Pico Technology PicoScope PS6403D advertises a 1-GS buffer space, one of the largest available. With the PC-based software you can configure a number of parameters; however, it always seems to limit the sample buffer to about 500 MS.  I do admit it’s fairly impressive that this still works at the 5 GS/s sample rate, since that suggests a memory bandwidth of 40 Gb/s! Using the segmented buffer (discussed later in this article) enables use of the full sample memory, but it cannot record a full continuous 1 GS trace, which you might expect based on the sales pitch.

Topic 2: FFT Length
Oscilloscope advertisements often allude to their ability to perform in a “spectrum analyzer” mode. In reality, what the oscilloscope is doing is performing an FFT of the measured signal. One critical difference is that a spectrum analyzer typically has a “center frequency” and you are able to measure a certain bandwidth amount to either side of that center frequency. By sweeping the center frequency, you can get a graph of the power present in the frequency system over a very wide range.

Using the oscilloscope’s FFT mode, there is no such thing as the center frequency. Instead you are always measuring from 0 Hz up to some limit, which is usually user-adjustable. The limit is, at most, half the oscilloscope’s sample rate but may be further limited by the oscilloscope’s analog bandwidth. Now here is the trick—the oscilloscope will specify a certain “FFT length,” which is how many points are used in calculating the FFT. This will also define the number of “bins” (i.e., horizontal frequency resolution) in the output graph. Certain benchtop oscilloscopes may have very limited FFT lengths, such as those containing only 2,048 points.  This may seem fine for viewing the entire spectrum from 0–100 MHz. But what if you want to zoom in on the 95–98 MHz range? Since the oscilloscope is actually calculating the FFT from 0 Hz, it will have only ~60 points it can display in that range. It suddenly becomes apparent why you want very long FFT lengths—it allows you to zoom in and still obtain accurate results. You can set the oscilloscope sample rate down to zoom in on frequencies around 0 Hz. So, for example, if you want to accurately do some measurements at 1–10 kHz, it’s not a big issue since you can set a low enough sample rate so that the 2,048 points are distributed between 0–20 kHz or similar. And when you zoom in you’ve got lots of detail.

In addition to the improved horizontal detail, longer FFT lengths push down the noise floor.  If you do wish to use the oscilloscope for frequency analysis, having a long FFT length can be a huge asset. This is shown in Figure 1, which compares an FFT taken using a magnetic field probe of a microcontroller board. Here I’ve zoomed in on a portion of the spectrum, with the left FFT having 2,048 points, the right FFT having 131,072 points.

Figure 1: When zooming in on a portion of the fast Fourier transform (FFT), having a larger number of points for the original calculation becomes a huge asset. Also, notice the lower noise floor for the figure on the right, calculated with 131,072 points, compared to the 2,048 used for the figure on the left.

Figure 1: When zooming in on a portion of the fast Fourier transform (FFT), having a larger number of points for the original calculation becomes a huge asset. Also, notice the lower noise floor for the figure on the right, calculated with 131,072 points, compared to the 2,048 used for the figure on the left.

A note on selecting a unit: The very low-cost oscilloscopes with small data buffers will obviously use a very small FFT length. But specifications for some of the larger memory depth oscilloscopes, such as the Rigol Technologies DS2000, DS4000, and DS6000 models, show they use smaller FFT lengths.  These models use only 2,048 points, according to a document posted on Rigol’s website, despite their large memory (131 MS).  PC-based oscilloscopes seem to be the best, as they can perform the FFT on a powerful desktop PC, rather than requiring it be done in an embedded digital signal processor (DSP) or field-programmable gate array (FPGA). For example, the PicoScope 6403D allows the FFT length to be up to 1,048,576 points.

Topic 3: Segmented Buffer
A feature I consider almost a “must-have” is a segmented buffer. This means you can configure the oscilloscope to trigger on a certain event, and it will record a number of waveforms of a certain length. For glitches that occur only occasionally (which is, 90% of the time, why you are troubleshooting in the first place), this can speed up your ability to find details of what the system is doing during a glitch.

Figure 2 shows an example of the segmented buffer viewer on the PicoScope software, where the number of buffers can be configured up to 10,000. Similar features exist in the Rigol DS4000 and DS6000, which call each segment a “frame” and can record up to 200,000 frames! Once you have a number of segments/frames, you can either manually flip through looking for the glitch, or use features such as mask limit testing to highlight segments/frames that differ from the “usual.”

Figure 2

Figure 2: Segmented buffers allow you to capture a number of traces and then flip through them looking for some specific feature. Using mask-based testing will also speed this up, since you can quickly find “odd man out”-type waveforms.

Certain oscilloscopes might make the segmented buffer an add-on. For example, only certain Agilent Technologies 3000 X-Series models contain segmented buffers by default; others in that same family require you to purchase this feature for an extra $800! Of course, always review any promotional offers—Agilent has recently advertised that it will enable all features on that oscilloscope model for the price of a single option.

Topic 4: Remote Control/Streaming
One more advanced feature is controlling the oscilloscope from your computer. If you wish to use the oscilloscope in applications beyond electronics troubleshooting, you should seriously consider the features different oscilloscopes provide.

PC-based oscilloscopes tend to have a considerable advantage here, as they are typically designed to interface to the computer. It seems most PC-based oscilloscopes from popular suppliers come with nice application programming interfaces (APIs) for most languages: I’ve found examples in C, C#, C++, MATLAB, Python, LabVIEW, and Delphi for most PC-based oscilloscopes. Some of the “no-name” PC-based oscilloscopes you find on eBay do not have an API, so always check closely for your specific device.

Most of the stand-alone oscilloscopes also have a method of sending commands, typically using a standard such as the Virtual Instrument Software Architecture (VISA). However, I’ve found these stand-alone oscilloscopes seem to have a considerably slower interface compared to a PC-based oscilloscope. Presumably for the PC-based oscilloscope, this interface is critical to overall performance, whereas for the stand-alone it’s simply an “add-on” feature. This isn’t a sure thing, of course—for example, see the PC interface for the Teledyne LeCroy oscilloscope, as described in a company blog post. It looks to give you access to features similar to those of PC-based oscilloscopes (multiple windows, etc.).

Beyond just controlling the oscilloscope, another interesting feature is streaming mode. In streaming mode data is not downloaded to an internal buffer on the oscilloscope. Instead it streams directly over the PC interface (typically USB or Ethernet). This feature is considerably more complex to work with than simple PC-based control, as achieving fast streams via USB is not trivial. However, using streaming mode opens up many interesting features. For example, you could use your oscilloscope as part of a software defined radio (SDR). If you wish to use such a feature, be sure to carefully read the specification sheets for the streaming mode limitations.

Topic 5: Decoding Serial Protocols
Decoding of serial protocols is another useful feature. If you have a digital logic analyzer, it will almost certainly include the ability to decode serial protocols. But it can be helpful to have this feature in the oscilloscope as well. If you are chasing down an occasional parity error, you can use the oscilloscope’s analog display to see if the issue is simply a weak or noisy signal.

While most oscilloscopes seem to support this feature, many require you to pay for it. Typically PC-based oscilloscopes will include it for free, but stand-alone oscilloscopes require you to purchase it. For example, this feature costs $500 for the Rigol Technologies DS4000 series, $800 for the Agilent Technologies 3000X, and $1,100 for the Tektronix DPO/MSO3000 series. Depending on the vendor, it may include multiple protocols or only one. But if you wish to enable all available protocols, it could cost more than your oscilloscope! It would typically be cheaper to purchase a PC-based logic analyzer than it would be to buy the software module for your oscilloscope.

This is one of the major reasons I prefer PC-based oscilloscopes: There tends to be no additional cost for extra features! Without the decoding you can look at the signal and see if it “looks” noisy, but having the decoding built-in means you can easily point to the specific moment when the error occurs. I’ve got some examples of such serial decoding in my video below.

Topic 6: Software Features
I’ve already mentioned it a few times in passing, but you should always check to see what software features are actually included. You may be surprised to find out some features require payment—even some models adding the FFT or other “advanced math” features require payment of a substantial fee.

There is hope on the horizon for getting access to all features in stand-alone oscilloscopes at a reasonable cost. As I mentioned earlier, Agilent Technologies recently announced it would be providing access to all software features for the cost of one module in the X-2000, X-3000, and X-4000 series. Once this goes into effect, it means that it’s really just $500–$1,500 for decoding of all serial protocols and all math features, depending on your oscilloscope. They sell this as saving you up to $16,500. (Which to me just shows how insanely expensive all these software add-ons really are!) With luck, other vendors will follow suit, and perhaps even finally include these software options in the selling price.

If you’re looking at PC-based oscilloscopes, you’ll often be allowed to download the software and play with it, even if you don’t have an instrument. This can give you an idea of how “polished” the user interface is. Considering how long you’ll spend inside this user interface, it’s good to know about it!

Closing Comments
This week I covered a number of features revolving around the software running the oscilloscope. Next week I’ll be looking into a few remaining features such as external trigger and clock synchronization, which will round out this guide.

Author’s note: Every reasonable effort has been made to ensure example specifications are accurate. There may, however, be errors or omissions in this article. Please confirm all referenced specifications with the device vendor.