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.

Ohio-Based “Design Dungeon”

“Steve Ciarcia had a ‘Circuit Cellar.’ I have a ‘Design Dungeon,’” Steve Lubbers says about his Dayton, OH-based workspace.

“An understanding wife and a spare room in the house allocated a nice place for a workshop. Too bad the engineer doesn’t keep it nice and tidy! I am amazed by the nice clean workspaces that have previously been published! So for those of you who need a visit from FEMA, don’t feel bad. Take a look at my mess.”

Steve Lubbers describes his workbench as a “work in progress.”

Steve Lubbers describes his workbench as a “work in progress.”

The workspace is a creative mess that has produced dozens of projects for Circuit Cellar contests. From the desk to the floor to the closet, the space is stocked with equipment and projects in various stages.

Lubbers writes:

The doorway is marked “The Dungeon.” The first iteration of The Dungeon was in my parents’ basement. When I bought a house, the workshop and the sign moved to my new home.

The door is a requirement when company comes to visit. Once you step inside, you will see why. The organizational plan seems to be a pile for everything, and everything in a pile. Each new project seems to reduce the amount of available floor space.

Lubbers_Floor

Lubbers’s organization plan is simple: “A pile for everything, and everything in a pile.”

“High-tech computing” is accomplished on a PDP-11/23. This boat anchor still runs to heat the room, but my iPod has more computing abilities! My nieces and nephews don’t really believe in 8” disks, but I have proof.

The desk (messy of course) holds a laptop computer and a ham radio transceiver. Several of my Circuit Cellar projects have been related to amateur radio. A short list of my ham projects includes a CW keyer, an antenna controller, and a PSK-31 (digital communications) interface.

Lubbers_Desk

Is there a desk under there?

My workbench has a bit of clear space for my latest project and fragments of previous projects are in attendance. The skull in the back right is wearing the prototype for my Honorable Mention in the Texas Instruments Design Stellaris 2010 contest. It’s a hands-free USB mouse. The red tube was the fourth-place winner in the microMedic 2013 National Contest.

Front and center is the prototype for my March 2014 Circuit Cellar article on robotics. Test equipment is a mix of old and new. Most of the newer equipment has been funded by past Circuit Cellar contests and articles.

Lubbers_Hero

“My wife allows my Hero Jr. robot to visit the living room. He is housebroken after all,” Lubbers says.

The closet is a “graveyard” for all of the contest kits I have received, models I would like to build, and other contraptions the wife doesn’t allow to invade the rest of the house. (She is pretty considerate because you will find my Hero Jr. robot in the living room.)

At one time, The Dungeon served as my home office. For about five years I had the ideal “down the hall” commute. A stocked lab helped justify my ability to work from home.

When management pulled the plug on working remotely, the lab got put to work developing about a dozen projects for Circuit Cellar contests. There has been a dry spell since my last contest entry, so these days I am helping develop the software for the ham radio Satellite FOX-1. My little “CubeSat” will operate as a ham radio transponder and a platform for university experiments when it launches in late 2014. Since I will probably never go to space myself, the next best thing is launching my code into orbit. It’s a good thing that FOX-1 is smaller than a basketball. If it was bigger, it might not fit on my workbench!

Lubbers’s article about building a swarm of robots will appear in Circuit Cellar’s March issue. To learn more about Lubbers, read our 2013 interview.

Using Arduino for Prototypes (EE Tip #121)

Arduino is an open-source development kit with a cult following. Open source means the software and hardware design files are available for free download. This begs the question of how the Arduino team can turn a profit, and the answer is the trademark and reputation of the Arduino name and symbol.

Arduino Uno PosterWhile there are now many Arduino clones, the original Arduino boards still outperform most. Arduino is very useful for prototyping. A recent example in my own work is adding a gyroscope sensor to a project. First, I purchased a gyroscope board from Pololu for a small amount. I plugged it into an Arduino breadboard shield purchased from eBay for roughly $5, and wired up the four pins: VCC (3.3 V), GND, SCL, and SDA. Pololu’s website has a link to some demo firmware and I downloaded this from GitHub. The library folders were extracted and renamed according to the instructions and then the example was run. The Arduino serial monitor then showed the gyroscope data in real-time, and the entire process took no more than 30 minutes.

Editor’s note: This EE Tip was written by Fergus Dixon of Sydney, Australia. Dixon runs Electronic System Design, a website set up to promote easy to use and inexpensive development kits. The Arduino Uno pictured above is a small portion of a full Arduino blueprint poster available for free download here.

Serial Carrier Card with Flexible I/O and Serial Technology

New 3U CompactPCI Serial Carrier Card from MEN Micro IntegratesThe G204 is a 3U CompactPCI Serial carrier card with an M-Module slot that combines fast CompactPCI Serial technology with flexible I/O options. The card serves as the basis for powerful 19″-based system solutions for transportation and industrial applications (e.g., data acquisition, process control, automation and vehicle control, robotics or instrumentation).

M-Modules are modular I/O extensions for industrial computers (e.g., embedded systems and high-end workstations). The M-Module slot enables users to interchange more than 30 I/O functions within a system. The M-Module, which needs only one CompactPCI Serial slot, is screwed tightly onto the G204 and does not require a separately mounted transition panel.

The G204 modular mezzanine card operates in a –40°C to 85°C extended temperature range for harsh environments and costs $483.

MEN Micro Inc.
www.menmicro.com

32-Bit Arm Cortex-M3 C Development Kit

ImageCraftThe CorStarter-STM32 is a complete 32-bit ARM Cortex-M3 C development kit. It includes hardware and software to develop and debug C programs in a simple to use package.

The CorStarter-STM32 base board is powered by a 72-MHz STM32 device with 256-KB flash and 64-KB SRAM. With 8-bit Arduino Shield-compatible headers, hundreds of Arduino Shields may be used to expand the system’s capabilities. Remaining I/O pins are brought out to the header, enabling access to full-power 32-bit embedded computing. The CorStarter-STM32 base board includes open-source hardware design files.

Fast code download and hardware debugging support are provided by either the industrial standard Segger JLINK-Edu or ST-LINK/V2. These JTAG/SWD pods enable full access to Cortex-M devices and seamless debugging without source code modification.

To complement the hardware, the CorStarter-STM32 kit also includes an ImageCraft non-commercial C compiler (ICCV8 for Cortex) license. The C compiler includes a professional IDE with an integrated flash downloader and a source-level debugger. The compiler can also be used for other Cortex-M development projects. The kit also includes example projects and libraries for various Arduino Shields.

The CorStarter-STM32 kit costs $99. The CorStarter-STM32 base board alone costs $55. For educators teaching embedded system courses, the kit costs $1.

ImageCraft
http://imagecraft.com