标签: waveform

My dad grew up during the depression and was in the Navy in the tail end of WWII. He had little use for Japanese products, and to this day sniffs at anything from China. If it’s not made in America he’s not interested in the product, and anything from China is, in his opinion, junk.   Thankfully he doesn’t read these columns, because Siglent sent me their very new SDG2042X arbitrary waveform generator (AWG), which is made in China, and, well, American competitors may find themselves squeezed by the product. It’s in a hefty metal box with a plastic handle and shock protectors and feels as solid as anything from the big guys. If “Siglent” were erased from the label I’d assume it was from Keysight or Tektronix. The difference is that the Siglent costs $500 and the others are (at least) ten times the price. I have one of Siglent’s lowest-end scopes and, while I like it, it’s noticeably more “plasticy” than my Agilent (though much less costly). The AWG hasn’t a bit of cheap or plastic feel to it.   Sexy looking thing, isn’t it? The unit has two independent and duplicate channels. So you’re getting two AWGs in one.   A hugely-important specification for any AWG is the sampling rate. Generally there’s no one number for a particular unit, which is the case with the SDG2042X. For the basic built-in waveforms memory is sampled at 300 MS/s. However, and possibly uniquely to this unit, another clock interpolates the data four times for each sample period. The result is an effective rate of 1.2 GS/s. Now, that’s on the built-in signals like sine, square, etc. For arbitrary waveforms the unit samples at 75 MS/s.   Vertical resolution is 16 bits, on a par with the best units available, with a 20 Vpp output. The unit they sent me is 40 MHz (sine waves; other waveforms are slower). An 8 million point buffer means it will generate just about any sort of signal. It includes all of the usual modulations like AM, DSB-AM, FM, PM, FSK, ASK and PWM. Each of those are controllable; for instance, the shape of the modulating signal is selectable. Normally one might sine-wave modulate an AM signal, but the unit can use any of a number of shapes, or even an arbitrary shape. I used the EasyWave software (more on that later) and created totally random waveforms which were stored in the AWG’s memory. Those can be recalled and used, not only to generate a signal, but as the shape of the modulating signal.   This is a sine wave modulated by an arbitrary waveform created on my PC. It will generate swept signals; one enters the start and stop frequency (or, center and span for RF folks who think in terms of spectrum analyzers), as well as the sweep rate. You can get some pretty cool-looking signals when sweeping through many MHz in a ms. Burst operation is supported as well.   Pulse rise and fall times are completely variable, with rise times down to 8.4 ns. Frequency precision is rated at ±1 ppm. At 40 MHz I measured a 0.5 ppm error. A frequency counter (the signal goes to a rear-panel BNC connector) is available that works even when both AWG outputs are operating. With a resolution of 1 Hz it was spot-on to the signal I provided.   The two channels can be linked in a variety of ways. One can have a frequency, amplitude or phase that is some multiple (integer or otherwise) of the other, or that can have some offset from the other. So channel 2’s frequency can be twice that of channel 1’s, or it could be 1.22345 Hz higher. In the following picture the scope shows the output of the two channels when one’s frequency is twice the other. If I had taken a single sweep instead of running continuously, the Lissajous curve on the top trace would be a simple sine wave with half the frequency of the blue trace. But this looks cooler:     The unit has a touch screen which works well, but I found it more convenient to use the buttons. The screen is bright and the letters very crisp. It does show a graphical view of the signal being produced. This is quite accurate even for arbitrary waveforms created using the included PC software. The one case I found where the display doesn’t mirror the output is for pulses when you’re adjusting the rise time. There’s so much control of that parameter it’s easy to turn a pulse into a ramp, but the screen always shows a pulse.   An AWG is conceptually pretty simple: just scan through memory tossing data to a DAC. The devil is in the details, including the analog. One quality metric is a sine wave’s harmonics. In the unit supplied to me these were an amazing 70 dB down from the 40 MHz fundamental. That’s 15 dB better than the spec and about equal to Keysight’s $5000 33612A.   The included “EasyWave” PC-hosted software can control the AWG. Documentation is scarce and I couldn’t figure out some of the functionality. There is a help file, but it was complete gibberish, most likely being a binary file of some sort. Fortunately, most of the features are pretty easy to figure out. One nit: to send a waveform to the unit one presses “Send Wave”, but then one must, every time, select USB or Ethernet, and then a device selection menu appears. What should take one click requires three.   The software has a lot of capability. A number of waveforms are built-in, and of course it will import a .csv file. It’s trivial to hand-draw a signal and download that to the AWG. Intriguingly, one can enter equations to create a signal; supported functions and operators are sin(), cos(), abs(), sqrt(), e, log(), pi, +, -, *, and /. The unit faithfully produces the desired signal, and that is shown on its display. The following is a screenshot of the EasyWave software displaying the signal 2*sin(x)+cos(x*pi):     What’s the worst thing I found about the unit? The big control wheel that selects parameters like frequency works well, with one glaring exception. Suppose the frequency is 25.100 MHz. If the “5” is selected, rotating the knob to the left decreases it exactly like you’d expect. Until it hits “0,” at which point the cursor jumps to the digit to the right of the decimal point. So while cranking numbers down you suddenly find yourself selecting the fractional part when most of the time you want to continue setting the integer part.   The SDG2042X does everything one would expect from a high-end AWG at a price that can’t be beaten. With 16 bits of vertical resolution, an 8 mega-point buffer, fast sampling, and a very complete set of features it’s a professional unit that will more than satisfy all but the most demanding engineer. I reviewed the 40 MHz model, but 80 and 120 MHz variants are available for $619 and $899 respectively. There’s more info here .   But don’t tell my Dad I like this Chinese product.

What is a pHz? According to Wikipedia, pHz stands for "picohertz," or 10-12Hz. fHz is "femptohertz"—three orders of magnitude smaller. A 1-fHz signal would take about 20 million years to complete a single cycle. Four cycles ago, dinosaurs roamed the Earth. When confronted with a new oscilloscope, the first thing I do is connect a probe to the calibration node and look at the resulting square wave. I tried this with the new MSO-X 3054A from Agilent Technologies and then pressed "Measure." The display showed the expected waveform, with current, mean, min, and max frequencies, all at 1.0012kHz. But it also showed the standard deviation as a few tens of fHz, and later pHz, prompting my dive into Wikipedia. The same sense of surprise dogged me for most of my evaluation of this product, which was a meta-surprise in itself. I don't expect much novelty in looking at scopes anymore. Until the advent of digital versions, the technology evolved very slowly. A World War II version wouldn't be much different, other than bandwidth, than one from the 1980s. Fifteen years ago HP startled me with their 54645D MSO—Mixed Signal Oscilloscope—which combined both a logic analyzer and a scope in one package. The brilliance of this device was its cross-triggering: it could start a sweep either on a combination of logic conditions or on a normal scope-like analog threshold crossing. Suddenly we embedded types could see how our digital circuits interacted with the analog components. A new day had dawned for debugging embedded systems. Later HP spun off their test equipment division into Agilent, a move that always puzzled me, since that group represented both the origins of the company as well as its finest engineering. But perhaps that was for the best since Agilent was spared the slings and arrows of Carly. First, let's get the boring part out the way. Agilent's new 2000X and 3000X line of digital scopes comprises, by my count, 26 models that cover the needs of most of us embedded systems engineers. At the bottom is a 70MHz 2-GSa/s (gigaasamples per second per channel) twin-channel model with 100k-deep storage costs just over $1,200; the top of the line MSOX3054A that startled me has seven times the bandwidth for about nine times the cost, with 4 GSa/s, four analog channels, 16 digital, and a 2 million sample deep buffer (4 million available). Options for all models abound, which can drive the cost up a bit while greatly increasing the functionality. The screen updates up to a million waveforms/second on the screen. A demo dramatically shows how a slower rate masks glitches. And in using the scopes, I consistently found them to be extremely fast, except when computing an FFT (fast Fourier transform). USB, GPIB, and Ethernet connectivity are available on the scopes, of course, but the Ethernet is LXI-compliant. LXI (LAN eXtensions for Instrumentation) is a standard aimed at the test and measurement world that specifies what kinds of capability, accessible via a standard LAN, is available in the instrument. For instance, it defines standards for triggering. If you have two instruments at opposite sides of, say, an airplane, the LXI standard ensures you can trigger both devices from the same signal. In the olden days, scopes had three basic bits of functionality: a vertical amplifier, time base, and triggering, all controlled by a sea of knobs and buttons. It took a lot of electronics to provide those features, so packages were quite large, yielding plenty of panel space for the controls. Tiny displays freed up even more space. No longer: These models occupy less than half a cubic foot and nearly half the panel is devoted to an enormous (8.5-inch (convert to mm except for displays) WVGA) screen. Like many other scopes today, soft keys are used to control many of the vast array of features, although all basic scope functionality has dedicated controls. Surprises! Agilent sent me two scopes representative of the bottom of the line (the DSO-X 2002A) and the top (MSO-X 3054A), which have identical form factors and look and feel. Both scopes came with all available options, which initially caused me some confusion and then delighted surprise. At first I couldn't find that 1kHz calibration square wave. These instruments have two scope compenzation lugs instead of the usual one, and somehow, due to excessive button pushing (who can resist all of those seductive knobs?), I'd enabled training mode. That generates one of a large variety of signals on these lugs ranging from a simple sine wave to RF bursts, runt pulses, and even streams of serial data like CAN and I2C. Training mode is targeted to the educational market and is accompanied by a quite large book that takes students from the basics of using an oscilloscope to quite advanced techniques. But at $500, I'd recommend buying the option even if you're an old pro. We've always used that 1kHz square wave to, among other things, provide a sanity check that the unit was set up properly. These scopes offer so much analysis capability that a similar sanity check is just as essential. Long ago, I worked with an engineer who had applied for a job at Cape Canaveral. He told me that he was given a tour of Cape Canaveral after his interview. The tour ended back at a panel of beautiful controls just crying out for some tactile interaction. A big bundle of wires trailing on the floor was cut, proving the box wasn't connected to anything, and my friend finally succumbed and twisted a knob. Klaxons suddenly blared all over the blockhouse! The panel was a test; management didn't want to hire someone who pressed buttons in a launch complex. He didn't get the job. I was pretty sure the MSO-X 3054A wasn't connected to a firing circuit, so pressed, twisted, and pushed to put the unit through its paces. Next surprise: All of the rotating knobs can be pushed in as well as twisted. Press a vertical or horizontal positioning control and the display returns to the zero position. A similar action on the trigger knob sets the trigger threshold to the signal's 50% level. For generations, vertical and horizontal controls operated in steps of 1, 2, and 5. The first position might be 1µsec/division, the next 2µsec/div, then 5, repeating at 10, 20, and 50. That's true on these scopes as well. But press the vertical or horizontal knobs and the devices switch to a "fine" mode where the detents select variable steps. For instance, at 100µsec/div this mode now clicks in 2µsec increments, letting one finely control how much of the waveform is shown on the screen. Another quick press and it returns to 1, 2, 5 steps. I got these scopes before their official introduction and presumably the instruction manual had not been complete, as none was included in the package. These are complex products and I was initially disappointed they had no manuals, but the next surprise mitigated my frustration. Hold any button in for two seconds and the screen shows pretty complete help information for that control. And this includes the soft keys. Triggering works as you'd expect, until you start pushing more buttons. It was no surprise that the unit can trigger off of a particular I2C or CAN packet; that's hardly unusual on an instrument with built-in protocol analyzer. But it's an essential feature for working with serial datastreams. The scope's ability to trigger on a variety of runt pulses, too, was expected and very welcome. But Agilent has taken the integration of analog and digital channels a step further. Sure, you can trigger on data patterns from the 16 digital inputs, but even with that entire section of the instrument turned off, the logic analyzer-like triggering features still exist. With just a few button presses one can start a sweep based on a rise/fall time being greater or less than a user-set value. Similarly, the units can trigger on specific pulse widths, programmable from 2 nsec to 10 seconds.