Scoping out new scopes

April 03, 2011

Jack Ganssle-April 03, 2011

After pressing, twisting, and pushing knobs, Jack Ganssle likes what he sees in a new mixed-signal scope series from Agilent Technologies.

What is a pHz? Wikipedia tells me pHz stands for "picohertz," or 10-12 Hertz. fHz means "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 1.0012 kHz. But it also showed the standard deviation as a few tens of fHz, and later pHz, prompting my dive into Wikipedia.

Click on image to enlarge.

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 70-MHz 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 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.

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