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Signal integrity is analog’s “reality show”

(An edited version of this column appeared in the March 19, 2007 issue of EETimes.)

You can't go anywhere in the engineering community now without hearing about signal integrity (SI). It was the main topic of exhibiters and papers at the recent DesignCon, it is very popular on the EETimes and related web sites, and it consumes a lot of engineers' time. We heard “a kiss is just a kiss, a sigh is just a sigh” in Casablanca's “As Time Goes By”, but in our world, that simple-looking signal path is really much more: it is a disguised transmission line, laden with complexities and subtleties.

There's good reason for the increasing focus on SI throughout design. As conceptually pure digital signals transition through the real world, real media, and real channels at GHz and higher rates, “analog” imposes its fabric of distortion, noise, dispersion, nonlinearities, and other imperfections. On one hand, I know it makes design harder; on the other, as an analog-centric engineer, I feel slightly gleeful and think, “you thought you could get away from us–well, you can't!” But since taking joy in the misery of others (aka schadenfreude) is not a long-lasting formula for happiness, I move on.

The problem of SI in circuits is not new. At the turn of the last century, Columbia University physics professor Michael Pupin (click here to read more) took the abstract electromagnetic theory of James Clerk Maxwell and Oliver Heaviside and developed loading coils, which extended the practical range of audio-band telephony cables. Whereas unloaded cables only worked for about a thousand feet, a properly loaded cable was good for three or four times that distance. (Ironically, these coils played havoc with the cables' ability to handle digitized signals 50 years later, necessitating complex equalization schemes, but that's another story.) Among other techniques engineers have used to adapt to, or counter, signal-integrity challenges include Dolby noise reduction, RIAA recording equalization, and FM pre- and de-emphasis.

But these measures were primarily the responsibility of analog, RF, and communications engineers. Designers of digital circuits and systems didn't have to worry about SI, except in the coarsest sense of the basic, good practices of keeping noise out and supply rails clean.

In contrast, almost every design today is fair game for SI problems. This means designers need to spend more time in up-front simulation with validated and accurate models.Nearly every PCB track is not just a short copper interconnect, but a transmission line whose placement, topology, and routing are cause for concern. The IC die, leads, and package also play a major role. Even the choice and placement of basic bypassing capacitors, and their models, is critical. Test equipment, probes, and associated procedures have to be upgraded.

It's all enough to make an engineer sometimes long for the all-analog, vinyl LP record and playback channel. But since that world is mostly gone, brush up on your SI skill set or find yourself working much harder on your project, or hardly working at all.

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