Signal Integrity Mailbag

My recent column on intentional clock modulation (EDN, Aug 3, 1998, pg 24) spurred some interesting responses from readers. Here are some of the best:

I couldn't agree more with your recent article in EDN concerning clock dithering. I work in the EMC department for Chrysler electronics. Our EMI requirements are quite stringent. What with the stringent requirements and the automotive industry's aversion to spending money, we have tried many of the more esoteric solutions, such as clock dithering. The major problem with dithering the clock is just as you said: Most CPUs that we use employ a PLL for generating the internal CPU clock. Modulating the reference just makes everything worse. It is a nice idea for a very small subset of EMI problems...

Does this mean there is a PLL in my automatic-braking-system module?

Your article about clock modulation was very refreshing. When I first heard about this practice, I was appalled. The radiated energy is not reduced, just the peaks in the frequency domain. This is like getting rid of a cow pie by stomping on it. Perhaps the regulations will be changed to stop this practice.

I always appreciate a good barnyard analogy.

I liked your article on clock modulation. I don't like intentional jitter either, but it does have its uses. In the case of PCs, it is a low-cost way of making EMC compliance without the cost of other solutions. But does meeting EMC compliance this way meet the letter of the law but violate the intent of the laws? For narrowband communication this spread-spectrum approach may be all right. But what about wideband signals, such as analog television?

Clock modulation has no effect on interference with wideband communications systems.

The one important point about clock modulation that many engineers may not realize it that it does nothing to reduce the actual interference that customers see in the field! It is a clever scheme to circumvent FCC regulations and nothing more. If a milliwatt of energy at exactly 60 MHz causes a certain amount of disruption to an audio preamp, smearing that same energy from 59 to 61 MHz causes almost exactly the same disruption. The only legitimate solution is to reduce the energy radiated or to shift it in frequency by a large amount, such as at least a factor of two or three.

The FCC regulations are not obstacles to be overcome. They are tools to help us solve real problems. When finding a ways to comply with the rules, we need to realize that rules are there to prevent problems, and simply complying to the exact letter of the rules may still leave the problems unsolved. The FCC rules sometimes need some common-sense interpretation. Unless we designers use our common sense, the FCC will have to rewrite the rules to eliminate loopholes such as clock dithering, which will make the rules more complicated and restrictive than they currently are.

Good point, Doug. Many analog circuits that are slightly nonlinear partially rectify high-frequency interference, turning an AM carrier into a noticeable (and objectionable) audio-frequency signal. This is a source of concern in all high-gain audio equipment.

Another way for clock modulation to assist in passing open-field emissions testing is to modulate the clock with some audio-program source. That way, when the emissions technician comes across your nasty, he'll miss it, thinking it's some local radio station...

I had a good laugh corresponding with Dan about this one. If anybody tries it, please don't say you got it from either of us.

Yet another way to pass the test is to park an unmarked white van next to the FCC open-field site. Have the van emit high levels of AM interference right on your clock frequency, masking the clocks.

Martin, did you ever work for the CIA?