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Spread-spectrum clocking to reduce EMI: clever or cheat?

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RWatkins
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
RWatkins   4/10/2012 3:38:48 PM
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One kind of sneaky question: Why, if the receiver noise is in fact a problem for measurements taken in accordance with test directives, do test directives still specify an unrealistic measurement bandwidth? It seems that the EMI test suite should be set for a bandwidth comparable to the expected receivers that would be in use for the frequency band being tested. If this was properly implemented, then there would not be a question of sneaky interference from "certified" or "verified" devices.

RWatkins
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
RWatkins   4/10/2012 3:34:22 PM
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Although what you say is probably true, it is the case with many of the "newer" schemes. Take a look at what is discussed with ultra-wideband to see an even more interesting discussion on these lines.

VIntage Bits
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
VIntage Bits   4/9/2012 5:54:38 PM
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I think spread spectrum clocking is a terrible thing to do in a system designed to interoperate with other components. When integrating such a system, I have seen configurations that were unstable with spread spectrum clocking in use. So, for "open" platforms I think it is a terrible method. Even in a closed system, it feels like a cheat, but at least the full system will have been tested together and, presumably, at least function properly.

HurinThalion
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
HurinThalion   11/14/2011 6:34:45 PM
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If you just want to pass a regluatory hurdle, I would make it my fix of last resort. However, the last time I saw someone use this was in a 90's automotive design. Everyone loved it until the platform folks did an FM radio listenting test. When you hit "seek the next higher station" button it would stop at an "empty" station and growl at you. I was the poor schmuck who was tasked with re-designing it. It is a blatant method of "fooling" the quasi-peak detector and spectrum analyzer sampling response time. We all better hope that CISPR 14 & 16 don't address this one day.

Rush.Hood_#4
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
Rush.Hood_#4   11/14/2011 6:10:18 PM
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Can't find the original citation, but my opinion is summed up by "This is like getting rid of a cow pie by stomping on it."

6J5
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
6J5   6/2/2011 8:26:56 AM
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In GPS & ESM circles this is called noise jamming. In the former spreading noise is tolerable due to distance to satellites but created the near-far problem when local augmentation pseudolites were proposed due to their relative proximity making precessing gain much less effective. If all and sundry went for SS ( cellular services, GPS/Galilleo (GNSS), EMI measures,TV broadcasting, etc) a rise antenna noise temperatures, and therefore noise floor, would begin to defeat the original attributes of SS. Too much of a good thing?

Bob Lacovara
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
Bob Lacovara   5/30/2011 5:55:38 PM
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WKetel, you have some right on your side: in some cases, spread spectrum is a means to dodge a regulation. But in an environment full of emitters, there has to be some level below which you, as the receiver, have to live with. Now if spreading your emissions widely enough puts potentially interfering emission far enough down, then that may be a case to accept the (admittedly) shady technique. At least until the requirements boys get a bit more clever and address SS techniques specifically.

WKetel
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
WKetel   5/29/2011 12:40:51 AM
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Spread spectrum is another "cheap trick" to get around a regulation. It does reduce some kinds of interference, but if a switching supply, or a microcontroller is radiating noise into my audio system, it is not going to make my problem any smaller, just a bit harder to locate the source. Noise at a constant frequency can be rejected by a tuned filter, but noise that is spread spectrum would need to be bypass filtered, or band-reject filtered, both of which are a bit more complex than a single frequency rejection filter system.

DU00000001
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
DU00000001   5/26/2011 7:35:21 PM
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Hej, from the shortwave listener's point-of-view, spread spectrum is cheating. It allows to pass EMI tests but - not reducing overall energy emitted - adds to the basic noise level. It is what makes live hard. Like "light pollution" making the astronomers' lives hard.

BeeGee
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re: Spread-spectrum clocking to reduce EMI: clever or cheat?
BeeGee   5/4/2011 9:02:41 PM
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The main reason for tight EMI emission standards is RF interference, and that only makes sense in the context of a given frequency. The fundamental issue isn't that your widget creates too much total emitted energy (unless your widget is very high power); but rather that the energy is concentrated at one frequency due to "flaws" in your product. These "flaws" are a side-effect of fast, synchronous designs and high-speed external connections. Spreading the energy just counteracts a side-effect of synchronous design.

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