[SI-LIST] Re: (EMI)spread spectrum clocking
- From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 6 Sep 2003 12:04:59 -0400
I sometimes though of spread-spectrum clocking as cheating, because it
doesn't reduce the energy radiated, it only reduces the narrowband peaks.
Because you measure the radiation with narrowband receivers (spectrum
analyzers), it seems to look better.
But it is true that this also helps reduce interference, in some cases. The
fact that the FCC endorses the practice, is some justification.
Yet, as Michael Poimboeuf points out, it doesn't work for everyone.
Dithering a clock may have an inconsequential effect on a purely digital
system (like a computer); but if there's any chance of the jitter being
demodulated, and the system has analog sections, there's a good likelihood
that the modulation signal falls within the analog bandwidth and corrupts
signals. Obviously, any system that requires a low jitter clock is not a
good candidate for SSC.
The original IBM PC did not include spread-spectrum clocking, but many
models today include it.
Regards,
Andy
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