Spread Spectrum

william chan

Modern PC Motherboards usually come with "spread spectrum" frequencies that are somehow related to the DDR RAM.

Anybody know what "spread spectrum" means?

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Mike Green

I know what "spread spectrum" is, but I can't see how it has anything to do with DDR memories.

Spread spectrum is a modulation technique for radio transmissions where the signal is frequency modulated in such a way that the transmitter energy is pseudo-randomly spread across a specified spectrum of frequencies so that it appears to be random low-energy noise, but, if you know the sequence of frequencies used and the timing for the changes, you can look for energy at the specific frequency and use that for your source of information.

I have heard the term used for computers in the sense of varying the clock of digital circuitry slightly over time so that the noise generated by the circuitry is spread over a spectrum of frequencies rather than being concentrated at multiples of a fixed clock rate.· The result is to reduce the amount of any radiated noise at any given frequency, thus making it easier to get the device approved for sale by the FCC.

Timmoore

Means that some of the high speed signals in a PC change frequencies around a center frequency e.g. a clock of 1GHz may actually continously vary from 995MHz to 1005Mhz. This removes the large amount of interference at the center frequency and spreads it around, normally used to get round FCC interference rules.

Beau Schwabe

Here is something that explains a little about Spread Spectrum clocking....

http://zone.ni.com/devzone/cda/tut/p/id/4154

...I understand this, but I think that determinism might be out the window or become much more complex with this approach.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.