Spread Spectrum & why are we disabling it?

[CHOPPER GOD]

thx

 

[CTho9305]

The FSB of the computer operates at some frequency (say, 200MHz). All the wires switching at the same time & frequency causes relatively high interference at 200MHz - in some environments, this can be a problem. Spread Spectrum basically jitters the clock and varies its period slightly so that instead of always running at exactly 200MHz, it operates in a range of +/- a few percent - this causes the interference to be less at a given frequency, and spread over a band of frequencies.

 

[CHOPPER GOD]

awesome ty

 

[beer]

Is the context of this question referring to 2.5 GHz PCI-E clocks?

 

[beer]

Speaking of which, the reference design(s)/platform(s) for PCI-E clocks I have seen indicate that you won't even be able to pass regulatory (FCC emissions) without SS enabled. This is in a mobile environment, but still, AFAIK the FCC, in all liklihood, would not certify a device in which a user could disable SS at 2.5 GHz, since the fundamental and harmonics (greatly) exceed FCC acceptable outputs. At 200 MHz it is a non-issue.

 

[Peter]

One last remark: The clock isn't being "jittered", it's being frequency modulated at quite a low frequency. The difference is that the FM method produces a slow, deterministic bobbing of the main frequency. Simple jittering would cause sudden jumps in frequency, which would upset the devices receiving the clock.

 

[CrispyFried]

And it should be OFF if you overclock, as the few percent it varies can put you over the top at an otherwise stable speed.

 

[harrkev]

I one wrote a short essay on this subject. What the others have said is true, but my essay goes into a little more detail.

Somehow, it found it's way HERE. It was originally on the MSI support forums, but I guess that somebody liked it 😉

 

[Peter]

Originally posted by: CrispyFried
And it should be OFF if you overclock, as the few percent it varies can put you over the top at an otherwise stable speed.

Typically it's .25% or .5%, no more. More would, in its upper extreme, already violate bus frequency specifications even when not overclocking. Also, some clock synthesizers offer a choice of "center" or "down" spread modes, with the latter avoiding exactly that (at the expense of a slightly lower average frequency).

 

[CrispyFried]

Originally posted by: Peter

Originally posted by: CrispyFried
And it should be OFF if you overclock, as the few percent it varies can put you over the top at an otherwise stable speed.

Typically it's .25% or .5%, no more. More would, in its upper extreme, already violate bus frequency specifications even when not overclocking. Also, some clock synthesizers offer a choice of "center" or "down" spread modes, with the latter avoiding exactly that (at the expense of a slightly lower average frequency).

I think youre right I remember reading something like that.

Its what I get pulling a number out of my *cough*