RE: [SI-LIST] : spread spectrum clock drivers

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Mon Oct 30 2000 - 07:02:33 PST


> has anyone used a spread spectrum zero clock drivers ?
 
Have not personally used them yet, but many others have.

(What is a "zero clock driver"? If you mean "zero skew", I don't think it
makes sense to put "spread spectrum" and "zero skew" in the same sentence,
since the former generates skew by its very nature.)

> any thing important
> one has to know before using it ?
 
By definition, they generate jitter. Just understand the impact of that on
all downstream clock consumers.

> how will such a driver handle a regular clock (non-modulated)?.
 
Um, I think that is how you are supposed to use them! If you fed a
modulated clock into one of them, the results might be "interesting", though
not necessarily a disaster. Or it might not be much different than what
you'd get with a non-modulated input.

> to my
> understanding, in such
> case it will act as any other PLL, but will be more susceptible to input
> clock noise/jitter
> due to its higher BW. am I correct ?
 
Why do you say they have a higher BW? If anything, one could reason that a
"spread spectrum" clock generator must have a somewhat narrow PLL loop BW,
so that it doesn't constantly correct its own modulation.

Any PLL that is downstream of the "spread spectrum" clock driver would need
to have high BW, so that it would track as closely as possible the jitter on
its input.

Andy

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