RE: [SI-LIST] : Differential Signalling and Crosstalk

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From: Fokken, Gregg J. (fokken.gregg@mayo.edu)
Date: Thu Apr 20 2000 - 15:34:07 PDT


I assume you're talking about the 1-0 transition on one line versus the 0-1
transition on the other line. If the two signals are 180 degrees out of
phase (which is definitely what you want), this is odd-mode coupling and
will be opposite in polarity, but remember you're "not" coupling noise,
you're coupling the "signal"! This is what you want (i.e. it is a good
thing, one of the very reasons you want to go differential in the first
place).

> -----Original Message-----
> From: Lisa Desandoli [mailto:Lisa_Desandoli@pmc-sierra.com]
> Sent: Thursday, April 20, 2000 5:07 PM
> To: 'SI'
> Subject: [SI-LIST] : Differential Signalling and Crosstalk
>
>
> Could anyone recommend guidelines for routing differential
> PECL signals at
> ~1.25GHz?
> What I'm concerned about is the relationship between coupling the
> differential pair and the cross talk induced because of that
> close coupling.
> I could be way off base here, but if the signals are closely
> coupled then
> there will be a lot of noise induced on both lines by the
> switching on the
> other. Since the signals are switching differentially, the
> noise induced on
> either line should be opposite in polarity. Since
> differential receivers
> are only good at rejecting common mode noise, wouldn't this be
> counterproductive? Is there a minimum spacing requirement
> for high-speed
> differential signals so that the cross talk noise is minimized?
>
> Thanks for your help,
>
> Lisa DeSandoli
> PMC-Sierra, Inc
>
>
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