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

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Mon Apr 24 2000 - 07:02:34 PDT


Lisa,

Hopefully Doug's ultracad link can provide you with the information
you need. The true and complement signals within the pair do
"crosstalk" with one another. However, a good differential driver
will switch the true and compliment signals out of phase with
one another at approximately the same instant in time. Therefore,
you will have only one predetermined crosstalk condition and it is much
easier to not think of the effect as crosstalk but as setting up an odd-mode

or differential impedance. If you are designing for Zo=50 ohms, for
example,
then for differential signaling you want to design the odd-mode impedance
for 50 ohms to a ground return. Often times 50 ohms to ground is instead
referred to as 100 ohms differential when referencing the impedance between
the
true and complement signals. Hopefully you have a EM tool that provides
you with the differential impedance (ours will when the number of conductors
is exactly equal to two).

Mike

_______________________________________________________________
Mike Degerstrom Email: degerstrom.michael@mayo.edu
Mayo Clinic
200 1st Street SW
Gugg. Bldg. RM 1042A Phone: (507) 284-3292
Rochester, MN 55905 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
_______________________________________________________________

> -----Original Message-----
> From: Doug Brooks [mailto:doug@eskimo.com]
> Sent: Thursday, April 20, 2000 6:03 PM
> To: si-list@silab.eng.sun.com; Lisa_Desandoli@pmc-sierra.com
> Subject: Re: [SI-LIST] : Differential Signalling and Crosstalk
>
>
> Lisa,
>
> There is an article on our web site entitled
> "Impedance Terminations: What's the Value?"
> It specifically describes the coupling that occurs when two
> traces are
> routed close together, and why this mutual coupling can be
> beneficial in
> the case of differential signals.
>
> http://www.ultracad.com
>
> Doug Brooks
>
>
> At 03:06 PM 4/20/00 -0700, you wrote:
> >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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> .
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> UltraCAD Design, Inc. http://www.ultracad.com
>
>
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