[SI-LIST] Re: Serpentining one side of a differential pair

If you change the path timing of one leg of the differential signal by adding a 
serpentine you have changed the differential impedance (and the single ended as 
well) in that section of the differential net.  The reflection caused by the 
discontinuity will also change the timing.  Have you time compensated for that 
effect also?

Tom Dagostino
Modeling Manager
Mentor Graphics Corp.
SAE
tom_dagostino@xxxxxxxxxx
503-685-1613


-----Original Message-----
From: Knighten, Jim L [mailto:JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Friday, September 21, 2001 1:24 PM
To: zanella_fabrizio@xxxxxxx; SI List (E-mail)
Subject: [SI-LIST] Re: Serpentining one side of a differential pair



Fabrizio,

The dominant generation mechanism of the common-mode of a differential
transmission line seems to be delay skew, from our investigations.  Hence,
to minimize common-mode generation, minimize the path length skew and live
with the short section with a change in the line-to-line coupling.  I can
point you to a paper or two in the archives of the IEEE EMC Symposia.

Jim Knighten
NCR
San Diego, CA


On board designs with nets running Gigabit signals, I constantly see that
one side of the differential pair is serpentined/lengthened to perfectly
match it to the delay of other side of the differential pair.  This causes
the positive and negative lines to not be coupled for a few hundred mils,
which can result in common mode noise.   
My question is, which of the two evils is worse, having a little bit of skew
within the pair or having the pairs uncoupled for a portion of the run?

Thanks very much,

Fabrizio Zanella
Signal Integrity
EMC Corporation
508-435-2075, x14645
fzanella@xxxxxxx


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