[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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