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

Tom,

What an interesting question.  Questions often cause me to think about an
issue in a different way than you are used to thinking.

The fundamental answer to your question is, No, we have not compensated for
timing.

In the world that we live in, our differential transmission lines are high
speed (> 1Gb/s), loosely coupled, and the data is serial with embedded
clocks.  A pertinent example is Fibre Channel.  

We devote effort to minimizing path length differences, i.e., delay skew.
Realize that if you choose to have unequal length lines in order to preserve
the degree of line-to-lie coupling along the length, or if you choose to use
a serpentine section to achieve path length equality, you will have a short
section of line that has a different impedance than the rest of the
differential line, hence reflection.  We have had no timing issues related
to this.

I know that delay skew has very little effect on the differential signal
waveform, even in the extreme.  On the other hand, delay skew has a dramatic
effect on the common-mode waveform on a differential transmission line.

Jim


Jim Knighten, Ph.D.
Teradata, a Division of NCR             http://www.ncr.com
17095 Via Del Campo
San Diego, CA 92127
USA
Tel: 858-485-2537
Fax: 858-485-3788
jim.knighten@xxxxxxx

 -----Original Message-----
From:   Dagostino, Tom [mailto:tom_dagostino@xxxxxxxxxxx] 
Sent:   Friday, September 21, 2001 1:48 PM
To:     Knighten, Jim L; zanella_fabrizio@xxxxxxx; SI List (E-mail)
Subject:        RE: [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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