Differential Pair Skew

Don Abernathey (dla@pyramid.com)
Thu, 25 Jul 1996 17:29:01 -0700

Hello!

I was reading the Fibre Channel spec (FC-PH) and I noticed that they
spec the skew of cables rather tightly. I don't understand why and I'm
hoping you folks can clue me in....

Imagine a differential serial interface transmitted on a twisted pair cable.

Imagine that the + and - conductors in the cable are of different
lengths resulting in a propagation delay difference, aka skew.

Question:
What impact does the +/- pair skew have on the received signal's
characteristics?

Thoughts:
I don't see a change in the received pulse width, since the
differential receiver sees an equal skew from + to - and - to +
transitions. The skew in a single cable is fixed and doesn't move
around (jitter). There might be a change in noise margin, resulting in
a form of jitter, in applications using AC coupled receivers since the
+ signal swing will above and to, and the - signal swing below and to
the receiver's reference.

*************************
Thank you |
Don Abernathey |
(503)690-6234 |
dla@pyramid.com |
*************************