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.
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Thank you |
Don Abernathey |
(503)690-6234 |
[email protected] |
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