In such an arrangement, you can get enough skew
between wires so that the signal received on the +
wire, at the end of a long cable, is more than one
bit removed from the signal on the - wire. This
totally scrambles the signal. As you approach this
BER performance drops. This is the effect of what
you refer to as a deterioration in the noise margin.
>Errors-To: [email protected]
>From: "Don Abernathey" <[email protected]>
>Date: Thu, 25 Jul 1996 17:29:01 -0700
>To: [email protected]
>Subject: Differential Pair Skew
>
>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 |
>[email protected] |
>*************************
>
>
_________________________________________________
Howard W. Johnson, Olympic Technology Group, Inc.
U.S. tel (206) 556 0800 // fax 206 881 6149 // email [email protected]