Differential Pair Skew

Howard W. Johnson (howiej@wolfenet.com)
Thu, 25 Jul 1996 18:06:19 -0700 (PDT)

I've come to understand that in certain
twisted-pair cables, like IBM type-I shielded twisted pair,
the two wires of the pair are rather weakly coupled
to each other. That is, there is a LOT of
capacitance per unit length between each signal
wire and the surrounding shield, and not so much
between the two wires. Basically, what you get
here is almost like having two independant
transmission lines, each registering 75 ohms
to ground, rather than a single 150-ohm balanced
twisted arrangement.
Try it out: send a signal on one wire, and **nothing**
on the other. You'll pick up some crosstalk on the other
wire. If the wires had been tightly coupled, you
would have gotten a full-sized, but opposite polarity,
signal at the receiving end of the second wire.

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: si-admin@silab.Eng.Sun.COM
>From: "Don Abernathey" <dla@pyramid.com>
>Date: Thu, 25 Jul 1996 17:29:01 -0700
>To: si-list@silab.Eng.Sun.COM
>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 |
>dla@pyramid.com |
>*************************
>
>
_________________________________________________
Howard W. Johnson, Olympic Technology Group, Inc.
U.S. tel (206) 556 0800 // fax 206 881 6149 // email howiej@wolfenet.com