Re: Differential Pair Skew

Don Abernathey (dla@pyramid.com)
Fri, 26 Jul 1996 11:36:25 -0700

On Jul 25, 6:06pm, Howard W. Johnson wrote:
> Subject: Differential Pair Skew
---snip---
> 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.

Hello!

The part about the effect of a shield around the conductor pairs is
true, but it helps to remember that often when somebody builds a
"differential" driver today the + and - outputs are separate buffers
and therefore quite capable of driving separate coax cables.

I'm still confused on the role skew plays in signal degradation. If
you think of skew as phase shift for a moment, its makes sense that a
180 degree shift would mess things up. But what harm would 90 degrees
cause?

I have this "skew" question because Fibre Channel and IEEE P1394 are
both point to point interfaces and both spec skew limits significantly
smaller than the bit window. Neither of them explain why. I've read a
bunch of reference material and none of it talks about skew.

Help me out folks, I need an explanation.

Thank you

>
>
>
>
> >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
>
>-- End of excerpt from Howard W. Johnson