RE: [SI-LIST] : Cable skew

Charles Hill (chuckh@csn.net)
Wed, 25 Mar 1998 17:20:34 -0700

Muzahid,

You made no mention of how you measured the skew in the cable. If you =
used a TDR technique, then was the risetime the same amongst the pairs? =
It may be that the bandwidth of the different pairs is not the same and =
you may be seeing the effects of limited bandwidth. This may be a case =
where a network analyzer would be more useful.

Chuck Hill, consultant

chuckh@altaeng.com

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From: Muzahid Huda
Sent: Monday, March 23, 1998 7:40 PM
To: si-list@silab.Eng.Sun.COM
Subject: [SI-LIST] : Cable skew

I have just finished measuring the intra-pair skew in seven samples of a
6-foot twin-ax cable. This cable consists of 4 twisted pairs (ie. 8
lines) and an individual shield for each pair (ie. 4 shields). The =
entire
bundle is enclosed by an outer shield.

In all seven samples, the measured intra-pair skew in three of the pairs
was within 50ps. The fourth pair consistently exhibits at least
150ps-200ps of skew. In these samples, it is always the <underline>same
pair</underline> that consistently measures this variation.

I have checked my measurement technique and equipment to eliminate any
systematic errors. I have also cut open a cable to check for physical
length differences, etc. The cable assembly has a molded ferrite choke
on one end; I have even removed this choke and remeasured the skew.
Nothing seems to make a difference.

Any suggestions/ideas/theories will be greatly appreciated.

Muzahid Huda

Staff Engineer

Silicon Image, Inc.

Tel: (408)873-3111 X132