[SI-LIST] : Re: Cable Skew

rbmccammon@mmm.com
Tue, 24 Mar 1998 14:49:53 -0600

I suspect it is the mechanics of twisting.
You can twist two or three bundles together and have them
assume stable symmetric positions such that each is the same length.
Four however will not assume a stable symmetric relationship.
I suspect that three of the cables formed a tight triad with the forth
bundle winding around the triad.

Muzahid Huda wrote:

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 enti=
re
bundle is enclosed by an outer shield.

In all seven samples, the measured intra-pair skew in three of the pair=
s
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
=