From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Nov 21 2000 - 11:27:40 PST
The twisting of differential pairs is almost entirely done for crosstalk.
Yes, the amount of twist will have a small effect on impedance, but I have
to believe it's very minor. The different twist rates are to alleviate
Engineers learned how to do this a century ago. If you can find any old
railways with standing telephone poles, still strung with dozens of
open-wire lines, take a train or car ride and watch the wires. You will see
that they transpose the wires of each pair every so often, using a very
specific (binary?) pattern for crosstalk reduction. Dissect a modern
multiconductor cable bundle and you'll find the same thing.
Relative to the dimensions of the pair's cross-section, the gradual twist is
so much larger that I think the effect on impedance is in the noise, you can
treat it as uniform in the Z dimension. Having no twist at all might have a
slightly higher impedance because the twist helps pull the pair closer
together, but beyond that, I have to think that it's pretty negligible.
If the purpose of the different twist rate was to vary the impedance, the
next question is, why? Is there any advantage to having slightly different
impedances per pair? I can't think of one.
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