From: Charles Hill (email@example.com)
Date: Tue Nov 21 2000 - 13:28:27 PST
In addition to Andy's remarks, twisting is basically a field containment
strategy. Think of the H field alternating direction between twists. So
to control crosstalk in bundles, the twists per length must be
varied. Also the number of twists per wavelength need to be 5-10 (I don't
remember the reference). I would expect a non-uniform characteristic
impedance at 1-2 twists per wavelength (but check my intuition).
Also, it seems the magnetic coupling between the pairs could increase as a
result of the twisting. That would reduce the per unit length inductance
and lower the characteristic impedance. Perhaps someone could confirm that.
At 02:27 PM 11/21/00 -0500, Ingraham, Andrew wrote:
>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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