Re: [SI-LIST] : Twisted Pair Impedance

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From: Derrick Stikeleather (dfs-rls@charter.net)
Date: Wed Nov 22 2000 - 04:51:04 PST


The lay of the cable in twisted pair (Cat5, 5e, 6, and 7) *is* done that way for crosstalk reasons. Cat6 cable must not have more than 38.3 dB of crosstalk with 17.3 dB of return loss at 250 MHz (last I checked).

If you take a bonded-twisted pair and note the impedance on a differential TDR, then take the twist out of the wire, you will not see a significant change of impedance. If you separate the pairs from each other, the impedance will change dramatically. This is why, when terminating UTP to a punch down block (or any termination for that matter), you want to leave the twist right up to the termination.

It is interesting to note that when one pairs' impedance is measured in the jacket with the other pairs, and then taken out of the jacket and measured alone, the impedance of that pair changes.

Also, as you would expect, some of the best twisted pair cable I have seen was individually shielded pairs with a foil wrapper in mesh.

Derrick Stikeleather
  ----- Original Message -----
  From: Barry Ward
  To: SI-LIST@silab.eng.sun.com
  Sent: Tuesday, November 21, 2000 1:17 PM
  Subject: [SI-LIST] : Twisted Pair Impedance

  I have read about the newer Cat 6 & 7 twisted pairs with their gigahertz data rates. From transmission line theory I understand that the distance between the conductors and the dielectric constant of the intervening material are the primary factors determining the capacitance and thus the impedance. But the new Cat 6 & 7 cables have very tight specs for the lay, or twisting, of the pairs. I am forced to believe that the lay and its consistency is therefore also very important. I assume some of this is for crosstalk reasons (the longer the distance between any repetition of the lay the less the x-talk), but is some of it also for impedance reasons? What is the math behind any of this (assuming a single pair surrounded by air for an infinite distance). Approximately how does this math change when multiple pairs are cabled together?

  Any answers would be greatly appreciated. I work for a cable manufacturer where we build cables with twisted pairs. We work in relatively low frequencies (up to 10 MHz), and the lay does not seem to be too critical. We untwist the pairs for a couple of inches at the ends of the cable to terminate the wires at connectors; this does not seem to make the cable unusable.

  Again, thanks for any information.

  Barry Ward
  GeoSpace LP
  bward@geospacelp.com
  713-939-7093

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