[SI-LIST] : Twisted Pair Impedance

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From: Barry Ward (bward@geospacelp.com)
Date: Tue Nov 21 2000 - 10:17:17 PST


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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