600 ohm Twisted Pair Telephone Line

To All,

(Sorry - I posted this to ABSE by mistake. It should have come here)

A while back we were discussing the 600 ohm impedance of telephone lines, and John L. mentioned the resistance of 24 ga wire had a significant effect on the impedance. Turns out he was right. The resistance dominates at low frequency for long lines.

Howard Johnson discusses this in "High Speed Signal Propagation: Advanced Black Magic". One chapter is "Performance Regions" which describes the critical regions in transmission lines, how they affect transmission loss, and how to overcome some of the loss problems.

At audio frequencies, long lengths of twisted pair act like an rc circuit, and the resistance of 24 ga works out to about 640 ohms.

This is discussed in section 3.5, "RC Region", at

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The entire chapter is provided online courtesy of Prentice Hall. It is available at

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and also

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Very interesting reading.

Mike Monett

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Mike Monett
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[snip]

Hello Mike. I haven't read the articles you mentioned, but similar assumptions were standard in old telecomms textbooks, (well, the same in the two I have anyway).

For a telephone cable the conductors were small diameter and close together, so R>>wL and wC>>G. For such lines, Zo can be shown to approximate to.... Zo = sq-root[R/wC] at an angle of -45.

--
Tony Williams.
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Tony Williams

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