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600 ohm Twisted Pair Telephone Line

Discussion in 'Electronic Design' started by Mike Monett, Jul 10, 2005.

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  1. Mike Monett

    Mike Monett Guest

    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

    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

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

    and also

    Very interesting reading.

    Mike Monett
  2. 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.
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