A short tutorial on characteristic impedance.

Discussion in 'Broadband' started by The Natural Philosopher, Jun 21, 2014.

  1. What is characteristic impedance?

    Its the INSTANTANEOUS impedance a cable looks like to a generator.
    Before the signal;has had a chance to travel to the other end ( and
    possibly back).

    Does it vary with frequency?

    No. Not theoretically, and very little in practice.


    How is it calculated?

    By a deeply dull calculation comprising Henrys per meter and Farads per
    meter. Since these do not vary with frequency, you can see the
    characteristic impedance is in fact constant with frequency. The more
    inductance per meter the higher the impedance, the more capacititance
    per meter the lower the impedance.

    Why is it important?

    Only really in one way at all. If you match a cable to the correct
    terminating resistance the same as its characteristic impedance the
    cable wont reflect signals from its ends, and so it won't RESONATE at
    any particular frequency. Apart from that it has *absolutely no meaning
    or use at all*.

    So what does '600 ohm audio impedance' mean?

    Almost **** all. Way back when it was discovered that running cables for
    audio at high terminating impedance was a recipe for 'snap, crackle and
    pop' and 600 ohms was settled arbitrarily as the 'sort of impedance to
    terminate an long audio cable at' (or a long telephone one).

    Outside of massively long copper lines, the reflections back from the
    far end are utterly irrelevant.

    the speed of light = 3.33564095 nanosecond per metre and its not a lot
    less in a cable. So to get a delay of - say - 50us which might make a
    difference above 10Khz, requires a cable length of around 8,000 meters.
    (16m there and back)

    So if you WERE running a shielded twisted pair to carry hifi audio 8km
    you WOULD need to match to the cables characteristic impedance to get
    full audio.

    The characteristic impedance of most common cables you will find range
    from 50 ohm air spaced dielectric coax, through 75ohm antenna coax
    through twisted pair in the 100-200 range, and 300 ohm 'antenna ribbon'
    cable. No cable I know of has a 600 ohm impedance.

    Please, chaps, don't 'nod knowledgeably' if you do NOT understand the
    engineering and physics.

    Telephone cables are around 200 ohms IIRC and should be terminated at
    that fir RF (DSL) use. On the audio side of the splitter/filter it
    doesn't matter a damn.

    High resistant joints make a BIG difference to these 'perfect twisted
    pairs' and the Openreach engineers carry reflectometers than can
    identify high impedance joints.

    Yes SOME RF will jump an open circuit, BUT there will be horrible nulls
    in the frequency response and you will lose a lot of bandwidth. High
    resistant joints have the same effect putting ripples in the frequency
    response leading to loss of some DSL frequencies.






    --
    Ineptocracy

    (in-ep-toc’-ra-cy) – a system of government where the least capable to
    lead are elected by the least capable of producing, and where the
    members of society least likely to sustain themselves or succeed, are
    rewarded with goods and services paid for by the confiscated wealth of a
    diminishing number of producers.
     
    The Natural Philosopher, Jun 21, 2014
    #1
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  2. The Natural Philosopher

    Peter Able Guest

    Typos apart, I can't argue with that - but I've missed what has driven
    you to this cri de coeur ;-}}

    BTW, 600 ohm twin is not uncommon as an RF transmission line.
     
    Peter Able, Jun 21, 2014
    #2
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  3. The Natural Philosopher

    Graham. Guest




    Date: Sat, 14 Jun 2014 18:22:50 +0100
    From: Vir Campestris <>
    Newsgroups: uk.telecom.broadband
    Subject: Phone cable and networking
     
    Graham., Jun 21, 2014
    #3
  4. |:)



    --
    Ineptocracy

    (in-ep-toc’-ra-cy) – a system of government where the least capable to
    lead are elected by the least capable of producing, and where the
    members of society least likely to sustain themselves or succeed, are
    rewarded with goods and services paid for by the confiscated wealth of a
    diminishing number of producers.
     
    The Natural Philosopher, Jun 22, 2014
    #4
  5. The Natural Philosopher

    grinch Guest

    Surely that's characteristic impudence
     
    grinch, Jun 22, 2014
    #5
  6. The Natural Philosopher

    grinch Guest

    grinch, Jun 22, 2014
    #6
  7. The Natural Philosopher

    Peter Able Guest

    Thanks Graham. I tend to skip OPs with such a respect for grammar,
    syntax and logic. Persisting through to the 18th June though, I got to
    the nub - and now all is revealed. A cri-de-coeur fully justified.
     
    Peter Able, Jun 22, 2014
    #7
  8. and inductance is when you offer tea and chocky biscuits to the engineer
    as an inducement to do a decent job :)
     
    Mike Tomlinson, Jun 22, 2014
    #8
  9. and resistance is what the Talk Talk dor to door cold calling salesman
    mmets.
     
    brightside S9, Jun 23, 2014
    #9
  10. The Natural Philosopher

    Jim Guest

    Not to mention those who don't know the difference
    between meter and metre.
     
    Jim, Jun 23, 2014
    #10
  11. The Natural Philosopher

    Chris Kirby Guest

    and admittance is when you let him/her into the house.
     
    Chris Kirby, Jun 29, 2014
    #11
  12. The Natural Philosopher

    Bob Eager Guest

    and conductance is when you usher them out again.
     
    Bob Eager, Jun 29, 2014
    #12
  13. The Natural Philosopher

    Mike Guest

    Only after a lot of coercivity!

    A LOT!
     
    Mike, Jun 29, 2014
    #13
  14. The Natural Philosopher

    198kHz Guest

    and reactance is his 'thank you very much', and capacitance is the
    quantity of victuals consumed.
     
    198kHz, Jul 6, 2014
    #14
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