Characteristic Impedance of a cable calculation

Discussion in 'General Electronics Chat' started by Gibson486, Oct 8, 2015.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    12
    I have this cable...

    http://alphawire.com/en/Products/Cable/EcoGen/EcoCable/77109

    However, when I try to calculate the characteristic impedance (sqrt of L/C), I do not get their impedance that the have on their specs.

    Am I making the wrong assumption that I just add the ground and wire capacitance?

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, your assumptions are not quite right.
    The wire-wire capacitance contribution from the capacitance to ground is 1/2 of the wire capacitance to the shield (ground) since the capacitance from each wire to the shield are in series (draw the capacitances of the wire to see this).
    Thus the characteristic impedance would be √[.18μH / (27pf +1/2*49pF)] = 59.1Ω (close to their value of 58Ω).
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I don´t see any specs of that kind, can you post a datasheet?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Click on the Properties box on the referenced webpage.
     
  5. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Thanks....it is still a little off on other cables, but I guess that means sqrt of L/C is really just a ballpark estimate?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    I would think being within less then 2% of their value is better than "ballpark". :rolleyes:
    The equation √(L/C) is for an ideal lossless line.
    There are likely some factors in the real cable that make its impedance slightly different from the ideal.
     
    Last edited: Oct 9, 2015
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