Dumb cable impedance question

Discussion in 'Archived Threads 2001-2004' started by Bob Weissman, Mar 29, 2002.

  1. Bob Weissman

    Bob Weissman Auditioning

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    I'm not a newbie, but this is a question I've wondered about for a while and never seen answered.

    We know that AV cables should have the proper impedance. TV coax, such as RG-59 and RG-6, is said to have 75Ω "nominal impedance." Component video cables are the same.

    Here's my question. How can, say, a 1m cable have 75Ω impedance and a 2m cable also have 75Ω impedance? Resistors in series impose the sum of their resistance, so wouldn't a 2m cable have 150Ω impedance? The same as two 1m 75Ω cables connected in series? What am I missing? Is this really impedance per unit length?
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    Impedence on a cable is length-independent, while DC resistance is dependent on the length.
    Think of it this way: it has to do with how signals on the center conductor generate an EM field and "leak" into the outer shield.
    So it has to do with the dielectrict constant of the insulator between the center conductor and the shield.
    If a signal pulse does not penetrate to the shield in the first few inches, it does not matter how long the cable is, the signal wont penetrate. (The signal does not get stronger as it travels.)
    Does this help?
     
  3. Bob Weissman

    Bob Weissman Auditioning

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    Aha, so the impedance is measured between the conductor and the shield? That makes sense. I was assuming it was measured end-to-end on the conductor. Thanks for explaining.
     
  4. Greg_R

    Greg_R Screenwriter

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    The terminations (connectors) need to have the appropriate impedance as well...
     

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