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Cable Shielding: Foil or braid

Discussion in 'Electronic Basics' started by qu1nn, Jul 29, 2004.

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  1. qu1nn

    qu1nn Guest

    Hello

    I am under the impression that if you wanted to protect a sensitive
    electronic device from external coupling (aka nearby cables near a
    switch or a sensitive cable near a noisier one) that it would be best
    to use shielded twisted pair with 100 shielding. Generally when you
    have 100% shielding it would be a foil with a bleeder cable strands.
    However if you get a cable with braids you only get 90% coverage.

    So if one is to ignore the ENDS of the cable and you are only looking
    at protection from nearby em fields would it not make more sense for
    100% shielding? or is it that the foil shield does not really "perform
    as well"?


    Thank you for your time
    Qu1nn
     
  2. Bob Myers

    Bob Myers Guest

    "Shielding" a cable is actually a bit more complicated than this. Not
    only are you trying to prevent capacitive coupling of noise into the
    signal path (which is what a "shield" does), you're also trying to
    prevent inductive coupling (achieved through minimizing the open loop
    area of the complete signal path, both "forward" and "return") AND
    you're trying to keep noise from entering the system through at least
    one other path - noise voltages/currents showing up on the system
    ground via the "shield" connection. If the "shield" of a cable is
    tied to the "ground" at both ends, it is a possible path for currents,
    including noise currents - and if that path shows a significant impedance,
    those currents result in a noise voltage (potential difference) between
    the two "grounds," which in some circuit configurations can lead to
    problems. So it's not just a matter of the physical coverage of the
    signal conductors by the "shield," but often also a question of the
    resistance
    or impedance seen by currents within that ground connection. Foils
    may not be very good in this respect, so often the best performance
    will be obtained from a combination foil/braid "shield."

    Bob M.
     
  3. Just curious......but are there any circumstances where grounding the
    shield at both ends would be required or helpfull. I seem to recall
    reading some material on shielding and I seem to remember there were
    some instances where on would connect both ends of the shield.....
     
  4. Rich Grise

    Rich Grise Guest

    With RF, and coax connectors, the shield is connected, but it's a
    sort of different animal than your original question, I think. I don't
    know the relative merits of braid vs. foil, but I'd bet that foil is
    considerably cheaper.

    Cheers!
    Rich
     
  5. Bob Myers

    Bob Myers Guest

    Consider that when you connect the "shield" of any cable to
    the ground at both ends, what you're really doing is tying the
    grounds of the two pieces of equipment together. This means
    that the "shield" may no longer BE just a shield, but possibly
    is a return current path. Sometimes, that's what is wanted, and
    sometimes it isn't. Connecting the shield at one end only means
    that it really IS just acting as an electrostatic shield (and if there
    isn't a return path already provided within the cable assembly,
    then you likely have problems because that current IS going to
    return SOMEHOW, or the thing just won't work). Both
    conditions (connected at one end or connected at both ends)
    have their uses.

    Bob M.
     
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