Re: Cable Shielding: Foil or braid
From: Bob Myers (nospamplease_at_address.invalid)
Date: 07/29/04
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Date: Thu, 29 Jul 2004 18:28:37 GMT
"qu1nn" <qu1nn@buffalo.com> wrote in message
news:417b2fd1.0407290859.181bb12e@posting.google.com...
> 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.
"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.
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