Sergio,
A "foil & braid" shield is quite common on high-speed cables. If a cable is
properly terminated and you don't have common-mode problems, most of its
radiated emissions will be from holes in the shield. Thus "optical coverage",
the percentage of the shield's nominal area that is actually covered by
wires/conductive foil, is a reasonable approximation to the shielding
effectiveness.
It is very difficult to braid wires in a way that achieves over 95% optical
coverage. A foil shield, with the overlap folded over so the conductive
surfaces touch, can easily achieve 100% optical coverage, but is fragile. If a
foil-shielded cable vibrates, or is repeatedly bent, the foil will eventually
tear. Even if end-to-end continuity is retained, this hole in the shield can
cause a great increase in radiated emissions. By braiding wires over the foil,
you start out with 100% optical coverage, and if/when the foil tears degrade in
just that area to the 90-95% optical coverage of the braid.
We used to use a type of parallel cable for Electromagnetic Compatibility (EMC)
testing that had a foil shield. We would get about three weeks use out of these
before they went bad and had to be thrown away because of excessive radiated
emissions. I helped develop and release an IEEE-1284 parallel cable in 1994
(Lexmark partnumber 1329605) that used a foil & braid shield, and we put these
in our EMC lab. It took nine months of heavy use before the first of these
cables exhibited a noticeable increase in emissions over brand-new cables.
John Barnes Advisory Engineer
Lexmark International
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