Nicely put, but I want to emphasize one point. The foil in most cables
doesn't have a metal to metal contact. Where the foil overlaps itself the
polyester on the "underside" is what is in contact with the foil "outside".
The type cable you are mentioning is called an "e" fold if I remember
correctly, but I don't kno why. A lot of folks don't realize that they are
not getting foil to foil contact, just and overlap of material.
Gary
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, November 07, 2000 8:58 AM
To: [email protected]; [email protected]
Subject: Re: Coaxial cable
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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