Richard,
The term that I ran across many times while researching my book was "optical
coverage"-- as though you put a light bulb inside the shield and measured what
percentage of its light leaked out. The basic assumptions are:
1. Electric and magnetic fields inside the shield are totally blocked by the
"picks" (conductive wires/strips/foil in a
group), so the only leakage is through the holes between picks.
2. The fields that leak out are incoherent, and thus add as scalars (sum of
magnitudes) for the peak leakage.
This is a first-level approximation, and is closely related to a discussion of
the shielding effectiveness of arrays of holes/ honeycomb on this mailing list a
couple of weeks ago. Over a wide range of frequencies, and far enough away from
the shield that the openings seem to blur together, the leakage is approximately
proportional to how much of the inside/ other side of the shield is exposed to
our view. But at specific frequencies, or if we get very close to a hole in the
shield,
we get diffraction and constructive-/distructive-interference that cause lobes
and notches in the leakage fields.
Some companies have tried to take advantage of this for special (high-priced)
single-function cables. "Optimal braiding" selects the gauge and number of
wires in each "pick" (group of wires laid parallel to one another) and carefully
controls their crossing angle during construction of the cable. The idea is to
create holes with a certain size and shape, and thus polarizability, and with a
certain spacing lengthwise and around the cable. The authors of the articles
claimed that at the design frequency they would get distructive interference,
just like a diffraction grating, making the holes in the shield appear smaller
than they really were.
The problem that I saw with this scheme was that at other frequencies, or if you
bent/pulled/deformed the cable in any way, the geometry changed and you would
get an *increase* in emissions. So it always seemed more sensible to me to try
to completely seal the electric and magnetic fields inside the cable/shield and
not optimize just one tiny operating point.
John Barnes Advisory Engineer
Lexmark International
author of Electronic System Design: Interference and Noise
Control Techniques
(Prentice-Hall, 1987)
richardg%[email protected] on 11/07/2000 12:11:36 PM
To: [email protected]
cc: (bcc: John Barnes/Lex/Lexmark)
Subject: RE: Coaxial cable
John,
Couldn't quite follow the "optical coverage" reference. Sounds like it
should be "optimal coverage" for copper wire cable verses optic cable.
Thanks.
Richard Georgerian
Technical Committee 8 Product Safety (TC-8), Vice-chair
Colorado Product Safety Technical Committee (CPSTC), Chair
Product Compliance Engineer
Exabyte
1685 38th Street
Boulder, CO 80301
USA
tel.: 303-417-7537 fax: 303-417-5710 mailto:[email protected]
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, November 07, 2000 9:58 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
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
[email protected]
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: [email protected]
Michael Garretson: [email protected]
For policy questions, send mail to:
Richard Nute: [email protected]
Title: RE: Coaxial cable
John,
Couldn't quite follow the "optical coverage" reference. Sounds like it should be "optimal coverage" for copper wire cable verses optic cable.
Thanks.
Richard Georgerian
Technical Committee 8 Product Safety (TC-8), Vice-chair
Colorado Product Safety Technical Committee (CPSTC), Chair
Product Compliance Engineer
Exabyte
1685 38th Street
Boulder, CO 80301
USA
tel.: 303-417-7537 fax: 303-417-5710 mailto:[email protected]
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, November 07, 2000 9:58 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
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
[email protected]
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: [email protected]
Michael Garretson: [email protected]
For policy questions, send mail to:
Richard Nute: [email protected]
