Jeff,
I pass this along in hopes that it helps you with your question.
Jim Knighten
------------------------------------------------------------------------
---------------------------
Dr. Jim Knighten
NCR
17095 Via del Campo
San Diego, CA 92127
Telephone: 619-485-2537
Fax: 619-485-3788
e-mail: jim.knigh...@sandiegoca.ncr.com
----------
From: DiBene, Ted
Sent: Friday, July 31, 1998 9:11 AM
To: Knighten, Jim
Subject: RE: Twisted Shielded Pair
Jim; I have an answer but I am not sure it is the answer he is looking
for. I believe he is really talking about the reduction in overall
attenuation from increasing the braid coverage to reduce return loss.
In going from 85% to 90% shield coverage the change in return loss is
not significant. Neither is it the dominant source of conductive
losses. Even if this was a single-ended communication link with low
impedance (say 50 ohms) the return loss would still be only a fraction
of the overall conduction loss. Since we are now talking about a
twisted pair and a differential communication path, if the signals are
well balanced then the signal and return paths are intrinisically the
primaries. However, if there is strong even mode coupling in structure
then there will be currents on the shield that will contribute to the
overall conduction losses. This, of course is very much dependent upon
this modal impedance which is construction dependent. If the signals
are imbalanced then, as you know, the common-mode current return path is
the shield and the overall losses may be increased slightly due to
signal energy loss.
Basically, this is a very difficult question to answer accurately since
it involves so many variables relating to construction and signal
propagation. However, all the theory aside, there is a simple formula
he can use that is really for approximations to coaxial line return
loss. The differential mode will be much different and thus the loss
contribution will be different however if he is looking for a swag, here
it is:
Assuming that your friend knows how to determine the attenuation
constant as function of resistive losses one can use the simple formula
below;
where rb is the radius of the inner shield and ra is the radius of the
primary conductor. If he takes the first part of the formula and ratios
this with second he can get a feel for the percentage of loss
differences. Again, this is assuming the frequencies are high enough to
consider the dominant losses to be due to skin effect. The DC
resistance calculation is much more difficult with a twisted wire and
braided shield construction. The formula above is a very crude estimate
and will almost always be worst case but I think it is a good start.
Ted.
----------
From: Knighten, Jim
Sent: Thursday, July 30, 1998 5:33 PM
To: DiBene, Ted
Subject: FW: Twisted Shielded Pair
Importance: High
Ted,
You may have an answer to this guy's question. If so, send it
to me and I will post it.
Jim K.
------------------------------------------------------------------------
---------------------------
Dr. Jim Knighten
NCR
17095 Via del Campo
San Diego, CA 92127
Telephone: 619-485-2537
Fax: 619-485-3788
e-mail: jim.knigh...@sandiegoca.ncr.com
----------
From: duval...@gvl.esys.com [SMTP:duval...@gvl.esys.com]
Sent: Thursday, July 30, 1998 3:13 PM
To: emc-p...@ieee.org
Subject: Twisted Shielded Pair
I am looking for a good rule of thumb regarding attenuation
versus shield coverage for a twisted shielded pair cable. Specifically
what level of attenuation change will be expected in going from a 85%
coverage to 90% coverage.
Thanks in advance
Jeff Duvall
