David,
Ah!! I see.
Kevin's original query asked about "attenuation." I interpreted that as
signal attenuation and I fired it off to my colleague Ted, since that is
his bag (signal integrity). But, perhaps you are right and Kevin was
really inquiring about shielding attenuation, i.e., attenuation of the
inside fields as they try to get out and vice versa) not attenuation of
the signal. In that case, you are absolutely correct, the parameter of
interest is surface transfer impedance, Zt, which is a function of
frequency and braid construction.
As a practical matter, there will be very little difference in surface
transfer impedance (hence "shielding effectiveness) between 85% and 90%
optical coverage. Theoretically, there will be more difference at high
frequencies than at low frequencies, but it probably isn't a
significant difference.
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: Brumbaugh, David [SMTP:david.brumba...@pss.boeing.com]
Sent: Friday, July 31, 1998 1:47 PM
To: 'emc-pstc list server'; 'Knighten, Jim'; DiBene, Ted
Subject: RE: Twisted Shielded Pair
The original message sounded to me like Ted was looking for
changes in
the shielding effectiveness when going from 85% to 90% coverage.
However, the answer is the same, the change is not significant.
I have commonly seen 20 dB used as a "rule of thumb" shielding
effectiveness for a TSP. However, I am the first to admit that
this can
be overly simplistic, and that the correct approach is to
determine
transfer impedance of the shield to determine coupling to and
from the
protected circuit. There are lots of papers in the open
literature (IEEE
EMC transactions and EMC symposia) over the last 30 odd years
which
address transfer impedance.
DB
> ----------
> From: Knighten,
Jim[SMTP:knigh...@exchange.sandiegoca.ncr.com]
> Sent: Friday, July 31, 1998 9:26 AM
> To: 'emc-pstc list server'
> Cc: DiBene, Ted
> Subject: FW: Twisted Shielded Pair
>
> 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
>
