------------------------------------------------------------------------
---------------------------
Dr. Jim Knighten
NCR
17095 Via del Campo
San Diego, CA 92127
Telephone: 619-485-2537
Fax: 619-485-3788
e-mail: [email protected]
----------
From: DiBene, Ted
Sent: Friday, July 31, 1998 2:43 PM
To: 'emc-pstc list server'; 'Knighten, Jim'; DiBene, Ted;
Brumbaugh, David
Subject: RE: Twisted Shielded Pair
Actually, that is not correct. I was simply trying to answer
the question directly which was what type of signal attenuation changes
would one expect if one altered the braid coverage from 85% to 90%.
This may be more of a conductive loss phenomenon and less of an EMC
issue (Again, my assumption of the original question). The issue you
are referring to has more to do with aperture coupling and braid
porpoising then signal integrity. Changing the braid coverage for this
issue can often improve shielding effectiveness but the braid structure
has as much to do with this improvement (e.g. overlay geometry, web
angles, etc.) then simply percentage increase. It is, I completely
agree, a complex subject which has been written about fairly
extensively. Your recommendation about papers on cable transfer
impedance is also I believe dead on (assuming this was the gist of Mr.
Duvall's question). Dr. L.O. Hoeft has written similar articles the
past 30 years on this subject and if this is the direction of the
question then I think it would be wise to look up his papers or others
of similar vein. However, I was under the impression that the question
was centered around attenuation and not radiation. If I am in error
here, I then stand corrected.
Ted.
----------
From: Brumbaugh, David
[SMTP:[email protected]]
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:[email protected]]
> 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: [email protected]
>
>
> ----------
> 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: [email protected]
>
>
> ----------
> From: [email protected]
[SMTP:[email protected]]
> Sent: Thursday, July 30, 1998 3:13 PM
> To: [email protected]
> 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
>
