You might try capacitively coupling the cable shield at one end. We had some
success doing this at a company I worked at some years ago. We mounted the
connector shell to a copper plane at the component side of a thin PCB. the
bottom plane of the PCB was in contact with the chassis of the unit. The PCB
functioned as a plate capacitor, with several small ceramic capacitors
connected between the planes to increase coupling. It worked quite well in
our application.
I hope this may help
Scott Lacey
Test Engineer
The Foxboro Company
sla...@foxboro.com
-----Original Message-----
From: rbus...@es.com [SMTP:rbus...@es.com]
Sent: Wednesday, November 04, 1998 10:56 AM
To: emc-p...@ieee.org
Subject: Cable Shielding
My company manufacturers image generation equipment (large
mainframe
computer) which is typically placed at a significant
distance from the
display device. These may be as much as 50 to 150 feet
apart. In my EMC
tests, I have determined that due to common mode, harmonic
noise, both
ends of this shielded cable must be terminated to the
respective device
chassis. Unfortunately, this has the potential to create
significant
ground loops which severely compromise the quality of the
video signal.
To add insult to injury, the signals within the cable are
twisted pairs,
differentially driven. This eliminates (as I understand it)
the
possibility of terminating the signal return to chassis
ground at the
point at which the cable exits the enclosure. So, on the
surface it
appears that I have the choice of meeting EMC regulations or
meeting
equipment performance.
My questions are as follows;
1. Is it reasonable to provide single end termination of the
overall
(outside) cable shield?
2. Can differentially driven signal grounds be successfully
grounded to
chassis return? Should they be?
3. Is it reasonable to request or demand that both devices
are sourced
from the same power AND/OR bonded to a common power grid?
4. Is it desirable for signal returns to terminate at the
chassis ground
at the point at which the cable exits the device.
Thanks in advance
Rick Busche
Evans & Sutherland
rbus...@es.com
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