Robert,
My experience has been that separating logic ground from the
D-sub shell on a plug-in card is not necessarily a good
idea. When you evaluate this card, please check it not only
on a system you know is a good performer for EMI, but _also_
on one that is no better than acceptable. You will find the
card sensitive to things like placement on the riser card,
impedance of the riser card to motherboard, buss frequencies
on the ground _at_ the video card due to riser card
impedance, placement within the chassis as a driven cavity,
and so on. These problems will follow the card from one
system to another _even if the card is not the source of the
problem frequency_. Video signals are not the only potential
problem, and you need to consider the others as well.
Cheers,
Cortland
Disclaimer: The opinions expressed in this message are my
own, and not necessarily the opinions or policies of my
employer.
______________________________ Reply Separator _________________________________
Subject: PCI Video Card
Author: Robert Terry <[email protected]> at internet
List-Post: [email protected]
Date: 8/29/96 7:52
We are currently in the process of designing a new PCI bus video card. This
card has 1 video D-Sub port. We have seen application notes indicating that
the best way to minimize potential radiated emissions from this port would
be to tie the D-Sub connector shell (chassis) to the logic ground on the
circuit board. Thus minimizing the ground return impedance. However, our
experience has indicated that we perform better (EN55022) when this D-Sub
Shell (chassis) and the logic ground on the PCI card are separated.
I am interested in what you folks think about this particular problem. Any
responses would be appreciated. Thanks in advance for you replies!
Please contact me if you have any questions or comments!
Thank you!
Robert L. Terry
[email protected]
313-994-0591 Ext.235
Fax: 313-994-8408
