The military world has always specified that functional ground and
chassis ground be isolated. The minimum resistance there is 1Mohm. The
best solution tends to be the use of a chassis ground layer (instead of,
or in addition to, circuit ground planes) on each board. This becomes
the high frequency noise return that can be 'tacked down' at as many
places as possible. Unfortunately, it is an expensive approach, albeit
effective.
The other approaches include transmission line design and controlled
impedance boards, or, in the case of large chassis, the shotgun approach
of enclosure design and filtering. Because you don't have complete
control of the final design ("The chassis will have 7 slots for plug in
cards
that can be provided by anyone."), you will probably have to do a little
of all of the above.
Bob Martin
ITS-Boxborough
r...@itsqs.com
----------
From: je...@ibus.com
To: emc-p...@majordomo.ieee.org
Subject: CompactPCI
List-Post: emc-pstc@listserv.ieee.org
Date: Monday, January 26, 1998 12:12PM
I am seeking advise and comments concerning EMC testing and approval of
CompactPCI chassis.
My background is in personal computers and similar ITE equipment. The
current practice is to tie logic ground and chassis ground together as
often as possible. Many circuit boards tie logic ground and chassis
ground
together at every point where the board comes in contact with the
chassis.
The traditional notion of single point grounding has been shown time and
again to be inadequate for preventing radiation in systems that are
running
frequencies of 100 MHz or faster.
My concern is for the portion of the CompactPCI specification that
requires
that logic ground and chassis ( Frame ) ground be isolated from one
another. Frame ground and logic ground can be tied together at only one
point, the power supply. The specification even gives a value of
9M Ohms of impedance between logic and frame ground ( measured with a
100V
DC source ).
We are working on a CompactPCI chassis that will include; passive
backplane, processor board, I/O companion board and power supply that
will
plug into the backplane. The chassis will have 7 slots for plug in cards
that can be provided by anyone.
Has anyone worked with a CompactPCI chassis built to this specification?
Has anyone worked with similar constructions that employ similar
grounding
schemes? What problems can the single point grounding cause? Any
comments
or suggestions would be appreciated.
Thanks in advance for your input.
Jeff Busch
Compliance Engineer je...@ibus.com
I-Bus 619-974-8470
San Diego 619-268-7863 fax
