[SI-LIST] : Electromagnetic shielding

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From: Tmtokar@ra.rockwell.com
Date: Wed Nov 15 2000 - 04:17:12 PST


Everyone,

I haven't seen a recent discussion of electromagnetic sheilding on this
list so I have
the following request.

I'm working on a design involving an ASIC with an 80 MHz clock and SDRAM.
My
concern is electromagnetic radiation. Because of constraints, I can't use
spread
spectrum techniques and the product won't go into a metal enclosure. I'd
like to
enclose a portion of the circuitry (the ASIC and SDRAM) under a small metal
cover
mounted to the circuit board. I'm doing this in anticipation of radiation
problems.
I don't have earth ground directly available so I suppose the cover will be
connected to digital ground.

Does anyone have (or know of any ) guidelines for using a cover like this
to restrict
electromagnetic radiation? I need answers like 1. how close to the board
does the
cover need to be, 2. how many ground contacts are needed around the
perimeter
of the cover and 3. what special considerations are there for the PCB
directly under
the edges of the cover?

I've looked in a few book references but don't see a treatment of this
subject.

Any help is greatly appreciated.

Thanks,

Tom Tokar
Rockwell Automation

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