From: Ken Cantrell ([email protected])
Date: Thu Nov 16 2000 - 15:05:49 PST
Tom,
Are you battery powered? Answers will be slightly different for a mobile
product. Assuming you are not (the easier case), you can either reflow for
a continuous shield, or stitch at the correct aspect ratio. I prefer the
former. Pressure fits are trouble prone, and fatigue over time. You have to
be connected to earth ground (virtual ground in the battery powered case),
and that ground has to have enough surface area (low inductance)to damp the
high frequency currents. The dimensions of the can cannot be at a resonant
frequency (major diagonal,i.e. slot radiator) which can often be overlooked
until the end. Minimization of current loop areas are imperative, and cut
your current down as much as possible. Provide a low inductance path to
ground. Keep adding surface area to your ground interfaces, i.e., bigger
vias, ground pads, connectors (also increase ground interleaving with signal
pins), and any thing else you can think of. Concentrate on the point where
the pin(s)hit ground. That has to be a big enough pool to allow laminar
current flow at that junction, and that junction has to be as close to the
pin as you can get it. At some magical point the tron flow will not be
turbulent, and your problems will dissapear. It's almost digital when you
cross the threshold. Now you see it, now you don't.
Ken
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of
[email protected]
Sent: Wednesday, November 15, 2000 5:17 AM
To: [email protected]
Subject: [SI-LIST] : Electromagnetic shielding
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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