🚀 go-pugleaf

----Munching line----

Hi,

I used to make tempest-proof things for a living.  It sure was a
heck of a lot easier to make things clean when it was designed in
from the ground up.  Ground planes on the P.C. board and the
like...

The most difficult part is attempting to prevent the EMI from
escaping through the holes in the case.  Obviously, putting the
whole thing in a steel box is pretty effective at stopping
everthing except very low frequency magetics, but it doesn't do a
lot of good for being able to view the CRT.  Connectors are also a
very nasty proposition, as they allow both conducted and radiated
noise to escape, and filtered connectors are enormously expensive.
I don't know what was inside, but even little 5 pin connectors cost
as much as $100.  A few of those replacing $0.29 DB-9s and you can
imagine what happens to the price.  Screen mesh fronts for CRT
faces are also pretty expensive-- at least for the ones that really
do stop RF.

Computer keyboards are also notorious, and offer a lot of surface
holes for emission.  A typical solution is to coat the whole set of
keys with conductive plastic, which is useful if the thing is going
to be used in a jungle any way.  One interesting thing is a
computer terminal I used, whose main box was pretty much a stock
HP 2645 with conductive piant on the inside of the case; the
keyboard, however, was a big block of steel with the regular
keyboard inside, and the key caps on metal toothpicks sticking in
the press the original buttons.

Floppy disk drives can also be quite nasty.  They're usually dealt
with by putting them behind a metal door which can only be opened
when the equipment is off.  RF-proof air filters can also be pretty
exciting.

I can't understate how much easier it is to work with things that
have been designed from the ground up to be electrically quiet.
Keeping noise down in equipment is a fine art that begins with a
lot of science.  A good case in point are apple computers.  I have
an early model 2+ that will wipe out a TV clear across the house; I
also have a 2e that can sit right next to the TV with negligable
effect.  The newer case design and PC are really simpler than the
old.

Bill
N. E. Ohio Universities College of Medicine
Division of Basic Medical Sciences