You may be heading for an uphill battle.
It will mostly depend on how good the motherboards etc are to start with.
The main problem is going to be how do you know when it is satisfactory? Do
you have spec levels for conducted & radiated emissions or is it just going
to be a case of "No, not good enough yet, try again". Or even worse - "We
have a set in XXXX that is interfering with his locally made radio system.
Your equipment needs to be fixed. Now. " (XXXX being a place very far away,
hard to get to and uncomfortable to live in)

If you do have spec levels to meet, you are going to have to get the beast
tested. This can be an expensive proposition, particularly as you are
likely to need several trys.

And what about susceptability to the RF transmissions from the vehicle? You
don't want it to reset every time they transmit. Again, more expensive
testing.


On the plus side, computers are manufactured to meet emission standards and
hence the addition of ferrite cores to video cables etc over recent years.
Some motherboards offer the ability to dither the clock to produce a
spread-spectrum noise - rather than a single frequency. I would suggest
that you check if this is available as it can improve (measured) output
noise by ~10dB or so. (The noise is still there but the average over the
measurement bandwidth is lower).

Add-in filters to the power cables, video, keyboard etc lines may be a
proposition but require careful layout and well specified components. And
more testing.

EMC is more of an art than a science and what works in one situation can be
useless in others. My advice would be to talk to  someone with as much
experiance in the field as possible. Then design,  build, test, get more
advice and repeat. It will cost but not as much as trying to start from
scratch.

As a starting point, try to get a firm specification sorted. This needs to
include  frequency ranges.  "Radiating almost no RF " will trip you up
sooner or later.

Sorry I can't be more encouraging - Best of Luck!

Richard P




This is not really a PIC question, but I really need some quick pointers
so apologies for a slightly OT posting...

I have just been asked to put together a design for an In-Car PC system
to go into emergency service vehicles, and which must radiate almost no
RF. I'm planning to go for a Mini-ITX form factor motherboard (the
CL1000 mobo) and a battery backed 12v PSU / UPS controlled by a PIC.

The whole caboodle will go into a custom made metal enclosure with a lot
of attention to RF shielding, to ensure that no RF leaks out of the vent
holes etc. I'm worried about how to prevent RF from escaping out of the
Kbd, VGA, Serial etc cables and was considering a 'filter-pcb' to filter
all communications to the outside world.

I know absolutely zilch about RF / EMC issues, so if anyone can point me
towards any URL's where I can learn the tricks of the trade, or just
advise what works and what doesn't, that would be great.

Thanks, Peter Moreton

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