> So a PIC development board with a 20MHZ ceramic resonator may be a problem
> with the FCC?   What is the solution?  Put the thing in a metal box?  Are
> there tricks to keeping the radiation down such as using chokes, trace
> length etc.?  Are metal can crystals less noisy than ceramic
> resonators?  What type of device would a stray 20MHZ signal interfere?
Ok
> enough with the 20 questions :)

EMI issues can get pretty tricky and require a lot more than 20 questions to
understand.

You have to think about everything in analog, and every wire and trace as an
antenna, including the shields of cables connected to your device.  There
are no easy answers, but a few basic strategies that should help.  A board
with a ground plane is probably the single most effective way to reduce EMI.
That way all the signal currents have an immediate low impedence return path
with a small loop cross section.  Shielding, if done right, is also very
effective although expensive.  This is usually considered the "last resort"
approach.  Note that some conducting boxes can actually cause problems by
creating slot antennas or resonant cavities.  You also have to make sure the
there is no current thru the shielding.  In other words, it is connected at
one point to the ground, preferably right where the external connections
enter the box.  Also watch external ground loops very carfully.  The list
goes on and on and on ...

If you're not comfortable with RF and the regulatory minefields, you should
consider bringing in a cosultant for those issues.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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