[sdiy] CE approval, was MOTM over time

[Jay Schwichtenberg]

Paul,

First off I've enjoyed your WEB site, thanks.

Second as I sit here typing this the UPS guys shows up with my DigiKey order
with all sorts of synth parts. COOL!

What they are really looking for is signals above ~50 mHz if I remember
right (it's been about 2 years now). There are 2 commmercal companies here
that do testing.

First you start out in a closed chamber that is EMI sealled both from the
inside and outside. It is a metal faraday cage with conductive foam that is
shaped like pyramids all around it. This keeps things from reflecting inside
the cage and forming EMI hot spots. With all this mobile comm stuff even in
a sealed room like this they can still have problems. A good techician can
look at the plots and tell if a cell phone is the problem or not. They have
turntables that rotate 180 degrees one way and then 180 the other. You place
your gear on a table and do a spin. Antennas in the room gather the EMI.
It's ran to a pre-amp, then to a spectrum analyzer with a computer taking
the data from the spectrum analyzer. The computer then prints out a report
on what the spectrum is, usually with indicators on what the acceptable
level is. They do this with the antennas both horizontal and vertical. If
you fail the test then you have to do the song and dance to get your product
to work. When you are below 4 to 6 db below the limits you do an "open field
test" to actually get the certification. The one time I did this phase of
testing it was literally open field. There was a large canvas tent set up in
a secluded gully in the Oregon coast range faceing the ocean. The test was
pretty much the same.

The people we dealt with would charge you by the hour. Depending on how busy
they were it would be $75 to $100 an hour. A minimal session for us would be
2 hours but we had a fair amount of setup time.

Being in the PC cards will pick up every thing inside the PC and radiate it
out the connectors or the backplate. Details of how grounding is done and
other things become extermly critical. Your project may not be as sensitive.
Here are some suggestions that might help:

Look at some products that have simular electronics to yours.
1) Are they in metal or plastic coated metal.
2) What do they have for connectors and how are they mounted to the case or
PCB.
3) Look at the PCBs and pay particular attention to the grounding on them.
Grounding is a very critical part of getting through EMI testing. If you are
doing high end audio and EMI testing they also seem to be mutualy exclusive.

Be generous with your bypass caps on the chips. Get them as close as
possible to the IC pins. Also run the power to the cap then to the pin. DO
NOT run power to the pin and then the cap! This forms little antennas that
radiate EMI.

Put EMI filters/chokes on the PS inputs.

If you have high freq. clocks running around put series resistors at the
clock source before it goes anywhere or is used. I think the run of thumb
here is less than 100 ohms, we used 33 ohms.

Hopes this helps.
Jay