I've been working on a multimedia board in a standalone box
that connects to a PC parallel port. It also has an external
DC power supply, and connections for audio and video
input and output.
To pass FCC class B, I had to:
1. Shield the box with a conductive coating connected to the board
ground
2. Connect the board ground through the DC power supply's DC return to
Earth Ground
3. Add a substantial ground connection between the board ground and
the parallel port connector shell.
Some of my colleagues have expressed concern because this creates a ground loop
between the PC (through the parallel port shield) and the DC power supply Earth
Ground,
not to mention potential ground loops to the audio/video input/output devices.
One suggestion has been to use AC paths (i.e. capacitors) to the board ground
for
everything except the power supply return. I have serious reservations about
this approach,
(I'm convinced it won't pass FCC) but I don't really know what to say about the
ground-loop argument. I'm well aware of the importance of common-point
grounding,
but I haven't found any other way to bring the EMI emissions down to class B
levels.
Any suggestions?
(Directions to any good reference material would be greatly appreciated.)
Arlen Olive
Hardware Engineer
FutureTel, Inc.
1092 East Arques Avenue
Sunnyvale, CA 94086-4601
Phone: 408-522-1472
FAX: 408-522-1439
