Arlen, first I would try eliminating the ground connection on the external power supply. I'll bet it will still pass class B due to the ground connection through the PC. If so, specify the external power supply to be protection class II (double insulated). Chances are the external power supply is class II construction anyway or can be changed with very little effort.
<---- Begin Forwarded Message ----> From: Arlen Olive <[email protected]> To: "[email protected]" <[email protected]> Cc: "arleno (E-mail)" <[email protected]> Subject: Grounding vs. Ground Loops List-Post: [email protected] Date: Thu, 11 Dec 1997 11:17:40 -0800 Reply-To: Arlen Olive <[email protected]> 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 <---- End Forwarded Message ----> Best Regards, Lou Aiken 27106 Palmetto Drive Orange Beach, AL 36561 USA tel 1 334 981 6786 fax 1 334 981 3054
