> From: 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?
What type of signals are you using? Analog, digital, hybrid? Edge rates? Freqs, clocks... Could you discuss a little more about the actual circuit layout and board construction? Analog and digital on one board? Multi-layer board, stackup, controlled impedance ...
