Max, At what frequency or band of frequencies is the problem occuring?
When you take the video cable off at the system end, the emission does not change? If you remove the video cable and the emission does not change, are there still other cables attached to the system? If there are other cables attached to the system, have you tried removing them one at a time or in groups to see if there is any one or two that are radiating the emissions? Is there video modulation in the signal or does it look like a pure clock emission (narrow band, CW)? How well is the I/O panel of the connector bonded to the system back panel and how well is the D-shell of the connector tied into the return plane on the board. What method is used to isolate analog and digital power and what type of decoupling are you using on the analog power plane? Are there traces that cross the gap between analog and digital power and what is the nearest return path in the board stackup for each of these traces , if any? Are the return paths on a layer immediately adjacent to these signal traces and directly underneath (i.e. a ground plane?). While from your answer, it sounds like you've done a number of things to determine that it is the card/box combo without cables attached, I am doublechecking. Not that this can be fixed through this e-mail forum, but more info is better. It's good to see that you are smart enough to consider a qualified consultant as another resource to bounce the design off of. Thanks, [email protected] ---------- From: Max To: emc-pstc Subject: Need EMC Help With Video Card List-Post: [email protected] Date: Wednesday, August 28, 1996 9:24AM I'm presently having an EMI problem with dot clock emissions from a PC video card. Adding additional ground layers isn't an option since I'm already at the maximum. The return plane is connected to the chassis on this card and the video cables are very good. The emissions are coming right out of the box and not from the monitor or the I/O panel, or cables, etc. All of the signal layers are routed between a power and ground plane. The power plane is, however broken up into a couple of island to accomodate different voltages. All of the unused areas on the signal layers have been filled in with ground pads. The power to the RAMDAC is well isolated and physically separated from the digital signals. The digital and analog grounds are not isolated. The particular RAMDAC being used doesn't facilitate this. I'm only have a problem at one frequency and am presently measuring about 1/2 dB under the limit. I would like to be about 4 to 6 dB under. The PC that I'm using is very good for controlling emissions. The problem is with my card and not the PC. The next step that I am considering is to add guard traces along the signals carrying harmonics of the problem frequency. I'm also thinking of covering a couple of chips with cans. Another possibility is to put capacitor mounting pads at the load ends of the signal lines and try to clean up the signals a little bit. I don't have much faith in this idea though. If I did this it would be on series terminated lines with series resistors. I was thinking of using resistor mounting pads that would also accomodote ferrites and then swapping back and forth between the two. I was also think of isolating the power (with a ferrite and lots of caps) on the chip that is sythesizing the frequency and on the ones that are doing a lot of switching at the dot-clock frequency. I would appreciate any tips or ideas anyone might have. In addition, I would consider opening a PO and paying consulting fees to a PC video card guru. Max [email protected]
