%>
%>
%>Max,
%>
%>At what frequency or band of frequencies is the problem occuring?
Our video clock operates at 126 Mhz. The frequency synthesizer seems to be
deriving this from a 31.5 Mhz clock, so it produces harmonics at 31.5,
63.0, 95.5, 126.0, 157.5, 189.0 220.5, 252,0, 283.5, 315.0, 346.5, etc.
The only frequency that I am having problems with right now is 189.0.
However, this is an interim version of the card. So, presumably, different
harmonics could pop up on the next version.
%>
%>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?
I still have the problem when I remove all of the cables from the system,
including the printer cable, etc. The emissions are coming out of the
holes in the PC cabinet and this is a very good PC and is well sealed. The
largest emissions seem to be coming from the fan vent holes.
%>
%>Is there video modulation in the signal or does it look like a pure
%>clock emission (narrow band, CW)?
Narrow band.
%>
%>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.
I checked this yesterday as the result of someone elses suggestion.
Everything is properly bonded and well bonded. This particular PC also has
EMI gasketing where the card panels mate with the cabinet panel.
%>
%>What method is used to isolate analog and digital power and what
%>type of decoupling are you using on the analog power plane?
A series ferrite with one 33uF and two .01uF and two .1uF caps on the chip
side and about fourteen .01 and .1 uF caps on the otherside of the ferrite.
Come to think of it, I wonder If I should have at least one larger cap in
there--like maybe a 330uf tantalum for instance?
%>
%>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?).
The stackup is:
S
S
P
S
S
G
S
S
And the power plane is broken up into two different voltage islands. So
the signal layers on the power plane side of the board have a long ways to
go to find a return path. I suspect that this is my problem. One idea
might be to put ground traces on the signal layers where the two islands
meet?
%>
%>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.
I presently have a dozen different ideas in the fire and little time to
devote to this, so I don't mind paying a good consultant. I have picked up
some good ideas from this group, though. The ones that seem most
reasonable to me are to put an RC filter on the clock lines and to try to
figure out a way to provide a better return for the clock and signal lines.
%>
%>Thanks,
%>[email protected]
Max
[email protected]
