From: Scott McMorrow ([email protected])
Date: Wed Feb 09 2000 - 08:42:28 PST
I agree with Lee. Having split ground planes is a disaster for
emi. I would suggest that you do the following:
1) unify the ground planes where possible.
1b) if not possible, thou shalt never allow a signal to cross a ground plane
2) provide ample via stitching between the layer 2 and layer 7 ground planes
3) at clock signal transistions from the top layer to the bottom layer
place ground stitch vias around 4 corners of the layer transtition via
(this will close the ac return loop and keep clock excited plane waves from
propagating from between the power and ground planes.)
4) Series terminate the clock drivers to the characterisitic impedance of the
5) Be absolutely sure that all areas of the board contain decoupling capacitors.
for each power plane that needs to be decoupled.
Your most likely source of emissions is due to clock layer transitions from
the top layer to the bottom layer. These transitions will cause parallel
plate mode propagation to occur when partial mode conversion of the
TEM wave occurs while passing through the vias on inner layers. Parallel
plate mode waves then propagate out circularly from the via and most likely
are radiating at the slot created in the ground at the analog/digital splits, or
out the end of your board.
-- Scott McMorrow Principal Engineer SiQual, Signal Quality Engineering 18735 SW Boones Ferry Road Tualatin, OR 97062-3090 (503) 885-1231 http://www.siqual.com
Lee Ritchey wrote:
> Put the ground planes back together. > > Gary Steinkogler wrote: > > > I went to a Certification/EMI House to find out where/why we were failing > > radiated emissions. There are 13 cables attached to my system - some > > shielded and some not. All the cables were hot - especially the video and > > audio cables.. After testing we determined it was our grounds on the pc > > board. We have 2 ground planes, with one of them being split, digital > > ground, audio gound, and video ground. The other ground is digital ground. > > You can view my system at www.dmida.com. Any type of filtering did > > nothing to reduce the emissions. I do not have a simulation package. Any > > suggestions? > > >
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