RE: [SI-LIST] : 20-H Rule for Power Planes

Douglas McKean (dmckean@corp.auspex.com)
Fri, 28 May 1999 13:04:22 -0700

At 09:09 AM 5/28/99 -0700, Todd Hubing wrote:
>
>I am not saying the 20-H rule is not a good idea. It can be very
>effective
>at eliminating EMI problems resulting from near field coupling off the
>edge
>of the board. However, it does not generally reduce EMI at power bus
>resonant frequencies by making the power bus a less efficient radiator.

I would like to underscore the comments from Dr. Hubing.

I have had to work with the near field coupling from the
edge of boards not using the 20-H Rule and it was quite
frustrating to say the least. It does happen and it can
be a nuisance. The specific details I am prohibited
from discussing. Anecdotal information proves nothing,
so take it as you wish.

Reduction of the power bus efficiency as a radiator is
also very true. One thing though that I've wondered
is yet another mechanism heretofore unmentioned - the
edged effect as far as plane inductance is concerned.

As circulating return currents move away from the edge
of the ground plane, i.e. the edge of the board, so too
does the inductance of the plane and thus the inductance
seen by those return currents. Intuitively speaking, the
inductance of the ground plane as *seen* by a trace in
the center of the board should be half the inductance
as seen by a trace at the edge of the board. Moving
the edge of the power plane away from edge of its
associated ground should lower the effective inductance
as seen by return currents in the ground plane and
thus the noise level ... in an ideal world of course.

Yet, another possible side effect is the slot effect.
Moving the edge of the power back from the edge of
the ground plane makes the *slot* between the power
plane and ground plane less well defined. And
hopefully less resonant.

There's several *possible* mechanisms at work here.

Strictly my hypothetical opinion of course ...

Regards, Doug McKean

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