Re: [SI-LIST] : 20-H Rule and Self-Resonant Frequency of Power Pl anes

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From: rbmccammon@mmm.com
Date: Thu Apr 20 2000 - 10:33:01 PDT


I also have been waiting for clarification.
It being lacking, I'll speculate.

1. I think we can assume that the wave lengths are much
smaller than the separation between the planes. Thus
a TEM wave reaching an un setback edge would produce an
evanescent field at the edge that would not radiate if
it didn't couple to something. Setting back one edge
probably encourages more power to be radiated and
coupled. This would agree with the previous posting
that said the set back decreased resonant effects.
Resonant effects would be decreased by improving the
"match" at the edge.

2. Increased coupling from the edge to the external
world sounds like a bad idea. But, resonant circuits
can match impedances. So, by improving the match at
the "output side" of the ground plane, and spoiling the
resonance, you can keep less noise from being pumped
into the ground plane in the first place. That's good,
because ultimately, that noise has to go somewhere. It
is either absorbed as heat, or it radiates.

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