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

Mark Freeman (msf@stratos.com)
Tue, 25 May 1999 10:09:37 -0700

Now and again I come across references to the "20-H Rule" for reducing =
radiation from power planes. This rule states that the power plane =
should be smaller than the ground plane; The power plane edges should be =
back from the power plane a distance of 20-times the plane spacing. =
This reduces fringing fields from the power plane and reduces coupling =
to adjacent planes and free space.

Best I can tell, this rule originated with Mike King. The earliest =
reference I found is Mark Montrose's "Printed Circuit Board Design =
Techniques for EMC Compliance," pg. 26. I have not found any numbers - =
analytical, simulation or measurement - which indicate the effectiveness =
of this technique over frequency. Intuition (a dangerous thing for this =
digital designer to rely upon) tells me that the dimensions of the =
fringing fields are small, thus only affecting GHz-range signals. Is =
this technique currently only of interest to cell 'phone designers, or =
do we need to begin applying this technique to digital PBW design?

Mark Freeman
msf@stratos.com
Stratos Product Development, LLC

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