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

Mellitz, Richard ([email protected])
Tue, 25 May 1999 12:48:01 -0700

I found stitched picture framing works better. It cuts near field edge
radiation. 20 H cuts to much in to routing area. 20 * 25 mils is 0.5 inch!
Richard Mellitz
Intel

-----Original Message-----
From: Mark Freeman [mailto:[email protected]]
Sent: Tuesday, May 25, 1999 1:10 PM
To: '[email protected]'
Subject: [SI-LIST] : 20-H Rule for Power Planes

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
[email protected]
Stratos Product Development, LLC

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