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

ajmani@us.ibm.com
Tue, 25 May 1999 14:57:10 -0600

I too have read the 20-H Rule in the book PCB Design Techniques for EMC
Compliance by Mark Montrose. However, I could not use it because our board
dimensions do not allow us to provide so much clearance space, as it directly
affects the available routing area.

I would think that this rule will be critical if you have standing waves on the
power planes, with high currents in the peripheral regions of the board. Again,
if you have any I/O cable in the vicinity of these high current regions then
these currents can cause common-mode noise radiation from the board. It may be
possible to model this with a good modeling tool. Even in these cases,
placement of edge terminations will help. Istavan Novak had made a presentation
on the subject of edge terminations some time ago, which could be downloaded
from his web site (I do not remember the site address).

Regards,

Ravinder Ajmani
Email: ajmani@us.ibm.com
***************************************************************************
Always do right. This will gratify some people and astonish the rest. ....
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Mark Freeman <msf@stratos.com> on 05/25/99 10:09:37 AM

Please respond to si-list@silab.eng.sun.com

To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
cc: (bcc: Ravinder Ajmani/San Jose/IBM)
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
msf@stratos.com
Stratos Product Development, LLC

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