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

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From: John Howard (jhoward@emcguru.com)
Date: Thu Mar 30 2000 - 06:39:57 PST


Jim & John,
Agreed! Isn't this how we distinguish science from myth?
John Howard
EMC Consultant

"Knighten, Jim L" wrote:

> John,
>
> You have put your finger on one of the negative features of this book - -
> rules without any backup or guidance as to the physics behind it. Not very
> helpful advice, is it?
>
> Jim
> ________________________________________________________
> Dr. Jim Knighten e-mail: jim.knighten@ncr.com
> <jim.knighten@ncr.com>
> Technical Consultant - Design
> NCR
> 17095 Via del Campo
> San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com>
> Tel: 858-485-2537
> Fax: 858-485-3788
>
> -----Original Message-----
> From: jrbarnes@lexmark.com [mailto:jrbarnes@lexmark.com]
> Sent: Thursday, March 30, 2000 5:06 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : 20-H Rule and Self-Resonant
> Frequency of Power Planes
>
> SI-LIST'ers,
> Page 54 of Mark I. Montrose's book "Printed Circuit Board
> Design Techniques for
> EMC Compliance" (IEEE Press, 1996) has the following
> statement:
> " Many multilayer PCBs generally have a self-resonant
> frequency between 200 and
> 400MHz. Use of the 20-H rule (see section 2.2) may increase
> the self-resonant
> frequency by a factor of 2 to 3."
>
> No where else have I found a mention of this effect.
> Following the 20-H rule
> on a board with 0.010" spacing between layers would amount
> to making the power
> plane 0.4" smaller in length and width than the ground
> plane. I don't see how
> such a small change in dimensions could cause such a huge
> reduction in
> inductance*capacitance that the second sentence suggests.
> Or could there be
> some kind of impedance matching because of the difference in
> the fringe fields
> at the edge of the power planes?
>
> Have any of you observed this effect, in measuring actual
> boards or in
> modelling? Thanks!
>
> John
> Barnes Advisory
> Engineer
> Lexmark
> International
>
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