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

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From: Knighten, Jim L (JK100005@exchange.SanDiegoCA.NCR.COM)
Date: Thu Mar 30 2000 - 13:59:16 PST


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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