**From:** *[email protected]*

**Date:** Fri Mar 31 2000 - 16:19:29 PST

**Next message:**[email protected]: "Re: [SI-LIST] : Coax connection to a CPW guide"**Previous message:**Ray Waugh: "RE: [SI-LIST] : Coax connection to a CPW guide"**Maybe in reply to:**Mary: "[SI-LIST] : 20-H Rule and Self-Resonant Frequency of Power Planes"**Next in thread:**Dr. Edward P. Sayre: "Re: [SI-LIST] : 20-H Rule and Self-Resonant Frequency of PowerPlanes"**Reply:**Dr. Edward P. Sayre: "Re: [SI-LIST] : 20-H Rule and Self-Resonant Frequency of PowerPlanes"

Response to SI inquiry of 3/30/00, Re: [SI-LIST] : 20-H Rule and

Self-Resonant Frequency of Power Planes

Sorry, guys, although I'm always open to newfound knowledge, my first

reaction to raising the PCB self-resonance by a factor of two to three times

is "horse pucky!"

Assuming a 12-inch PCB, the only possible support I see for the claim in

Mark's book would be the observance of surface/external waves (using air as

the dielectric) which would exhibit a fundamental frequency of about 492 MHz,

and support higher harmonics of same. Or perhaps a unique situation where a

surrounding enclosure reflected radiated emissions at a higher harmonic of

the PCB and added to the resonance strength. Let's look at two approaches...

First:

Consider a 12-inch PCB (i.e., its longest dimension). For c = 2.998E8 m/s =

9.836E8 ft/s and FR-4 with a relative dielectric constant of 4.5, the basic

fundamental resonance of a pair of ground/power planes with interplanar

excitation would be 231.8 MHz (ignoring any edge setback of the planes).

When the PCB is stuffed with components at reasonable density, the added

capacitance of the many necessary vias alone reduces the board resonances by

a factor of two to four based on the added interplanar distributed

capacitance. So the example PCB would tend to exhibit a fundamental (half

wave) resonance in the 60 MHz to 120 MHz range, plus at harmonics of the

fundamental frequency. I have observed this phenomenon in over 40 different

designs from as many vendors over the last decade.

When one considers the influence of the (typical) myriad of 0.1 UF bypass

capacitors, their distributed capacitance impact is primarily observed below

200 MHz because of the limiting effects of self-resonance. In other words,

the reactance of their connecting inductance becomes sufficiently high to

isolate their effect from the interplane waves. Therefore, the capacitors

become only a secondary consideration on the effective Er of the dielectric

above 200 MHz.

The surface waves I alluded to earlier would treat the plane pair as a single

conducting plate; hence, the potential for exhibiting higher frequency

resonances independent of the dielectric between the planes.

Second:

Basic transmission line theory demonstrates that when the line length is

shorter than 1/4 wavelength, a shorted line will exhibit inductive reactance

(varies with frequency and length of course) and an open line will exhibit a

capacitive reactance (which I've used in many RF designs). By the same

effect, adding capacitance to an open line will make it appear longer. If we

cut back on the power plane at the edge of a PCB, the length is shortened

which will raise the self-resonant frequency of that plane a bit. However,

the fringing capacitance now added by the ground plane being longer than the

power plane will make the shortened power plane look longer. Hence, the

effective cutback (power plane length) is actually less than the magnitude of

the physical cutback, and the PCB resonances will be minimally affected.

BOTTOM LINE:

I'm either missing something or Mark is quoting a special EMC circumstance

like we have all seen and scratched our heads at.

Michael L. Conn

Owner/Principal Consultant

Mikon Consulting

*** Serving Your Needs with Technical Excellence ***

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**Next message:**[email protected]: "Re: [SI-LIST] : Coax connection to a CPW guide"**Previous message:**Ray Waugh: "RE: [SI-LIST] : Coax connection to a CPW guide"**Maybe in reply to:**Mary: "[SI-LIST] : 20-H Rule and Self-Resonant Frequency of Power Planes"**Next in thread:**Dr. Edward P. Sayre: "Re: [SI-LIST] : 20-H Rule and Self-Resonant Frequency of PowerPlanes"**Reply:**Dr. Edward P. Sayre: "Re: [SI-LIST] : 20-H Rule and Self-Resonant Frequency of PowerPlanes"

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