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

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From: Dr. Edward P. Sayre (esayre@nesa.com)
Date: Mon Apr 10 2000 - 11:19:21 PDT


Mike:

I agree with you. This inter-plane stuff makes no sense to me; neither
does Mark.

ed

======
At 07:19 PM 3/31/00 EST, you wrote:
>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
>
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