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

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From: [email protected]
Date: Fri Apr 21 2000 - 12:56:37 PDT

In a message dated 4/21/00 10:16:54 AM Pacific Daylight Time,
[email protected] writes:


<< Concerning radiation from the edge in absence of any other traces/metal
 besides the pair of planes:
 It sounds like you're saying that one of the launching antennae (and perhaps
 the dominant when no other traces/metal are present) is really ALONG the
 edges of the plates rather than ACROSS the edges of the plates. Is this
 interpretation correct?>>

Yes, but with many caveats. Key factors include WHERE along the edge the
wave first impinges, what FREQUENCIES are contained in the wave, what the PCB
RESONANCES are along that edge (both in the dielectric and on the outer
surface of the copper), whether OTHER REFLECTIONS of the offending wave
source hit the edge in question in a reinforcement or destructive
interference manner, etc. We are generally concerned with those excitations
that contain harmonics at the resonant frequencies of the PCB as a universal,
all encompassing analysis would be a nightmare. If the offending wave DOES
excite a PCB resonance and depending on the consistency of the source of the
wave, a standing wave will be observed on the PCB edge and a peak in radiated
emissions can be expected.

<< It seems the 20H rule would have little impact on this mechanism involving
 transfer from ACROSS plates to ALONG edges. True or false?>>

False. The energy flowing along the edge will generate fields that will seek
a return. The adjacent dielectric and extended ground plane will provide the
preferred path for concentrating (by means of an Er higher than air) and
terminating (on the ground plane) those fields that (if the setback were not
there) would be launched into the surrounding ether/air. Therefore, the use
of a power plane setback (20H or other) from the ground plane will still be a
positive field interception mechanism.
As you noted, this is a multifaceted subject that IS complicated, even for
seemingly simple cases. The many contributing factors (including different
physical PCB constructs) and their interactions contributes to the confusion
on this subject.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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