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

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From: [email protected]
Date: Wed Apr 19 2000 - 15:49:25 PDT

Mary and Jim:

I was waiting to see some further questions about the "clarification" on this
topic presented a few days back. There are several statements in the
clarification that add to the confusion. There are perhaps THREE fundamental
effects going on that are being erroneously integrated and (IMHO) leading to
more confusion.

FIRST, the setback of a power plane from the edge of a nearby ground plane is
an intuitively obvious method to contain EM energy propagating between the
two planes. When propagating waves (picture TEM) hit an unfettered edge,
electric fields will literally spray into the surrounding ether and attempt
to seek out a return reference. Although an inefficient process for this
condition, significant (relative to allowable limits) radiation is the
typical result. Setting the edge of the power plane (selected because it is
typically the noisiest and most isolated from chassis or earth reference)
back by a respectable ratio to the dielectric thickness between the planes
(20H is arbitrary, but easy to remember) allows the electric fields hitting
the plane edge a considerably better picture. Specifically, the sprayed
fields immediately see a nearby ground reference plane on which to terminate.
 Not only that, but the concentrating factor of the relative dielectric
constant of the dielectric (which must also be continued beyond the power
plane edge for greatest effectivity) literally sucks up the potentially
radiating field lines and captures/contains them. This method of field
interception (and therefore containment) is particularly effective at higher
frequencies. An excellent verification of this effect was published in the
Motorola MECL System Design Handbook (one of the true "bibles" of signal
integrity engineering) way back in 1980 where it was demonstrated that the Zo
of a microstrip line over a limited width ground trace was essentially
identical to using an infinite ground plane if the ground reference trace was
>3 times the signal trace width. (For completeness, this finding only
applies for signal trace widths greater than the dielectric thickness between
the two traces.)

Therefore, the recessed edge of the power plane stands on its own merit as a
simple, viable method of reducing the EM radiation launched from a
non-recessed planar construction. It's a good thing.

SECOND, the INTERPLANAR SRF (self-resonant frequency) of the
dielectric-loaded structure will be (because of distributed bypass capacitors
and component pin parasitic capacitances) much lower than the simple
calculation using 1/(sqrt Er) and the physical dimensions of the printed
circuit board (PCB). No matter what one does with a few tens of mils around
the outer edge of the power plane, it will have little affect on the
interplanar SRF. Enough said.

THIRD, invariably there will be some EM energy coupling to the SURFACE of a
reasonably stuffed PCB structure. This effect is accomplished by parasitic
leakage from component leads, connector structures that rise from the PCB
(either sockets or pins), surface traces, etc. This energy will propagate
associated currents (including harmonics) on the planes nearest to the
surface of the PCB. Because the surface dielectric material (only soldermask
in some cases) is thin, and interfaces directly to air (not another plane or
conducting medium), the surface dielectric has negligible affect on the
magnetic fields that are created in the air adjacent to the PCB surface.
Excitation of these resonances reasonably accurately based on the physical
dimensions of the PCB excluding dielectric effects, and significant radiation
efficiencies can be achieved. Note that this SURFACE SRF will NOT be
directly loaded/lowered by bypass capacitors and may behave more like a solid
planar conductor referenced to earth ground. This condition is similar to
surface currents flowing on cable shields, which are typically excited by
just such a mechanism... we call it common mode excitation... and we pay the
penalty for it.

BOTTOM LINE: EMC/EMI is a messy, complicated science subject to much
interpretation (and confusion). The above is my partial take on the
apparently mixed subjects discussed under this one topic.

I hope everyone has a great three-day weekend.



Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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