From: Chris Padilla (email@example.com)
Date: Thu May 11 2000 - 12:49:22 PDT
Actually, the purpose *it seems* for the 20H rule is to reduce the
reflected energy between planes (power or ground, it does not really
matter). With two planes even, one tends to get more energy reflected
rather than pour out. With uneven adjacent planes, one tends to get
LESS energy reflected because one radiates more.
So now it seems 20H causes more radiated emission from the edges. Is
this necessarily a bad thing? Maybe, maybe not. If your bord edge
is in a good metal enclosure, you might not have anything to worry
I believe I have a case right now where the even-edge of planes and
a ground via fence are causing too much relfected energy that is then
coupling onto my I/O vias and heading outside my chassis. I haven't
proven it yet but I am working on it.
There were questions after the presentation about the percentage of
radiated energy but the speaker had yet to compile that data. Stay
At 09:42 AM, you wrote:
>Thanks for the nice summarization. Please allow me to raise some more
>The intention of 20H rule is to reduce the radiation from the space between
>pwr plane and gnd plane. However, "The simulations presented showed that 20H
>structures actually resulted in more emission at the board edge." Did
>anybody measure the radiation from the board edge to validate the 20H rule
>I would do my best to reduce common mode voltage in pwr/gnd planes when
>designing PCB, and pay less attention to differential mode voltage between
>pwr and gnd planes. Does the radiation from board edge come from CM or DM
>voltage? If DM, does it make main contribution to the radiation of the whole
>Sent: Wednesday, May 10, 2000 9:39 AM
>Subject: RE: [SI-LIST] : May 9th Presentation: "Radiation from Edge
>Effects in Printe...
>Here is what I think I heard at the EMC society presentation. The
>simulations presented showed that 20H structures they examined actually
>resulted in more emission at the board edge. However, the presenters were
>neutral on whether this was detrimental to compliance for the system as a
>whole, pointing out that EMC solutions are rarely universally applicable. I
>hope they post this on the web, I found it most instructive.
>The main points I remember:
>1) Time-varying currents on vias can inject radial TEM-mode waves into the
>space between planes.
>2) The energy thus injected bounces around the cavity volume between the
>planes. The board edge is a discontinuity in the medium and so results in
>partial reflection of the propagating wave and partial transmission, i.e.
>radiation from the board edge.
>3) Fencing the board edge with grounded vias is equivalent to changing the
>PCB-edge discontinuity to a short to ground, so the reflection coefficient
>becomes -1 and all energy is kept inside of the fenced area.
>4) By contrast, a 20H-rule example showed that that structure, which looks a
>little like a patch antenna, allows for more efficient radiation from the
>edge. The exposed area allows a propagation mode where energy can travel
>around the outside edges of the board also. Thus less energy is trapped
>within the board area and more gets radiated.
>5) Is this good? Energy bouncing around between planes can be picked up by
>structures like the one that initially injected it, e.g. vias, and then
>travel along conductors to outside surface components where it can be
>emitted. This is not especially desirable. On the other hand, the more
>efficient radiatiing edge (20H) puts more energy into the system chassis,
>which moves the problem one level higher.
>6) Closely spaced ground vias all across the board had the effect of fencing
>in the injected energy to a small area. This seems to cut radiation from the
>edges drastically. I would like to know more about this particular case.
>The data was obtained from an FDTD simulation of a small board with a single
>off-center via as the point of injection. Excitation was with both a
>continuous sine-wave at 1GHz and also a Gaussian-derivative pulse.
>Dielectric losses were included in the FR4 model. The results of lengthy
>simulations were presented as captivating animations, with color variation
>showing the magnitude of the Poynting vector across the whole board. The
>test setup was of necessity rather artificial but it did help to give a feel
>for the physical behavior underlying radiation from board edges.
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