Re: [SI-LIST] : 20-H Rule for Power Planes

Michael E Vrbanac (mvrbanac@cisco.com)
Fri, 28 May 1999 17:37:41 -0700 (PDT)

I second all that, John and Todd.

I think you have said it much better than I!

The radiation problem seems to require two necessary conditions:
1. a fringing field that is set up between two planes
2. a conductor placed in the "near field" to that fringing field
which has access to a susceptible circuit or can carry the energy
outside the system to "radiate".

Without those two things together.... "nothing happens" (and could
be a valid reason why some folks haven't seen it before).

As Todd noted (and others and I agree with), in the "two plane model",
the 20H rule does not stop the "power" plane's ability to radiate.

Additional "ground" planes provide better field capture to the problem
presented by the "two plane model" (when the "power" plane is between
the "ground planes" and inset by 20H the distance to the nearest
plane). This reduces the probability for significant near-field
coupling to ANY nearby conductors. This is easily implemented in
multi-layer stackups.

As far as the "un-balanced" question somebody raised earlier, I
haven't thought about that much but perhaps the additional ground
planes would help "rebalance" the things compared to the two plane
model.... any thoughts?

To me, the 20H rule is a "field manipulation technique", not a "source
suppresion technique" .... as the field not particularly "reduced" but
more or less moved or reoriented. I usually do the source suppression
stuff first to try to eliminate the problem but I won't overlook an
opportunity to use field manipulation if I need to.

Michael E. Vrbanac

>
> Thanks Todd,
>
> I think you may have put your finger on the source of the confusion here.
> There was a lot of talk of reducing emissions by shrinking the power planes
> slightly. That may have created the impression that emissions directly from
> the power planes were reduced rather than reducing the power plane coupling
> to something else that was radiating. In the products where I have had
> problems that caused me to cut back the power planes it was always coupling
> to a trace near the edge of the power plane, or the power plane at the
> board edge coupling to part of the chassis or an air vent. These cases
> always involved small distances (fractions of inches).
>
> John Lockwood
> Juniper Networks
>
> At 09:09 AM 5/28/99 -0700, you wrote:
> >Wow! What an interesting discussion! Since we have recently been
> >investigating this issue, I can't resist adding my own 2 cents worth.
> >
> >First of all, the 20-H rule was developed years ago, before radiation
> >directly from the power planes was a common problem. As a couple of
> >people
> >have pointed out, pulling the power plane in away from the edge of the
> >board
> >reduces near-field coupling to other boards, cables, or the enclosure.
> >This
> >can be a very good thing, because it keeps energy from coupling to the
> >things that may be good antennas.
> >
> >However, in a board with only 1 return plane, pulling the power plane in
> >away from the edge of the board does not reduce the power bus
> >structure's
> >ability to radiate. In fact, slightly more power can be radiated when
> >the
> >power and ground planes are not of equal size. (I liked Larry Smith's
> >intuitive remarks regarding the loss of balance.)
> >
> >Placing a ring of return trace around the perimeter of a board and
> >stitching
> >it to the return plane also does not reduce radiation directly from the
> >power bus. This is something we have experimented with in our lab. The
> >gap
> >between the power plane and the return ring becomes the new "edge" and
> >radiates just as effectively.
> >
> >I am not saying the 20-H rule is not a good idea. It can be very
> >effective
> >at eliminating EMI problems resulting from near field coupling off the
> >edge
> >of the board. However, it does not generally reduce EMI at power bus
> >resonant frequencies by making the power bus a less efficient radiator.
> >
> >Todd Hubing
> >University of Missouri-Rolla
> >
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