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

S. Weir (weirsp@a.crl.com)
Sun, 30 May 1999 02:08:09 -0700

I don't think so. The geometry of the planes themselves still has the
abrupt edge. All we have done is to move the center point of the fringing
pattern so that the field gradients across any susceptor antennae, (
traces, metalwork ) near the board edge is less. To do what you suggest, I
believe that we would need a material where we could grade the dielectric
constant. Does anyone feel differently?

Steve
At 11:23 PM 5/29/99 -0700, you wrote:
>Can the implementation of the 20-H rule not be viewed as replacing an abrupt
>impedance discontinuity at the edge of the board, with a gradual
>discontinuity? If so, it will have the effect of "smearing" the resonant
>peaks and reduce
>the radiated power at any given frequency.
>
>Vinu
>
>Michael E Vrbanac wrote:
>
>> 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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