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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