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

S. Weir (weirsp@a.crl.com)
Sun, 30 May 1999 20:03:59 -0700

Vinu,

It sounds like you are suggesting that the imposition of the remaining
substrate makes a significant difference in the field distribution.
Intuitively, I do not expect that to help a whole lot. It would be
interesting to see if anyone has actually run an FEA analysis on this to
quantify whether a useful amount of shaping does in-fact take place. Have
you run any test cases through a tool?

Regards,

Steve.
At 03:46 PM 5/30/99 -0700, you wrote:
>Grading the dielectric constant is one possibility. The other is to grade
>the dielectric thickness. I think a 20-H rule implementation achieves a
>gradual increase in the effective dielectric thickness at the edge of the
>board.
>
>Vinu
>
>S. Weir wrote:
>
>> 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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