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

Vinu Arumugham (vinu@cisco.com)
Sat, 29 May 1999 23:23:52 -0700

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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