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

Bill Dempsey (bdempsey@dnaent.com)
Wed, 26 May 1999 12:10:29 -0500

Michael,

What test data do have from either personal or company experiments
to substantiate the 20H rule that you stated so eloquently? Have you
been able to conclusively relatea 20H design to EMC compliance and a
non-20H design failing?

Until then I must agree with the nay sayers as I have never been able
to "see" an EMC difference. Of course I'm only running 1 GHz edge
rates and am not looking above that point...

Regards,

Bill Dempsey
DNA Enterprises, Inc.

Michael E Vrbanac wrote:
>
> Mark,
>
> Essentially the 20H rule derives from Gauss' Law for the Electric
> Field which simply states "the net electric flux density leaving an
> object is determined by its volumetric electric free charge density on
> it surface". I am sure that more "proof" is unnecessary to verify
> this as a physical phenomenon.
>
> The point that I believe applies to this question is "how much" is
> important in "the design situation" not whether the situation exists
> or not. The point of the 20H rule is to provide an all-around
> recommendation for a wide variety of designs which can be encountered.
> Obviously, if the power plane is closely coupled to the ground plane,
> the shorter the actual "20H" distance will be as I believe has been
> pointed out in another response to your question. It should be
> important to note that while the actual distance depends on the
> "conditions" present and the susceptibility of components/circuitry to
> those conditions, this principle should not be ignored. There can be
> cases made for 10H (or even 100H!) depending on the situation.
>
> I am puzzled that you have not been able to find any "data" on this.
> Perhaps you are being too literal... Here's a example of what I mean.
> Check Dr. Johnson's book "High Speed Digital Design" in Chapter 5, pg
> 190 and 191 I believe or thereabouts... While it doesn't specifically
> state the 20H rule, he gives an excellent equation on estimating
> crosstalk levels from a certain distance away from a conductor (he
> even supplies a very useful equation!). Now while that is not
> specifically a 20H rule dissertation, the fundamentals are quite the
> same when trying to estimate stimulus levels at the plane edges for
> re-radiation. From there, some meaningful outcomes with regard to this
> question can be fairly easily obtained. Again, the evidence and
> analytical work is out there for those who can see it. I would
> encourage anyone trying to work this through to consider taking a bit
> of time to think through the ramifications of what these men who have
> "explored" this territory before us were trying to tell us and perhaps
> take what they have done and expand the knowledge further (i.e. sounds
> like a great paper to write, too!)
>
> re: fringing fields too small... GHz range signals
> Well, some of us work with stuff like that.... and with digital, too.
> And we need to pay attention to electromagnetic emissions and immunity
> as well.... Would I need to apply that technique to what I do....
> you bet. Do you? Only you can answer that....
>
> Regards,
>
> Michael E. Vrbanac
> mvrbanac@cisco.com
>
> >
> > Now and again I come across references to the "20-H Rule" for reducing =
> > radiation from power planes. This rule states that the power plane =
> > should be smaller than the ground plane; The power plane edges should be =
> > back from the power plane a distance of 20-times the plane spacing. =
> > This reduces fringing fields from the power plane and reduces coupling =
> > to adjacent planes and free space.
> >
> > Best I can tell, this rule originated with Mike King. The earliest =
> > reference I found is Mark Montrose's "Printed Circuit Board Design =
> > Techniques for EMC Compliance," pg. 26. I have not found any numbers - =
> > analytical, simulation or measurement - which indicate the effectiveness =
> > of this technique over frequency. Intuition (a dangerous thing for this =
> > digital designer to rely upon) tells me that the dimensions of the =
> > fringing fields are small, thus only affecting GHz-range signals. Is =
> > this technique currently only of interest to cell 'phone designers, or =
> > do we need to begin applying this technique to digital PBW design?
> >
> >
> > Mark Freeman
> > msf@stratos.com
> > Stratos Product Development, LLC
> >
> >
> >
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> >
>
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