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

Michael E Vrbanac (mvrbanac@cisco.com)
Tue, 25 May 1999 15:38:58 -0700 (PDT)

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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