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

Mark Freeman (msf@stratos.com)
Wed, 26 May 1999 11:20:43 -0700

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-----Original Message-----
From: Michael E Vrbanac [SMTP:mvrbanac@cisco.com]
Sent: Tuesday, May 25, 1999 3:39 PM
To: si-list@silab.eng.sun.com
Cc: Michael E Vrbanac
Subject: Re: [SI-LIST] : 20-H Rule for Power Planes

.
.
.

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!)

Thank you for reminding me of this bit from Howard's book. In this case =
(coupling between signal traces) the impedance and energy of the source =
and victim are fairly well understood. Harried computer designers have =
some readily applied design guidelines (such as the "3-W Rule") to =
increase the odds of success, without being exceedingly conservative.

Power-plane coupling is not as simple of a model, as I can see from the =
discussion so far.

First, the energy sources are not well controlled or characterized. A =
simplified model could be one IC and all of the bypassing - but where do =
you locate the IC for worst-case conditions? Perhaps some "rule of =
thumb" could be reached for a particular situation: one bus width, edge =
rate and bypassing method.

Next, how does this correspond to currents along the edge of a PWB? =
There seems to be disagreement in this area.

Finally (for radiated EMI), how does this correspond to far-field =
radiation? Staring at a 2.4 GHz patch antenna, I have no idea how they =
arrived at the geometry of the thing, but it does look in some respects =
like a computer board. Comparing a computer board to a patch antenna, =
how would you locate the "feed point" (the highest-energy noise sources) =
and lay out the geometry of the planes, to make this an inefficient =
antenna?

Given enough room on the PWB, applying the 20-H rule obviously could =
help. When PWB real-estate becomes critical, however, it isn't clear if =
and when this might be worth the cost. I don't have an =
order-of-magnitude feel for it. In my hands, I have a 1993-era computer =
board which was designed under the guidance of Mike King - proponent of =
the 20-H rule - but which does not use this rule in its layout. It =
could be that space constraints did not allow application of this rule, =
or that 1993-era edge rates and bus widths did not warrant its use. At =
some point in the evolution of bus rates, the 20-H Rule may become an =
important guideline for computer board layout. The simple to ask, but =
difficult to answer question is: are we at the point, with 133 =
MHz/64-bit designs, where the 20-H rule should be considered, tested, or =
universally applied? =20

Mark Freeman
msf@stratos.com
Stratos Product Development, LLC

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