Don,

The "rule" for plane separation comes from a text by Mark Montrose , he
refers to the rule as the "20H rule" ... Power Plane should be set back 20 x
the dielectric separation between the planes. Montrose's analysis:

http://www.sigrity.com/papers/epep2000/epep_20h.pdf

Another paper referenced via the Speeding Edge website (Lee Ritchey) focuses
on experimental results and suggests that applying "20H" may actually make
the plane performance worse:

http://ieeexplore.ieee.org/iel5/15/30899/01433046.pdf

More can be found on Si-List:

http://www.freelists.org/post/si-list/20H-Rule-theoretically-investigated-with-simulation,3

Based on my research, I do not employ "20H" in any of my designs.

Hth,
Dave

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