I'm not sure I understand your concern about the impedance "between"
a pair of traces. Each trace has its own impedance. For a
differential pair, you would need to match those impedances, as well
as other characteristics such as delay.

No it does not matter if one plane is "hot" and the other is ground.
That is a DC definition. At high frequency, the power plane is just
another ground plane, and is an additional path for a high-speed
return signal. The capacitance between the planes does provide some
lead inductance decoupling

> Does anyone out there have the preferred configuration and equations for
> controlling the impedance between a differential pair of traces that are
> sandwiched between two power planes on epoxy-glass?
> 
> Also, does it matter if one of the power planes is ground and the other
> "hot", or will the inevitable in-circuit capacitance between them ensure
> a common ac reference at typical transmission line frequencies?
> 
 

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Alan Hilton, P.Eng.         Phone: (408) 982-0660 X213
R&D Engineer                Fax:   (408) 492-9170
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Emulation Technology, Inc.  
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