[SI-LIST] Diff Z for asymmetric coupled striplines

Hello,
I need to determine the differential impedance of pair of coupled stripline 
traces. They are edge coupled and are asymmetric in the sense that the 
distances to the 2 reference planes are different - both traces are
4 mils from the upper plane and 7mils from the lower.

Our pcb fabricator told us that this configuration would have a diff Z of 100 
ohms, but I have gotten a value from the cross section calculator in Cadence 
Allegro of 79 ohms. I'd like to find another method to settle this, as our 
board does have SI problems.

The diff Z formula for this configuration isn't in any of the well known SI or 
TL texts, although the symmetric case (H1 = H2) is. Can anyone either:
 1) Point me toward the correct equation  
 2) Suggest a free field solver that would let me calculate this value?

I've downloaded the student version of Ansoft Maxwell, and am trying it out. If 
it can extract the mutual 
capacitance and inductance, is there a general method of getting the diff Z 
from these?

Any help is greatly appreciated,
Jim


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