[SI-LIST] Re: Diff Z for asymmetric coupled striplines

Hi Jim,
   Give MMTL a try.  I have had pretty good luck with the results, and
it's free.  

http://mmtl.sourceforge.net/

Best of luck,
-Mike

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of otter30@xxxxxxxxxxxxx
Sent: Monday, October 20, 2008 10:16 AM
To: si-list@xxxxxxxxxxxxx
Subject: [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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