Ralph

I appreciate your problem.  We have also searched long and hard for
data on differential traces.  The best I can do for you is to
refer you to a book by Brian C. Wadell, "Transmission Line Design
Handbook", 1991 Artech House, Inc, ISBN 0-89006-436-9, page 226

        Artech House Publishers
        685 Canton Street               Tel: (617) 769-9750 
        Norwood, MA 02062               Fax: (617) 769-6334

For your information, Polar Instruments has already developed a simple
single ended Windows Impedance Calculator (with graphical output).  If you
are interested, a free disk can be obtained from Polar Instruments Inc.
Contact details are:

        Robert Bosley                   Tel/Fax:  (360) 398-1946
        Polar Instruments Inc           Helpline: (800) 328-0817
        5373 Guide Meridian, Bldg. D
        Bellingham, WA 98226

We are currently working on a differential version of the above and are
engaged in practical verification trials with a few PWB manufacturers in
order to prove the formulas.  I can offer you a prototype spreadsheet
which we developed in Excel (usual caveats will apply!), but our work to
date suggests that we can be fairly optimistic about the performance of
our broadside coupled equation.  Best contact me offline if you would like
me to email you a copy of DIFFZCALC.XLS

I hope this helps.

Best regards

Andy Burkhardt
Applications Engineer
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