[SI-LIST] Impedence of Differential Pair Not Over Plane (Revised)

Apologies for posting this twice. I made a fundamental mistake in the
original posting!
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 Is there a formula for calculating differential impedence over an
(adjacent)
co-planar pair of tracks that are NOT routed over a plane layer?

 I am looking for 100 Ohms (PCB is 1.6mm FR4).

As a guess (based on a 2-wire transmission line in air)  I would estimate
that a track width to spacing ratio of approx  4 is required. If this is
correct then quite wide tracks will be required.

Can a reasonable transmission line be made in this manner?

 Thanks for your help.


Best Regards,

Ray Gordon                      ray@xxxxxxxxxxxxxxxxxx

Scene Double Ltd.            Tel:  +44 (0)20 8386 9001
Vision House                    Fax: +44 (0)20 8386 9002
Station Road
Borehamwood
Herts.  WD6 1DE
UK.


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