Re: [SI-LIST] : Parallel tracks on Flexible PCB without Ground Plane

Mike Jenkins (jenkins@lsil.com)
Thu, 01 Jul 1999 13:53:43 -0700

Cai,

In case it helps you, here's a simple trick to estimate (dif'l)
impedance of two lines without a ground plane: Assume they are
symmetrical (i.e., same size conductors mirror imaged about some
imaginary line):
________
|________|

...............................

________
|________|

If they are also driven differentially, the fields would be
exactly identical whether or not there were a ground plane at the
dotted line. The dif'l impedance of the two lines is then twice
the impedance of one line over the ground plane. And there's lots
of formulas and programs for the line-over-ground-plane case.

I know that doesn't cover all the cases by a long shot. But it
does give a sanity check on more elaborate formulae.

Good luck,
Mike

-- 
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