[SI-LIST] : Differential Impedance of a Stripline w/ No Planes?

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From: Rede, James (JRede@taqua.com)
Date: Tue May 08 2001 - 14:18:10 PDT


To the SI Good Guys,

I have been asked to design an edge-coupled differential stripline with no
planes present
for FR-4. This design is necessary to pass a Hi-Pot requirement of 1500
volts.
The differential impedance is to be 100 ohms for T1 frequencies (1.544MHz).
Four layers are present but no layers are power or ground. Let the board
thickness
be 62 mils +/-10%. Use 1/2 oz copper and assume that Er = 4.1.

I cannot use my Hyperlynx field solver because THEY TOOK MY DONGLE.

When I use an impedance calculator found on the web and specify 5-7-6 mils
(Trace-Space-Trace, TST) and a plane-to-plane separation of 30 mils, the
differential
impedance is 100 ohms. If I then widen my plane-to-plane spacing to
30000000000 mils,
I get something like 255 ohms. By trial and error, I was able to find a TST
of 20-6-20
that would produce a 100 ohm differential impedance at the 30000000000 mil
plane-to-plane
separation.

Qs: Is this a reasonable way to approach this problem? Is there a better
way?
How should this problem be done, even a field solver will not give an answer
without ground planes present? What about accounting for the board thickness
then air?

J.J. Rede
Signal Integrity Engineer
Taqua Systems
Hyannis MA

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