# RE: [SI-LIST] : Upper limit of interplane capacitance

From: Chan, Michael ([email protected])
Date: Fri Jun 09 2000 - 10:03:28 PDT

I would like to point out that what would be the impedance look like when
you looks at it from the center of the two plates viewing from the top? Can
you still qualify it as a rectangular wave guide as the wave is spread out
in
360 degree other than in one particular direction. Instead of calling it the

traditional "characteristic impedance" I would prefer to see it as " driving

point impedance". Any comment from any guru ????

Michael Chan

-----Original Message-----
From: Doug [mailto:[email protected]]
Sent: Friday, June 09, 2000 11:36 AM
To: [email protected]
Subject: Re: [SI-LIST] : Upper limit of interplane capacitance

> Larry Miller wrote:
>
> Dr Howie Johnson did an article on this sometime back.
>
> Here is an excerpt. See what you think:

What would people expect the characteristic impedance
to be of a 4" x 6" x 4mil FR-4 structure?

Parallel plates are just two dimensional transmission
lines with L and C being in units of H/area and F/area.
The equations pop out pretty much the same as a one
dimensional transmission line, i.e.

vprop = 1/sqrt(LC)
Z0 = sqrt(L/C)

In fact, there's some pretty neat derivations
for parallel plates such as

Z0 = sqrt(mu/epsilon)*d/w

d = distance of separation
w = width

ignoring fringing effects.

Just was wondering if anyone has done any
research into correlating pure theory
versus reality.

- Doug McKean

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