Hi Doug,

My knee jerk reaction is to say "you can't do that".
It should seem plausible that one should be able to
adjust the construction of a stripline to end up with
a microstrip.

BUT, mathematically deriving capacitances from an "ideal"
construction uses several assumptions.  One of the
assumptions is that if a Gaussian surface is used,
that surface will basically remain the same throughout
the derivation. Another assumption can include a
constant charge.

So, IMO, I don't think you can do that.
It's a good question. So, I'd like to see
other responses.

Regards,  Doug

----------
> From: Doug Brooks <[log in to unmask]>
>
> Ooops. I apologize that the first message had
> two W's and no H's! (No wonder it didn't work!)
> A corrected message follows:
> Sorry for the inconvenience
> Doug Brooks
>
>
> There is a question I have had for several years;
> I though I understood the answer but now recognize
> that I don't. Perhaps some of our resident "Designer
> Council" experts or some IPC gurus can help.
>
> The problem involves the formulas for intrinsic
> capacitance of a trace, as published in IPC 317 and
> 317A. For stripline, the  formula for Co is:
> Co=1.41Er/Ln[3.81H/(.8W+T)]  (5.37 in 317A)
> for Asymetric stripline, the formula is:
> Co=2.82Er/Ln[2(H-T)/(.268W+.335T)]  (5.38b in 317A)
>
> This later formula can be manipulated to
> Co=2.82Er/[Ln[6(H-T)/(.8W+T)]
> which can be compared with Eq. 5.37
>
> Now assume a stripline case with
> H = 10  (mils)
> Thickness = 1 oz
> W = 8
> Er = 4.3
>
> Co is calculated as 3.8 pf/in
>
> Now, simply move the lower plane very far away and use the
> asymetric stripline formula. You get 6.42 pf/in, roughly
> twice as much. But since you have only moved one plate
> AWAY from the trace, intuitively you'd think the capacitance
> would go DOWN by a factor of two, not up.
>
> Is there something wrong with my thinking, or with the
> formulas?????
>
> Thanks for the help.
>
> Doug Brooks
> .
> ****************************************************
> Doug Brooks, President              [log in to unmask]
> UltraCAD Design, Inc.           http://www.ultracad.com
>
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