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
W = 10  (mils)
T = 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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