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Thu, 12 Mar 1998 10:07:25 -0800 |
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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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