[SI-LIST] : Trace impedance

casperdd (Dominic.Casperson@cern.ch)
Thu, 20 Aug 1998 11:14:51 +0200

Hi

I was hoping that someone might be able to shed a little light on a
problem that I have encountered regarding the impedance of stripline,
and microstrip PCB tracks.
I require a formula that is capable of predicting the impedance's of
stripline, and microstrip tracks. I have already found several from
different source, but none of them seem to agree with each other, my TDR
results, or the TDR results from "MECL system design handbook" by
Motorola. All the formulas that I have come across claim to have an
accuracy to within about 5%, but as far as I can tell the discrepancy
can reach up to 20%, depending on the dimensions involved.
The region of interest to me is:

Track width = 10 mils
Track thickness = 1.37 mils
Ground plane/Track separation (dielectric height) = 8 mils
Relative permittivity = 4.7
TDR edge rise time = 28ps

The most alarming problem that I have encountered is the natural
logarithm that most formula seem to have, this then returns a negative
number when its argument is smaller than one, even though most formula
claim to still be accurate at this limit.

I would appreciate it if anyone could show me where I have been going
wrong, or point me in the direction of an accurate formula.

Many Thanks

Dominic Casperson

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