# Re: [SI-LIST] : Trace impedance

Doug Brooks ([email protected])
Thu, 20 Aug 1998 09:23:00 -0700

As an aid, you might look at the freeware calculator at
The help file has an extensive discussion of the formulas and sources for
them.

Doug Brooks

At 11:14 AM 8/20/98 +0200, you wrote:
>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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>
>
.
****************************************************
Doug Brooks, President [email protected]