# Re: [SI-LIST] : Trace impedance

[email protected]
Thu, 20 Aug 1998 09:28:13 -0700

Hi Dominic,

In addition to the recommendations from John Philips, I would suggest you
refer to Microstrip Circuits by Gardiol. On page 44 is a curve fit
equation for the characteristic impedance of microstrip from Hammerstad and
Jansen which is claimed to have an error less than 0.03% over a very wide
range of w/h (width/height) which covers your situation.

Best regards,

Eric

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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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Eric Wheatley Ph.D. Electromagnetic Modeling Services
Alterra Technology Co. (760) 942-9426 (phone)
435 Dunsmore Ct. (760) 942-2366 (fax)
Encinitas, CA 92024 US [email protected]
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