# RE: [SI-LIST] : Trace impedance

John Philips ([email protected])
Thu, 20 Aug 1998 13:55:08 +0100

Domonic,

The 'MECL system handbook' is a good starting point but the stripline and
microstrip equations in it are simplistic and are only valid when the height
of the trace is 20 thou or so above the groundplane.

I would suggest that you have a look in 'High-Speed Digital Design a
Handbook of black magic' by Howard W Johnson and Martin Graham where you'll
find accurate equations for predicting trace impedances. Alternatively visit
http://www.sigcon.com/
where you can obtain the formulas in Mathcad format.

John

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of casperdd
> Sent: 20 August 1998 10:15
> To: [email protected]
> Subject: [SI-LIST] : Trace impedance
>
>
> 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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