The problem is What's Theoretical?  Every published equation I've ever
seen is only an approximate solution to solving the theoretical
solution.  Every equation out there should have an accompanying
statement that describes where it is valid.  Mathematically, the best
results I've seen come from using a field solver.  I use a program
called Apsim RLGC, but there are many others out there.

Even if you are using a field solver, you need to make sure your inputs
are valid.  Even a 100% theoretical solution gives the wrong answer if
the input variables aren't correct.  Most oscilloscopes for measuring
TDR have a sampling edge rate of approximately 10 GHz.  The dielectric
constant for FR4 you used is the value at 1 MHz.  The exact value of the
dielectric constant of FR4 at 10 GHz is difficult to come by.  There a
number of test methods out there all of which seem to give conflicting
results.  Empirically, I've found that at dielectric constant of 3.5 to
3.7 works well in matching measured values.  If I model your structure
with a Dk of 4.5, I get 49.9 ohms.  With a Dk of 3.7 I get 55.0 ohms.





> ----------
> From:         George Franck[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;George Franck
> Sent:         Monday, August 24, 1998 5:50 AM
> To:   [log in to unmask]
> Subject:      [TN] [Fab] Emperical Impedances
>
> Technet,
>
> Is it typical to have actual impedance reading which vary (10% or so)
> from
> those predicted from theoritical data?
>
> For example, in a cross section of a Stripline impedance coupon, I
> measured
> the following dimensions.
> ========= Ground plane
> ========= 11.9 mil Dielectric (4.5 Dk)
> ========= Circuit Layer  1.2 mil thick,  9.0 mil wide
> ========= 11.8 mil Dielectric  (4.5 Dk)
> ========= Ground plane
>
> Material is High Tg Epoxy.  The Dk is supplier estimate.  Theory would
> predict a value of 49 Ohms.  Actual TDR measurements show an impedance
> of
> 54.7.
>
> George Franck
> Raytheon Systems
> Falls Church Virginia
> "My Opinions are just that."
>
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