# Capacitance extraction

Discussion in 'Wireless & RF Design' started by Bero, Jan 18, 2015.

1. ### Bero Thread Starter New Member

Dec 17, 2014
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Hey People

I have a very basic problem. The measurements on an interdigitated capacitor are made as 1-port measurements (one side connected to RF signal and other side of capacitor to ground), using VNA. I have four Z11 (or Zin) values of a interdigitated capacitor, for different frequencies.
They are:
0.045GHz Z= -2K -j*8K
10.033GHz Z = 0.76 -j*54
18.02GHz Z= 0.478 -j*12
40GHz Z= 60 + j*154

The model of the capacitor also has ESR and inductance as it is a transmission line for such high frequencies. Using these values, I have to find actual capacitance of the interdigitated capacitor. Can you please help me out with ideas ?

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Postulate a lumped RLC model. Try different values of RLC to see if you can find a combo that matches your four measurement points. If you are lucky, and a simple RLC model works, you dont have to screw with transmission lines. OTOH, if the simple model is inadequate, then you can use this problem as your thesis...

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Don't see how a passive component can have a negative resistance component in the complex impedance.

4. ### Bero Thread Starter New Member

Dec 17, 2014
6
0
Yes, but I need some initial values for that. How to get them ?

Jan 21, 2015
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