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# Interdigital capacitorp in HFSS

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#### vfurlan

##### Full Member level 2
Interdigital capacitor in HFSS

Hi,
I designed IDC in HFSS (see attached pics and HFSS file). How can I see capacitance of such IDC?

P.S. this is IDC on alumina substrate, just to see how it works in HFSS. Later I will make it on high permitivity BST material.

#### Attachments

• pic1.gif
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• pic2.gif
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• IDC Alumina.rar
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Re: Interdigital capacitor in HFSS

can you get the s-parameters?

Eddy.zhu

### Eddy.zhu

Points: 2
Re: Interdigital capacitor in HFSS

Here, S11 and S12.

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• S11.jpg
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Again, but imag and real part.

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• S11 mag.jpg
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• S12 mag.jpg
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since you have s-parameters you should be able to get the capacitance. There's a relatively simple transformation to go from s-param to Y-param
and from that capacitance (or inductance). There should be a past post in this forum that describes this. Looking at your graphs I'd expect there
to be a complicated frequency dependence. I usually write out the s-params into an .s2p format file and process the data in matlab.

Here's a matlab sniplet : (needs rf circuit package)

Z0=intrinsic impedance (usually 50)

yp=s2y(s2p,Z0); % Y-params
y11=reshape(yp(1,1,,1,length(F));
C11=imag( y11)./(2*pi*F); % capacitance
L11=imag(1./y11)./(2*pi*F); % inductance

Last edited:
vfurlan

### vfurlan

Points: 2
Should I use S12 or S11 parameters? Can you explain? Thank you

Should I use S12 or S11 parameters? Can you explain? Thank you

you should use the full s-param matrix which in your case is a 2x2 matrix ((S11,S12),(S21,S22)) for each frequency point analyzed.
So the variable "s2p" above is a 2x2xN array : the last index runs over the number of frequency points analyzed.
The "s2y" function transforms "s2p" into another 2x2xN array ((Y11,Y12),(Y21,Y22)). Then Y11 can be used to get capacitance,...
Your "capacitance" could be frequency dependent (not constant)

vfurlan

### vfurlan

Points: 2
S parameteres have to be in re +i*imag form of course?!
How do you export files in s2p format from HFSS? I do not have this option, although it is not a problem, I can perfectly well export in txt.

S parameteres have to be in re +i*imag form of course?!
How do you export files in s2p format from HFSS? I do not have this option, although it is not a problem, I can perfectly well export in txt.

I'm running and older version (12.1) (don't have 13 unfortunately). What I use there is : (menu tabs)

HFSS -> Results -> SolutionData -> ExportMatrixData

There's also an EquivalentCircuitExport which I haven't used

vfurlan

### vfurlan

Points: 2
Why don't you use y parameters calculated by HFSS?

sure,also you can directly export the y-parameter to do it.

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