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Design >> RF Design >> How to measure a varactor's capacitanc
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Message started by Tim on Jul 10^th, 2007, 4:06pm

Title: How to measure a varactor's capacitanc
Post by Tim on Jul 10^th, 2007, 4:06pm

Hi, All experts

I am a beginner in RF design. I am doing VCO. Does anyone know how to
measure a varactor's capacitance (i.e. capacitance v.s. frequency) in cadence?
Someone told me I could use S-Parameter to measure it. But I still can't get it.
Anyone can tell me in detail. Thanks

Tim

Title: Re: How to measure a varactor's capacitanc
Post by topquark on Jul 12^th, 2007, 4:25am

Hi Tim

first off, you can already see the cap you're setting in the schematics, right? :D
For MOS varactors, you specify the W/L and you can see the cap value
well, cap vs. freq will be constant but impedance vs. freq is what you're looking for..uh?
If you go by S-parameters, you can get Zin/Zout as function of frequency 'n then calculate cap.

Or, simply run an ac sim with a standalone varactor 'n you can characterize Cap vs. voltage bias which will be required for your VCO tuning.

Hope this helps

Title: Re: How to measure a varactor's capacitanc
Post by neoflash on Jul 15^th, 2007, 5:41am

u should notice that in LC-tank, the capacitance matters is not small-signal capacitance, instead, it is the large signal averaged capacitance.

Title: Re: How to measure a varactor's capacitanc
Post by jlee on Jul 15^th, 2007, 7:01am

If small signal SP is performed, use the following equation to get C vs freq curve with the calculator in Spectre.

C=imag(y(1,1))/2/pi/xval(imag(y(1,1)))

Maybe more accuracy results can be obtained by PSS+PSP.