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how to simulate the varactor c-v curve

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sunn

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simulate varactor c-v

I did that before by using ADS, but you need a non-linear varactor model first.
you can search this board to get informations how to get a non-linear model if you don't have.
 

sunking

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how to simulate varactor

add a ac source for a freq, such as 1MHz
connect to a varactor cap, sweep the Vac to get the current.
 

rfsystem

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c-v curve of varactor

Sorry AC does not say you the true story!

Do it different than the others, do it right:

In a VCO the amplitude is not small in comparison to the VJ parameter of the diode capacitance over voltage model. That is why you get amplitude to frequency conversion effects. Via this way also differencies between PSS noise analysis and measurements exist. But it is not the only one.

The method:

Use a sinewave source (transient). If the varactor is differential driven use a sinewave with two controled sources 0.5 and -0.5 coeffcient. Use a amplitude similar to tank amplitude. Measure transient current trough the varactor. Do a FFT on the current. Take the imaginary part of the first harmonic. Calculate the effective capacitance by dividing the imaginary part of the current by the voltage and radian frequency. If you vary the amplitude the frequency of the tank will change.

Take into mind that also active devices have voltage dependend capacitance.
 

Soben

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sdd_examples_prj

I have done a similar thing in ADS , I am attaching a Doc with a schematic and Graphs. Basically doing what rfsystem says.
 

elbadry

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how to simulate varactor cap

rfsystem said:
Sorry AC does not say you the true story!

The method:

Use a sinewave source (transient). If the varactor is differential driven use a sinewave with two controled sources 0.5 and -0.5 coeffcient. Use a amplitude similar to tank amplitude. Measure transient current trough the varactor. Do a FFT on the current. Take the imaginary part of the first harmonic. Calculate the effective capacitance by dividing the imaginary part of the current by the voltage and radian frequency. If you vary the amplitude the frequency of the tank will change.

Take into mind that also active devices have voltage dependend capacitance.

I wonder if the same thing can be done by PSS followed by PAC

First place a sine source (port) with the dc-bias expected in the circuit and specify an AC magnitude for it. Make the amplitude of the sine a variable.

Using PSS sweep on that variable, then using PAC find ( i / v ) for each amplitude. Dividing ( i / v ) by the radial frequency, you can get the Variation of the Capacitance in response to the Amplitude variations...

I am not sure if this is correct. Hope it is...
 

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