When designing LC-tank based oscillator, we need to analyze the tuning curve of frequency.

Instead of directly running LC-tank with active circuits, I hope that I could have a look at the tuning curve of the varactor.

Instead of the DC tuning curve which is easily available, I would like to see the effective capacitance of the varactor at certain voltage waveform.

Can eldoRF do this? Can SpectreRF do this? Which document refers to the setup and theory?

Thanks,
Neoflash

Ken:

The average capacitance I want to observe is just like what is mentioned in Emad's book.

The capacitance obtained in your method is the small signal capacitance at certain DC biasing point. However, I thought there is also some way to get average capacitance. When we run that simulation, we need to specify input voltage amplitude and DC voltage. Thus, simulator will plot us a curve of average capacitance.

I see this mentioned in one recently pulished jssc paper, but i can not remember the title. I will check when I was back in office.

Thanks,
Neoflash

Ken:

Thanks for the information. But I can not locate any reference doc or application notes on this.

I guess I do not like got some results without knowing its physical meaning. Maybe I can try to acess Cadence AE, however, I doubt supporting groups in my region knows what I want.

It could be nice if you have some demo there.

The possible background of your question is to estimate the shift of the oscillation frequency of a tank dominated circuit. That requires that you calculate the effective capacitance for the first harmonic current. If the tank capacitane is primary current driven by the tank inductance you have to use a current source. Use a sine current source and extract the first harmonic voltage amplitude and phase. If the capacitance is not ideal you will observe phase differing from -90°. If you change the current amplitude you will observe a nonlinear change of the cophase component voltage. That indicate that the oscillation frequency will change with amplitude. Also known as AM2FM conversion effect.

I also observe that there is also bias shift effects in the circuit which are difficult to differentiate from the above effect. These DC bias shift too modify the oscillation frequency via the cap nonlinearity. Therefore the circuit is studied by selectively switching in/out the effect in the model and simulating the hole circuit instead of only the components.