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