I want to simulate the varactor in Spectre and plot the capacitor value vs. voltage.
How to do it?
Thanks.

Dear shuguang,

Description:
Set up a schematic with a voltage source connected to the varactor. Set AC Magnitude to "1".
Set up an AC-Simulation, sweep DC Voltage of the voltage source. Use 1/2Pi=0.159 as frequency.
Plot the current (AC current) of the voltage source = capacitance value. When using any other frequency than above, divide the current by 2Pi and the frequency.

Explanation:
AC computes small signal values, i.e. derivatives of the plotted signal vs. the AC sources. If plot the varactor's current IF("/V1/PLUS") the AC Analysis yields  dIC/dVC.
dIC/dVC = j*ω*C * vc = j*2πf*C * vc
where vc is the small signal voltage (AC Magnitude), vc=1.
Thus:
C = dIC/dVC / 2πf / vc = IF("/V1/PLUS") / VF("/V1") / 2πf

Understandable? If not - please ask.
Regards,
Achim