I am designing a LC VCO in 0.18u CMOS.

How do I plot the tuning characteristics of the VCO ( Freq vs control voltage) in spectre RF?

thanks

A few things you can do:

a) A sweep of the control voltage and then do an autonomous PSS
   to get the oscillator frequency. This is probably quite expensive
   in terms of simulation time though.

b) A transient analysis, using the Verilog-A model freq_meter from
   ahdlLib at the output of your oscillator - this measures the
   times between zero-crossings and outputs this as a frequency.
   The benefit of using the Verilog-A model is that the crossing
   is resolved in the simulator, and so minimises interpolation
   error. However, you need to sweep the control voltage slowly
   over time to avoid impacting the instantaneous frequency too
   much.

   You can then change the x-axis to be the control voltage,
   and plot the frequency against that.

c) As an alternative to using freq_meter,  a calculator function
   that I wrote can be used. I can't post it here (it is too long,
   and the system won't let me post it), but it is available
   on http://sourcelink.cadence.com as solution number
   11015938.

Regards,

Andrew.