Yomnat,

  It sounds like you want to simulate a VCO, an autonomous circuit,
with a driven source, the chopping source. As mentioned in a recent
append Spectre RF will support this analysis from MMSIM71.

from the thread, http://www.designers-guide.org/Forum/YaBB.pl?num=1227668750


Spectre will support the autonomous QPSS ( and corresponding small
signal analyses, like PAC, PXF, PNOISE, etc ) in MMSIM7.1. You can
specify a guess frequency for the LO in the GUI, it is very much like
what you do for the autonomous PSS simulation.

                                                                   Best Regards,

                                                                      Sheldon