I am working on a CMOS ring oscillator(VCO). I
am using 0.25u SOS technology. I am trynig to simulate the circuit using
Spectre RF in Cadence. I tried to run the PSS analysis for the sweep and
it failed to get a convergance for certain voltages. On the other hand I
could run the PSS for those voltages seperately ( I had to make some
changes like the "tstab" and guessed fundamental frequency). I could
also simulate the circuit for transient analysis at all the voltages
without any problem.

I went through the cadence spectre manual and used small capacitances at
the output but they didn't help. I tried to change the accuracy preset,
reltol. etc. but they don't help either.

To clarify the situation let me explain this:

For a particular guessed fundamental frequency and "tstab" values the
circuit does sweep for certain voltages and then fails beyond that,
but, if I run the PSS seperately for the failed cases they do
converge. The only changes I need to do are to make new guesses for the
fundamental frequency and / or "tstab".

I will appreciate if anybody could help me to get the plot for sweep for the
whole range using some techniques, that have not been explained in the
manual.