Hi everyone,


I am designing a mm-wave Colpitts VCO and am running into big discrepancies (around 7-8 GHz) between the calculated PSS Harmonic Frequency and the calculated frequency from the transient simulations. I use harmonic balance for performing my PSS simulations with an harmonic number of 10. The version of Cadence I am using is the 6.16 and MMSIM is 14.10.

My oscillator seems to start after approx. 4 ns and I therefore set the tstab value in the PSS to 10 or 15 ns when the oscillation has already stabilized. I also set the step and maxstep from both the PSS and the transient simulation value typically to 1 ps in order to capture the oscillation behaviour. If i don't do this, Spectre can't typically resolve out the oscillation and the transient simulation shows a damped out response.

I´ve saved the initial transients (tstab) from the PSS simulations as well. Looking at them, the calculated frequency/wave seems to agree very well with my transient simulation, but if I look at the time wave from the PSS results, it does not match the obtained graph from the initial transient/transient simulations. The difference in frequency is around 7-8 GHz being the PSS calculated frequency the higher one.

Until now I´ve just tried to reduce the maxstep in the PSS options as well as use the sigglobal accuracy option to check if that improves the results agreement, but no succes by now.

I would like to ask if anyone has run into such problems and how could I solve it or at least improve the matching between my results?


Many thanks in advance.

BR  :)

I have met the problem with ADS software and I set steady state time more than default. then it was better.
simulation of Colpitts oscillator has this problem.

I've run a transient simulation to check for stabilization, the oscillator amplitude stabilizes aroudn 3 ns, therefore I set a tstab in PSS of 5 ns, but I've already tried with values as high as 50 ns. I've found this issue not only in Colpitts but also Clapp oscillators, which I typically work with. This might indicate for me it is not an architecture related issue.

The transient saved by PSS agrees sometimes quite well with my transient waveform, but the PSS time wave does not agree with both of them at the end. I understand more or less how the HB works, but this does not explain why I get the discrepancy at the end.

Any feedback is really appreciated.

Thank you!