Hello,
I want to simulate the phase noise of a PLL in the output of the VCO. The VCO is a ring oscillator that comprises of transistor while the other elements such as the divider and the PFD are modeled in Verilog-a language, according to the papers found in this forum.
When I simulate the phase noise of the VCO alone I get a line falling by 20dB/dec as expected by the theory.
However when I close the loop, I perform a pss and pnoise analysis but the phase noise of the VCO is approximately the same and does not seem to be filtered by the loop filter.
I wonder what mistakes I am doing. Thanks in advance.

I have a relative question also. For simulating the VCO in a closed loop do I need to model it using for example verilog-a, or using the schematic is sufficient? I ask this question because I 've seen the approach of modeling in some papers. Thanks again.

-tromeros-

I suppose that the pll is locked since the control voltage of the VCO is a periodic signal and the pss analysis converges. However in the pnoise analysis that follows I get the warning message:

Warning from spectre during PNoise analysis `pnoise'.
   The Floquet eigenspace computed by spectre PSS analysis appears  to be ill-defined. PNOISE computations may be inaccurate.  Consider re-running the simulation with smaller reltol, different tstab(s) and method=gear2only'. Check the circuit for unusual components.

And the phase noise results are the same with the open - loop VCO.

I then apply some stringest requirements from the options menu of the simulation as proposed by the message but then the pss analysis does not converge even though I increase the stabilization time. My question after all is:
Can i use the Direct simulation method to predict the phase noise and if yes is it possible to estimate in advance the time required for the convergence?

Thanks a lot for the help.

Hi Ken,
I simulated the PLL using transient analysis with the strobe option and after some nanoseconds
the control voltage becomes a DC signal, which means that the PLL converges.
In the PSS analysis that follows there is in Fundamental Tone field the reference frequency signal. I also use the autocalculate option for the beat frequency. Finally I set logical values in tstab field according to the previous transient simulation.
The VCO I use is a ring oscillator block which contains transistors. It is a schematic and not an equivalent model.
I simulate the circuit without having the Oscillator option enabled and the PSS does not converge.
When I enable the oscillator option having as an Oscillator node the output of the ring circuit the pss analysis does not start, throwing the message that I have a periodic input signal(reference) which is inconsistent with autonomous circuits.
What can I do to overcome this problem? I suppose that the Oscillator option should be disabled but then, there  probably appear problems in the build-up of the signal.
Thank you for your interest