When you set up your PSS analysis you:
- Do NOT select the oscillator button.
You are applying a signal (vsource) to your ring oscillator which causes the oscillator to oscillate at the injection frequency. Thus it is a driven circuit - as it is driven by the injection source. This is why you don't push the oscillator button on the PSS choosing analysis form. SpectreRF considers this a driven circuit and will error out if you select the oscillator button on the PSS choosing analyses form.
- Set tstab to 100 periods.
You want the circuit to be stabilized, i.e. you need to get it close enough to the final oscillation frequency so that PSS can converge. This will depend on the time constant of the circuit. A high Q oscillator will take a long time to settle. A ring oscillator is lower Q by definition, so it should not be as problematic.
- Check the saveinit button.
The waveforms for the initial transient before steady state are saved and you can view them with the results browser.
- You may even want to try to run the circuit using transient analysis.
Look at the oscillator output compared to the injection source. Look for variations in the timing of the oscillator output compared to the locking source. Hopefully, the circuit will settle fairly quickly.
Now, once you can get PSS to successfully converge, you can look at things like pnoise jitter