Re: vco charactrization
PSS stands for periodic steady state. The VCOs fall under the category of Cyclostationary circuits whose properties are maintained from cycle to cycle. In these circuits PSS sim computes the operating point accurately after which the other analyses can be done.., viz noise -> pnoise, ac -> pac, stb -> pstb, etc. A non-converging PSS can indicate unsustainable oscillations. The spectral purity of the oscillator is accurately obtained with PSS which can be tricky to find out without it.
Hope this helps