SpectreRF uses a method that creates a linear time-varying model of the circuit for pnoise analysis. It is different from Hajimiri's approach, however. For details, see Appendix A "Oscillator Noise Analysis" of the SpectreRF Simulation Option User Guide. By the way, there is a small flaw in Hajimiri's reasoning, see http://www.designers-guide.org/Forum/YaBB.pl?num=1123060319/20#20 for details.