This paper reports comparative analyses of the phase noise predictions through Cadence simulations for two LC oscillator topologies: Colpitts and Hartley oscillators. The oscillators have been designed and phase noise performance have been derived by means of both direct Cadence-SpectreRF simulations and the Impulse Sensitivity Function (ISF). All the simulation steps for deriving the ISF have been discussed in detail. The ISF has been evaluated for a wide range of amplitudes of the injected current pulse and compared with simulation results obtained directly by Cadence-SpectreRF simulations. The comparative analyses carried out for a set of different injected pulse amplitudes show that the ISF provides accurate predictions across the entire amplitude range and that the phase noise performance of Colpitts are superior to Hartley oscillator. (5 pages)