diffrence between pss and transient analysis results !! why ??

elec_888

hello guys !im simulating a VCO using spectre .. my oscillation frequency using transient simulation is found to be 2.1 GHz.. but using pss i find my first harmonic to be at 2.8 GHz !!! how is this possible and whats wrong with the simulations to have such different results ??

thanks a lot