  We studied how the cryogenic sapphire resonator responds to fast variations
of the dissipated microwave power. The experiments were carried out with
sapphire resonators cooled to 6 K at frequencies around 11 GHz. We found that
the power-to-frequency conversion of the resonator depends on Fourier frequency
as the transfer function of the 1st-order low-pass filter with corner frequency
close to the resonator loaded half-bandwidth. Having measured the
power-to-frequency conversion of the cryogenic sapphire resonator, we predicted
the phase noise of the microwave oscillator based on such resonator.
