We consider various methods of measurements of the spectra of amplitude and phase noises of harmonic microwave signals and choose the most sensitive method of synchronous demodulation with two independent channels and the determination of the cross-spectrum. The results of measurements of the spectra performed by using a model of two-channel low-frequency analyzer, as well as the results of measurements of the spectra of phase noises of ultrahigh-frequency sources of various intensities, are presented and compared with the data of reference measurements.