A microwave-frequency source, generating an output electrical signal at an output frequency fM, comprises a pump laser source, an optical resonator, and a photodetector. Free spectral range vFSR of the optical resonator equals an integer submultiple of a Brillouin shift frequency vB of the optical resonator (i.e., vB=MvFSR). The pump laser source is frequency-locked to a corresponding resonant optical mode of the optical resonator. Pumping the optical resonator with output of the pump laser source at a pump frequency vpump results in stimulated Brillouin laser oscillation in the optical resonator at respective first, second, and third Stokes Brillouin-shifted frequencies v1=vpump−vB, v2=vpump−2vB, and v3=vpump−3vB. The photodetector receives stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies v1 and v3 and generates therefrom the output electrical signal at a beat frequency fM=v1−v3=2vB. The output electrical signal at the output frequency fM exhibits exceptionally low phase noise.