A microwave-frequency source for generating an output electrical signal at an output frequency f M, the microwave-frequency source comprising: (a) a dual optical-frequency reference source arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v 1 and (ii) a second optical reference signal at a second optical reference frequency v 2 > v 1; (b) an electro-optic sideband generator (200, 300) arranged so as to (i) receive the first and second optical reference signals (120a, 120b) and a sideband generator input electrical signal at the frequency f M and (ii) generate therefrom multiple sideband optical signals (210, 310) at respective sideband optical frequencies of the form v 1 ± n 1 f M and v 2 ± n 2 f M, wherein n 1 and n 2 are integers; (c) an optical bandpass filter arranged so as to transmit a subset of the multiple sideband optical signals (210, 310) including the sideband optical signal at a frequency v 1 + N 1 f M and the sideband optical signal at a frequency v 2 -N 2 f M, wherein N 1 and N 2 are integers; (d) an optical detector arranged so as to (i) receive the transmitted sideband optical signals (430a, 430b) and (ii) generate an optical detector electrical signal at a beat frequency f BEAT =( v 2 -N 2 f M)-( v 1 + N 1 f M); (e) a reference oscillator arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency f R; (f) an electrical circuit arranged so as to (i) receive the optical detector electrical signal and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit; and (g) a voltage-controlled electrical oscillator, VCO, arranged so as to (i) receive the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency f M, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator (200, 300) as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source, (h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator (200, 300) as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop. (a) a dual optical-frequency reference source arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v 1 and (ii) a second optical reference signal at a second optical reference frequency v 2 > v 1; (b) an electro-optic sideband generator (200, 300) arranged so as to (i) receive the first and second optical reference signals (120a, 120b) and a sideband generator input electrical signal at the frequency f M and (ii) generate therefrom multiple sideband optical signals (210, 310) at respective sideband optical frequencies of the form v 1 ± n 1 f M and v 2 ± n 2 f M, wherein n 1 and n 2 are integers; (i) receive the first and second optical reference signals (120a, 120b) and a sideband generator input electrical signal at the frequency f M and (ii) generate therefrom multiple sideband optical signals (210, 310) at respective sideband optical frequencies of the form v 1 ± n 1 f M and v 2 ± n 2 f M, wherein n 1 and n 2 are integers; (c) an optical bandpass filter arranged so as to transmit a subset of the multiple sideband optical signals (210, 310) including the sideband optical signal at a frequency v 1 + N 1 f M and the sideband optical signal at a frequency v 2 -N 2 f M, wherein N 1 and N 2 are integers; (d) an optical detector arranged so as to (i) receive the transmitted sideband optical signals (430a, 430b) and (ii) generate an optical detector electrical signal at a beat frequency f BEAT =( v 2 -N 2 f M)-( v 1 + N 1 f M); (e) a reference oscillator arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency f R; (f) an electrical circuit arranged so as to (i) receive the optical detector electrical signal and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit; and (g) a voltage-controlled electrical oscillator, VCO, arranged so as to (i) receive the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency f M, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator (200, 300) as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source, (h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator (200, 300) as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.