Abstract: A microwave synthesiser comprising: a plurality of oscillator sources (1, 13, 3, 15, 5, 17, 7), each for generating signals over a range of frequencies, said ranges of frequencies together extending over a total range of frequencies generated; phase locked loops (19, 31, 83, 37, 38, 41, 81) for phase-locking any selected one of said oscillator sources (1, 13, 3, 15, 5, 17, 7) so that it generates signals of a frequency at which it is set; a single low harmonic modulator (21) for modulating the signals generated by the selected one of said oscillator sources (1, 13, 3, 15, 5, 17, 7), the modulated signals passing to an output (27) of said synthesiser; and at said output (27) elements (25, 29) for detecting the power of the signals at said output (27), the modulation by said single low harmonic modulator (21) being controlled in dependence on the detected power thereby to control the power of the signals at said output (27).