Abstract: A frequency stable RF oscillator 10 comprises a variable frequency RF source in the form of a microwave cavity 24 having a Gunn diode 26 and a varactor 28 mounted therein, and produces an RF output frequency f.sub.o. The output of a frequency stable reference oscillator 14 having a frequency f.sub.r is impressed upon the RF source. Through self-mixing action of the Gunn diode, an IF whose frequency is f.sub.IF =.vertline.nf.sub.r -f.sub.o .vertline. is generated, n being a high harmonic number. The IF is detected by an IF amplifier 18 which forms part of a frequency lock loop controlling the frequency of the RF source.

Abstract: An injection locked RF oscillator comprises a variable frequency RF source 12 having a Gunn diode 20 and a varactor 22, and a frequency stable reference oscillator 14 whose output f.sub.i is injected into the source to cause injection locking of the source. The output of a low frequency perturbation oscillator 30 is applied to the varactor and the resultant variation in the current consumption I.sub.o of the Gunn diode detected by amplifier 28 and filter 36 to provide a control signal which is effective to shift the resonant frequency f.sub.o of the source 12 towards the frequency f.sub.i of the reference oscillator, or a harmonic thereof. A sweep-and-lock circuit 44 is provided to acquire injection locking upon switch-on.