The theory and performance are discussed of an automatic frequency control system based on a low-noise dual detector sampling the standing wave in a reference waveguide. The reference waveguide and detectors are temperature-compensated and constructed from readily available components. The stabiliser can control the frequency of a 9 GHz reflex klystron to 5 parts in 10⁷ per hour. With slight modification the method is applicable to solid state oscillators with narrow-band electrical tuning.