A signal generating system comprises a signal generator (14) for generating an electrical signal at a predetermined frequency; an impedance matching circuit (16), the electrical signal being supplied from the signal generator (14) via the impedance matching circuit (16) to a reactive load (10) in use; and an impedance matching control system (30) for detecting the electrical signal between the signal generator and the reactive load and for adjusting the impedance matching circuit (16) to achieve a predetermined condition. The impedance matching circuit control system (30) comprises a heterodyne circuit, and the system further comprises a heterodyne frequency generator (48) coupled to the signal generator (14) to generate a second, heterodyne frequency from the predetermined frequency from the signal generator. This second, heterodyne frequency is mixed with the detected signal to generate sum and difference signals. A filter (50, 52) passes the difference signal and a processor (Magnitude control; Phase control; 26; 28) responsive to the difference signal is used to adjust the impedance matching circuit (16) to achieve the predetermined condition.