A new type of threshold-extension f.m.-signal demodulator containing no feedback loop and no limiters is described. The f.m. signal and a delayed version of it are applied to a homoheterodyne mixer, the difference-frequency output of which is then demodulated by a synchronous-phase detector. The paper describes the basic principle of the demodulator and presents a theoretical analysis of its behaviour. Although the theory has been confirmed in the laboratory, no attempt is made to describe its practical design and measured performance; this will be the subject of a later paper.The new demodulator—termed ‘autocorrelation’ because the signal processing is analogous to deriving the autocorrelation function of the signal—offers some significant advantages in f.m. radio links. It should enable greater threshold extension to be achieved in satellite links, especially when channel signal/noise ratios of 30dB or less are acceptable. Moreover, the fact that no feedback loop is required should prove attractive in applications where baseband frequencies extend to many megahertz.