The paper describes the adaption of a v.h.f. surveillance receiver for reception of suppressed-carrier double-sideband signals by means of a synchronous demodulator of the 2Ftype employing a digital tracking filter. A theory of operation of the demodulator is developed and its performance under various conditions described. A discussion of the choice of automatic gain control is included. The performance is compared, both in theory and practice, with the performance of the receiver when using an a.m. and a f .m. demodulator.