A transmitter in which a complex low IF digitised signal is applied in quadrature to a complex phase comparator (32) together with a digitised frequency down-converted version of an analogue signal produced by a transmitter VCO (46). The low IF digitised signal serves as a reference against which the digitised frequency down - converted version of the transmitter VCO (46) is compared. The output of the complex phase comparator (32) is converted to an analogue signal which is applied as a control signal to the transmitter VCO. An analogue embodiment of the transmitter which uses zero - IF signals is also disclosed.