A transmitter in which a complex low IF digitised signal is applied in quadrature to a complex phase comparator together with a digitised frequency down-converted version of an analogue signal produced by a transmitter VCO. The low IF digitised signal serves as a reference against which the digitised frequency down-converted version of the transmitter VCO is compared. The output of the complex phase comparator 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.