An apparatus, a method and computer program, the apparatus comprising: a modulator (9) comprising a first input for receiving a data signal (27) and a second input for receiving an output signal (23) from a phase locked loop (5) wherein the modulator (9) is for modulating the output signal (23) from the phase locked loop (5) using the data signal (27) and providing the modulated signal (31) to an output for connection to an amplifier (13); and a phase shifter (11) configured to result in a predetermined phase shift to the modulated signal (31).