Selection of local oscillator frequency in a transmitter which transmits on multiple frequencies simultaneously

This application concerns a transmitter for transmitting data via a combination of a first signal modulated at a first carrier frequency, and a second signal modulated at a second carrier frequency. The transmitter includes a single local oscillator 1418 which is used to up-convert baseband (1002 Fig 10) or/and low IF signals (1004 Fig.10; 402, 404 Fig. 4) to create signals (1010, 1008; 408, 410) at the two carrier frequencies. The local oscillator frequency is selected in dependence on a required signal strength of the first signal relative to a required signal strength of the second signal. If both signals require similar signal strengths then the local oscillator frequency is set to a value midway between the two carrier frequencies so that the transmitter is operated as a direct conversion transmitter (as in Fig. 4). However, if significantly different signal strengths are required the local oscillator frequency is set to one of the carrier frequencies (as in Fig. 10). The invention finds particular application in carrier aggregation systems.