Embodiments of the present invention comprise a signal processing method and apparatus for use in a satellite payload in which an input RF signal received at a receiver antenna is modulated by using a single optical carrier at the input of an optical modulator. The optical domain signal is processed and is subsequently combined with a single unmodulated optical LO tone to provide an output RF signal for radiation by a transmitter antenna or for further digital processing by an on-board processor. This results in a clean generation of the frequency-converted RF signal at the output of the opto-electrical conversion stage.