A feed signal generator (10, 30, 40, 50, 60, 70, 100, 112, 120) for a phased array antenna, the feed signal generator comprising: an input (12, 72) arranged to receive an optical spectrum having first and second phase-locked spectral components, the first spectral component having a first frequency and the second spectral component having a second frequency different to the first frequency by a preselected frequency difference; wavelength selective separator apparatus (14, 74) arranged to separate the optical spectrum into a first optical signal being the first spectral component and a second optical signal being the second spectral component; an optical time delay element (16, 32, 42, 62, 92) arranged to apply a time delay to the first optical signal to form a delayed optical signal; optical amplitude control apparatus (18, 64, 82) arranged to set an amplitude of the delayed optical signal; and a heterodyning device (20, 78) arranged to heterodyne the delayed optical signal and the second optical signal, wherein the second optical signal is delivered to the heterodyning device (20,78) without the time delay being applied, to generate a feed signal for the phased array antenna at the preselected frequency difference, the feed signal having a power proportional to a product of an amplitude of the second optical signal and the amplitude of the delayed optical signal and having a phase proportional to said time delay; wherein the optical amplitude control apparatus is arranged to set the amplitude of the delayed optical signal such that the product of the amplitude of the second optical signal and the amplitude of the delayed optical signal causes the power of the feed signal to have a preselected value. an input (12, 72) arranged to receive an optical spectrum having first and second phase-locked spectral components, the first spectral component having a first frequency and the second spectral component having a second frequency different to the first frequency by a preselected frequency difference; wavelength selective separator apparatus (14, 74) arranged to separate the optical spectrum into a first optical signal being the first spectral component and a second optical signal being the second spectral component; an optical time delay element (16, 32, 42, 62, 92) arranged to apply a time delay to the first optical signal to form a delayed optical signal; optical amplitude control apparatus (18, 64, 82) arranged to set an amplitude of the delayed optical signal; and a heterodyning device (20, 78) arranged to heterodyne the delayed optical signal and the second optical signal, wherein the second optical signal is delivered to the heterodyning device (20,78) without the time delay being applied, to generate a feed signal for the phased array antenna at the preselected frequency difference, the feed signal having a power proportional to a product of an amplitude of the second optical signal and the amplitude of the delayed optical signal and having a phase proportional to said time delay; wherein the optical amplitude control apparatus is arranged to set the amplitude of the delayed optical signal such that the product of the amplitude of the second optical signal and the amplitude of the delayed optical signal causes the power of the feed signal to have a preselected value.