Photonic radio frequency, RF, signal generation apparatus (100) comprising: a coherent optical source (102) to output a coherent optical signal (104) at a carrier frequency, f0; an optical splitter (106) to split the coherent optical signal into a first and a second optical carrier signal; RF signal generation apparatus (108) to generate at least one RF signal (V(t)) at an initial frequency, fe; optical modulation apparatus (110) driven by the at least one RF signal to modulate the first and second optical carrier signals at the initial frequency to generate optical harmonic signals (114) around each optical carrier signal; a first optical filter (116) to select an nth order optical harmonic signal (118) of a first sign, +n, of the optical harmonics around the first optical carrier signal; a second optical filter (126) to select the nth order optical harmonic signal (128) of an opposite sign, -n, of the optical harmonics around the second optical carrier signal; and an optical combiner (130) to combine the selected +n optical harmonic signal with the selected -n optical harmonic signal to generate an optical beat (132) signal forming an output photonic RF signal.