A source of radiation comprises a first low frequency oscillator 200 for providing a reference signal and a plurality of phase shifters 210a, 210b, 210c coupled to the first oscillator. In addition there are a plurality of phase locked loops 230a, 230b, 230c, each phase locked loop having a respective Voltage Controlled Oscillator (VCO) 240a, 240b, 240c for outputting a signal. Each phased locked loop is coupled to a respective one of the phase shifters, so that in use each VCO is phase locked to a reference signal which has been phase shifted by a respective one of the phase shifters. In this way the phase of the radiation output by each VCO may be controlled indirectly by controlling the phase shift of the reference signal. In a preferred embodiment the phase shifters are adjustable to shift the phase by an adjustable amount.