A reference signal is split and input to first and second variable phase shifters 10, 20. The first and second variable phase shifters output to first and second inputs 31, 32 respectively of a phase comparator 30. Initially, the first and second variable phase shifters 10, 20 are preferably set to the same phase. The first and second phase shifters are then aligned, e.g. by adjusting the calibration of one or both of the phase shifters so that the phase comparator 30 indicates that they output the same phase. The phase of the first phase shifter 10 is then adjusted by one step and a phase delay section 60 is placed between the output of the second phase shifter 20 and the second input 32 of the phase comparator 30. The first and second phase shifters 10, 20 are then aligned again.