A phase detector for producing an output signal indicative of the relative phase of periodic transitions in a first signal and a further periodic signal comprising: a smoothing circuit (33); a variable gain amplifier (31) via which the first signal is applied to the smoothing circuit; and a control circuit (35) for varying the gain of the amplifier cyclically between positive and negative values at the frequency of the further signal.The detector finds particular application in a phase locked loop arrangement whereby a periodic signal generator is phase locked to periodic transitions occuring between an adjacent pair of channels in each frame of a multiplex time division signal, the other frames of which contain analogue data. The gain of the amplifier is then suitably varied in steps of equal magnitude between positive and negative values of equal magnitude, the time between adjacent steps being equal and the number of steps being equal to the number of channels in a frame.