A radio receiver or transmitter including a tunable filter constituted by an N-path filter (14) having a local oscillator (52). the centre frequency of which filter is displaced from the oscillator (52) frequency through the use of feedback (26). In the case of a radio receiver the feedback provides image rejection. Preferably an i.f. signal is taken from the low frequency section of the N-path filter. In the case of a radio transmitter, an i.f. signal is applied to a low frequency path within the N-path filter. The use of feedback avoids the generation of an image frequency transmission.