A multimode transceiver circuit comprises a four port coupler of a type which splits a signal applied to one port between two of the other ports. The split signals are phase shifted by a known amount. A first port of the coupler is for receiving a signal and for outputting a signal therefrom. A controllable oscillator is connected to a second port of the coupler and switchable impedances (Schottky diodes) are connected to the remaining ports. A controller is provided for controlling the operation of the oscillator and the impedances to determine the mode of operation of the circuit. In one mode of operation, the oscillator is inoperative and at least one of the impedances is switched to operate as a non-linear impedance such that the circuit detects amplitude-modulation of the signal applied to the first port. In a further mode of operation, the oscillator is operable and both impedances are switched to operate as non-linear impedance elements such that they operate as a mixer to produce an intermediate frequency signal in dependence on the signal applied to the first port. In yet a further mode of operation, the oscillator is operable, and the impedances are switched such that the oscillator frequency appears at the first port for transmission therefrom.