Receiver with automatic gain control by an alternating current closed loop

The disclosed receiver comprises an antenna 310 configured to receive a wideband signal in the terahertz frequency range and to couple this signal to a quadratic detector 320. The received signal is modulated with data that has been sampled at a sampling frequency. The quadratic detector may be a plasmonic FET (420, fig.4) or a mixer comprising Schottky barrier diodes (620, fig.6). The signal from the quadratic detector 320 passes to a low pass filter 340 for data recovery, and to a bandpass filter 330 centred on the symbol rate. The strength of the symbol rate signal is measured 370 and used to control 380 the power of a regenerated carrier signal from a carrier regeneration circuit 360. The level-controlled, regenerated carrier signal is supplied to the quadratic detector 320 together with the signal from the antenna 310.