A system comprising: a photomixer transmitter and homodyne receiver ; and two sources configured to emit electromagnetic signals at offsetting frequencies ω 1 and ω 2 for pumping the photomixer transmitter to transmit an electromagnetic signal to the homodyne receiver at each of one or more selectable difference frequencies (ω 2 - ω 1 ) in the terahertz region of the electromagnetic spectrum, the system including a cavity along a propagation path of the transmitted electromagnetic signal between the photomixer transmitter and the homodyne receiver ; characterised by a transmitter bias modulator configured to generate a modulated signal at a frequency ω m for biasing the photomixer transmitter , the electromagnetic signals emitted by the two sources also being for pumping the homodyne receiver to down convert the electromagnetic signal received thereat; and a frequency dither circuit coupled to either or both of the two sources and configured to apply a frequency dither to either or both of the electromagnetic signals emitted thereby, the applied frequency dither having a selected span of at least a minimum frequency period of an expected standing wave in the system that is based on distance between reflectors in the cavity, and a selected rate greater than a signal processing bandwidth for sampling a frequency spectrum including the one or more selectable difference frequencies; wherein the signal processing bandwidth has been determined based upon the modulated signal generated by the transmitter bias modulator , so as to process the transmitter bias modulator signal with minimal attenuation. a photomixer transmitter and homodyne receiver ; and two sources configured to emit electromagnetic signals at offsetting frequencies ω 1 and ω 2 for pumping the photomixer transmitter to transmit an electromagnetic signal to the homodyne receiver at each of one or more selectable difference frequencies (ω 2 - ω 1 ) in the terahertz region of the electromagnetic spectrum, the system including a cavity along a propagation path of the transmitted electromagnetic signal between the photomixer transmitter and the homodyne receiver ; characterised by a transmitter bias modulator configured to generate a modulated signal at a frequency ω m for biasing the photomixer transmitter , the electromagnetic signals emitted by the two sources also being for pumping the homodyne receiver to down convert the electromagnetic signal received thereat; and a frequency dither circuit coupled to either or both of the two sources and configured to apply a frequency dither to either or both of the electromagnetic signals emitted thereby, the applied frequency dither having a selected span of at least a minimum frequency period of an expected standing wave in the system that is based on distance between reflectors in the cavity, and a selected rate greater than a signal processing bandwidth for sampling a frequency spectrum including the one or more selectable difference frequencies; wherein the signal processing bandwidth has been determined based upon the modulated signal generated by the transmitter bias modulator , so as to process the transmitter bias modulator signal with minimal attenuation. <EOS>