A system comprising: a photomixer transmitter and homodyne receiver; and two sources (14a, 14b) 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 (14a, 14b) 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 (14a, 14b) 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 (26a, 26b) 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 (14a, 14b) 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 (14a, 14b) 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 (14a, 14b) 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 (26a, 26b) 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.