A system for use in relation to a continuous wave THz spectroscopic arrangement, comprising: a transmitter and receiver, the transmitter being configured to transmit an electromagnetic signal to the receiver at each of one or more selectable frequencies, the receiver being configured to receive the electromagnetic signal and another electromagnetic signal for mixing therewith, wherein the electromagnetic signal is a THz signal and the other electromagnetic signal is an optical signal, the system including a first propagation path of the other electromagnetic signal to the transmitter, and a second propagation path of the other electromagnetic signal to the receiver; two different laser sources (14a,14b) at two different frequencies, both of them injecting light in each one of the first and second propagation paths; an arrangement located along either of the first or second propagation paths (18,32), or along each of the first and second propagation paths (18,32), the arrangement configured to alter the length of a respective propagation path such that the difference of the lengths of the first and second propagation paths (18,32) is altered at a pre-selected rate during transmission of the electromagnetic signal from the transmitter to the receiver, and receipt of the electromagnetic signal and the other electromagnetic signal at the receiver, the pre-selected rate spanning an integer multiple of wavelengths of the electromagnetic signal transmitted at a respective frequency over a dwell time; a modulator configured to amplitude modulate the electromagnetic signal transmitted by the transmitter, the modulator being configured to modulate the electromagnetic signal amplitude at a frequency ω m; and a processor configured to recover an amplitude and phase of the transmitted electromagnetic signal, including being configured to receive a sequence of samples of the received electromagnetic signal, and Discrete Fourier Transformation process the sequence of samples at a first frequency of interest ω m- ω FS, and DFT process the sequence of samples at a second frequency of interest ω m + ω FS, wherein ω FS represents a path length modulation frequency. a transmitter and receiver, the transmitter being configured to transmit an electromagnetic signal to the receiver at each of one or more selectable frequencies, the receiver being configured to receive the electromagnetic signal and another electromagnetic signal for mixing therewith, wherein the electromagnetic signal is a THz signal and the other electromagnetic signal is an optical signal, the system including a first propagation path of the other electromagnetic signal to the transmitter, and a second propagation path of the other electromagnetic signal to the receiver; two different laser sources (14a,14b) at two different frequencies, both of them injecting light in each one of the first and second propagation paths; an arrangement located along either of the first or second propagation paths (18,32), or along each of the first and second propagation paths (18,32), the arrangement configured to alter the length of a respective propagation path such that the difference of the lengths of the first and second propagation paths (18,32) is altered at a pre-selected rate during transmission of the electromagnetic signal from the transmitter to the receiver, and receipt of the electromagnetic signal and the other electromagnetic signal at the receiver, the pre-selected rate spanning an integer multiple of wavelengths of the electromagnetic signal transmitted at a respective frequency over a dwell time; a modulator configured to amplitude modulate the electromagnetic signal transmitted by the transmitter, the modulator being configured to modulate the electromagnetic signal amplitude at a frequency ω m; and a processor configured to recover an amplitude and phase of the transmitted electromagnetic signal, including being configured to receive a sequence of samples of the received electromagnetic signal, and Discrete Fourier Transformation process the sequence of samples at a first frequency of interest ω m- ω FS, and DFT process the sequence of samples at a second frequency of interest ω m + ω FS, wherein ω FS represents a path length modulation frequency.