The spectral components of a multifrequency THz-wave may be characterized using the nonlinear electrical response of a low temperature grown GaAs photomixer. The heterodyne detection with the photomixer is exploited to down-convert a radiofrequency-modulated THz-wave to the microwave domain, by using a suitably tuned optical beat that acts as local oscillator. The photomixer is electrically driven with an alternative voltage oscillating at the modulation frequency and provides a phase-sensitive DC response signal indicating the demodulation of the THz-wave.