This contribution addresses theoretically and experimentally the photomixing signal in a low-temperature-grown GaAs (LTG-GaAs) device with a planar antenna that is driven with an alternative electric field superposed on a bias electric field. The total electric field is applied on the optically-driven nonlinear photoconductance of LTG-GaAs. A theoretical framework based on unidimensional carrier transport is developed to calculate the amplitudes and the phases of different spectral components of the current density. Radiofrequency modulation of the photomixing signal in the microwave domain is experimentally demonstrated. Conversely, the photomixer is exploited as an optically-driven detector. Heterodyne detection with a photomixer is experimentally demonstrated with modulated THz-waves. The photomixing approach allows to extract THz modulation parameters by measurements with a modulated optical beat.