We demonstrate that a two-dimensional electron system fabricated from a GaAs/AlGaAs quantum well in the presence of a magnetic field B possesses the ability to detect electromagnetic radiation in a broad frequency range. Irradiation of the sample with microwaves produces a dc-photovoltage which oscillates as a function of B. The amplitude and the period of the oscillations are proportional to the radiation power and the wavelength, respectively. Successful operation of such a detector/spectrometer is reported for microwave frequencies up to similar to150 GHz and temperatures up to similar to80 K. We do not anticipate any principal difficulties in extending the operation frequency further into the terahertz region.