A new continuously tunable submillimeter source for spectroscopy and other high‐resolution applications has been developed. In this source the optical spectrum of a mode‐locked femtosecond laser is downconverted into the submillimeter region by the demodulation process of a photoconductive switch. The power generated is subsequently radiated into free space by an antenna which is integrated along with the switch on low‐temperature grown GaAs. The very high resolution is ultimately traceable to the cavity length of the laser and the stable mode‐lock frequency which results. Among the most important attributes of the sources are straightforward absolute frequency calibration, very high spectral purity, and the potential for spectral multiplexing. ©1995 American Institute of Physics.