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.