  Terahertz sources based on intra-cavity difference-frequency generation in
mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the
first monolithic electrically-pumped semiconductor sources capable of operating
at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress
in power output, that now exceeds 1mW in pulsed and 10 {\mu}W in
continuous-wave regime at room-temperature, knowledge of the major figure of
merits of these devices for high precision spectroscopy, such as spectral
purity and absolute frequency tunability, is still lacking. Here, by exploiting
a metrological grade system comprising a terahertz frequency comb synthesizer,
we measure, for the first time, the free-running emission linewidth (LW), the
tuning characteristics, and the absolute frequency of individual emission lines
of these sources with an uncertainty of 4 x 10-10. The unveiled emission LW
(400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited to
operate as local oscillators and to be used for a variety of metrological,
spectroscopic, communication, and imaging applications requiring
narrow-linewidth THz sources.
