  Terahertz quantum cascade laser sources based on intra-cavity difference
frequency generation from mid-IR devices are an important asset for
applications in rotational molecular spectroscopy and sensing, beingthe only
electrically pumped device able to operate in the 0.6-6 THz range without the
need of bulky andexpensive liquid helium cooling. Here we present comb
operation obtained by intra-cavity mixing of adistributed feedback laser at
{\lambda} = 6.5 {\mu}m and a Fabry-P\'erot device at around {\lambda} = 6.9
{\mu}m. The resultingultra-broadband THz emission extends from 1.8 to 3.3 THz,
with a total output power of 8 {\mu}W at 78K.The THz emission has been
characterized by multi-heterodyne detection with a primary frequencystandard
referenced THz comb, obtained by optical rectification of near infrared pulses.
The down-converted beatnotes, simultaneously acquired, confirm an equally
spaced THz emission down to 1 MHzaccuracy. In the next future this setup can be
used for Fourier transform based evaluation of the phaserelation among the
emitted THz modes, paving the way to room-temperature, compact and
field-deployable metrological grade THz frequency combs.
