  We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz
which display bandwidths up to 950 GHz with a single stable beatnote. Devices
are spontaneously operating in a harmonic comb state, and when in a dense mode
regime they can be injection locked at the cavity roundtrip frequency with very
small RF powers down to -55 dBm. When operated in the electrically unstable
region of negative differential resistance, the device displays ultra-broadband
operation exceeding 1.83 THz ($\Delta f/f=50\%$) with high phase noise,
exhibiting self-sustained, periodic voltage oscillations. The low CW threshold
(115 A$\cdot$ cm$^{-2}$) and broadband comb operation ($\Delta f/f=25\%$) make
these sources extremely appealing for on-chip frequency comb applications.
