  A room-temperature mid-infrared (9 um) heterodyne system based on
high-performance unipolar optoelectronic devices is presented. The local
oscillator (LO) is a quantum cascade laser, while the receiver is an antenna
coupled quantum well infrared photodetector optimized to operate in a
microcavity configuration. Measurements of the saturation intensity show that
these receivers have a linear response up to very high optical power, an
essential feature for heterodyne detection. By an accurate passive
stabilization of the local oscillator and minimizing the optical feed-back the
system reaches, at room temperature, a record value of noise equivalent power
of 30 pW at 9um. Finally, it is demonstrated that the injection of microwave
signal into our receivers shifts the heterodyne beating over the bandwidth of
the devices. This mixing property is a unique valuable function of these
devices for signal treatment.
