The heterodyne receiver consists of a superconducting tunnel diode mixer mounted with a first intermediate frequency amplifier at the 4 K stage of the liquid helium Infrared Laboratory cryostat, of an ambient temperature second IF amplifier, and of a local oscillator. Local oscillator power is injected in the waveguide mixer input port through a cooled waveguide coupler. The mixer operates in the whole receiver band without any tuning, the only tunable part of the receiver being the local oscillator.