For mixing in a Josephson junction at infrared frequencies, we have shown that the available power from the junction increases as the intermediate frequency is increased. Following this result an infrared receiver has been developed incorporating a 9‐GHz maser preamplifier at the i.f. Using this system, the beat between the 401st harmonic of a high‐quality microwave source and a 3.8‐THz infrared laser has been observed. Also, for low‐order mixing at 3.8 THz, a comparison of beat signals from a Josephson junction and a room‐temperature mixer has been made.