Josephson heterodyne detection at high thermal background levels

Point‐contact Josephson junctions with a submicron contact diameter have been used as high‐frequency mixers (170–220 GHz) under high thermal background conditions. The junction is formed across a full height (0.56×1.06 mm2) waveguide between a 3–5‐μm thick Nb whisker and a 0.2‐mm diam Nb center conductor of a coaxial line. The best single‐side band mixer‐noise temperature that was measured is 165 K (± 25%) with a corresponding single‐side band conversion efficiency of 0.36±0.03 (a loss of 4.5 ±0.4 dB) at a signal frequency of 185 GHz, including losses in the coupling horn and the waveguide. The performance was measured with the hot‐cold source technique. The effects of nonheterodyne response in that situation were investigated in detail. The junctions have been operated in a coherent receiver that can be used on a telescope with only small adaptations. The instantaneous bandwidth of the receiver is about 20 GHz.