Heterodyne reception at 891 GHz using a Josephson harmonic mixer and a microwave local oscillator

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Abstract
The performance of Nb-Nb point-contact Josephson junction mixers was investigated experimentally with signals from a 891 GHz laser. The local oscillator (LO) was provided by the appropriate junction-generated harmonic of either a 74 or 99 GHz klystron. The intermediate frequency (IF) power varied linearly with signal power for signals less than 10−5 to 10−6 W. The optimum conversion loss (from 891 GHz to the IF) was not significantly different for the two klystron frequencies used: the lowest useful conversion loss for the whole mixing system including the signal input matching loss, was 42 (±3) dB. This is 10 dB greater than that observed with a 891 GHz laser LO and the same junction apparatus. Conversion loss of the mixer element alone was independent of IF with the latter varied up to 12 GHz. The noise at the output of the IF system was usually dominated by that of the transistor preamplifier with a noise temperature of about 500 K. Thus the input signal necessary to give a signal-to-noise ratio of one at the IF was equivalent to 10−16 W Hz−1 of IF bandwidth. This mixing system, which can be significantly improved, forms the basis of a fully tunable heterodyne receiver for the submillimetre region.