New elements for analysis of series arrays of superconducting junctions for submillimeter heterodyne detection

Abstract
Some specific features of series arrays of superconducting junctions used as mixing elements in radioastronomical spectroscopy are analysed. The influence of the area and insulating barrier inhomogeneities between the different junctions is examined with a simple and accurate technique. It uses the variation of the maximum Josephson currents of the individual junctions as a function of an applied external magnetic field. It is also shown with this technique that the possibility for some junctions of the array to trap some random flux can prevent stable and sensitive operation of the mixer. Some experimental performances of mixers working near 380 and 550 GHz with series arrays are given. The difficulty of fully suppressing the Josephson currents of each junction at a time appears to be partly responsible for lower sensitivity and instabilities. Another cause of degraded performance comes from the different embedding impedances of each individual junction at the working frequency. It is due to a spreading of the geometrical parameters either of the junctions or of the individual tuning circuits used to compensate the junction capacitance for the 550 GHz mixer. The best performance of the array is then the result of a compromise where all junctions do not operate with optimum efficiency.