NEW ELEMENTS FOR ANALYSIS OF SERIES ARRAYS OF SUPERCONDUCTING JUNCTIONS FOR SUBMILLIMETER HETERODYNE-DETECTION

Some specific features of series arrays of superconducting junctions u sed as mixing elements in radioastronomical spectroscopy are analysed. The influence of the area and insulating barrier inhomogeneities betw een the different junctions is examined with a simple and accurate tec hnique. It uses the variation of the maximum Josephson currents of the individual junctions as a function of an applied external magnetic fi eld. It is also shown with this technique that the possibility for som e junctions of the array to trap some random flux can prevent stable a nd 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 juncti on at a time appears to be partly responsible for lower sensitivity an d instabilities. Another cause of degraded performance comes from the different embedding impedances of each individual junction at the work ing frequency. It is due to a spreading of the geometrical parameters either of the junctions or of the individual tuning circuits used to c ompensate the junction capacitance for the 550 GHz mixer. The best per formance of the array is then the result of a compromise where all jun ctions do not operate with optimum efficiency.