Predicted performance of superconductor-insulator-superconductor mixers with inhomogeneous distributed junction arrays

The mixing properties of a new type of distributed superconductor-insulator-superconductor (SIS) junction array, which consists of different dimensions of junctions and spacings between every two junctions, have been theoretically investigated using the quantum theory of mixing. A set of dimensions of junctions and spacings between every two junctions of the distributed junction array have been determined so as to minimize the reflection coefficient of an S-parameter, S-11, at the input port in the array using microwave computer-aided design, assuming that each SIS junction is represented by a parallel combination of linear normal-state resistance and geometrical capacitance. When the array is composed of well optimized set of dimensions of junctions and spacings between every two junctions, it is demonstrated that excellent mixing properties are attained over a broad-band in spite of the low critical current density of an SIS junction. It is also found that the degradation of mixing properties, which is always observed at certain frequencies in a conventional array with the equal dimensions of junctions and spacings, can be considerably suppressed in the array with a well optimized set of dimensions and spacings of junctions.