Superconducting RF tuning circuits for low-noise submillimeter wave SIS receivers

Heterodyne receivers using SIS (Superconductor-Insulator-Superconductor) junctions are the most sensitive receivers for radioastronomy in the millimeter wave and submillimeter wave frequency range. These instruments use superconducting RF tuning circuits to resonate out the high shunt capacitance of the SIS junction in order to provide a good impedance match with the microwave envionment at the frequency of operation. An accurate model to determine the electrical propeties of impedance matching circuits has been developed. An analysis of superconducting microstrip lines and radial stubs taking into account the fringing fields of the thick electrode, the step discontinuities, and the Mattis-Bardeen complex conductivity for the superconductor surface impedance is presented. This model is used to calculate the resonance frequencies of the matching circuit with the capacitively-´shunted SIS junction. Good agreement is found between calculated and experimentally measured resonance frequencies at 550-600 GHz.