Monolithic high-temperature superconducting (HTS) K-band sub-harmonic mixer based on reverse-biased Josephson junctions

Abstract This paper presents a monolithic high-temperature superconducting K-band sub-harmonic Josephson mixer using a pair of reverse-biased Josephson junctions (JJs), which are biased using reversed DC current. The sub-harmonic mixer can provide higher conversion gain and stronger rejection of the even-order harmonics of the local oscillator signal. A matching circuit with several bandpass filters (BPFs) is designed to match the junctions and achieve a maximum conversion gain. Based on the circuit configuration and using the established Verilog-A JJ mixer model, transient simulation has been done in the circuit simulator. An electromagnetic (EM) model is built and simulated in a full-wave simulation environment, and the results reveal that the design realizes very low loss at 25 GHz, excellent port isolation and high conversion efficiency. A prototype has been fabricated and tested in a cryocooler at operating temperatures of 40 K and 20 K. Measurements have experimentally verified the novel radio frequency design and simulation results. Maximum conversion gains of −10.5 dB and −6.5 dB were obtained at 40 K and 20 K, respectively, with a bandwidth greater than 400 MHz and outstanding gain linearity. It is also predicted that the mixer performance can be further improved with higher critical current I c values and identical or similar I c values between two parallel junctions.