In this study , we designed and experimentally evaluated a series-connected array of superconductor-insulator-superconductor ( SIS ) junctions in the 100-GHz band mixer for the multi-beam receiver FOREST on the Nobeyama 45-m millimeter-wave telescope .
The construction of the junction chip comprised a waveguide probe antenna , impedance matching circuit , SIS array junction , and choke filter , which were made from a superconducting niobium planar circuit on a quartz substrate .
The multi-stage impedance matching circuit between the feed point and the SIS junction was designed as a capacitively loaded transmission line , and it comprised two sections with high ( \sim 90 \Omega ) and low ( \sim 10 \Omega ) characteristic impedance transmission lines .
The structure of this tuning line was simple and easy to fabricate , and the feed impedance matched with the SIS junction in a wide frequency range .
The signal coupling efficiency was more than 92 % and the expected receiver noise temperature was approximately two times the quantum limit for 75–125 GHz based on quantum theory .
The array junction devices with 3–6 connected junctions were fabricated and we measured their performance in terms of the receiver noise temperature and gain compression in the laboratory .
We successfully developed an array junction device with a receiver noise temperature of \sim 15–30 K and confirmed that the improvement in the saturation power corresponded to the number of junctions .
The newly developed array junction mixer was installed in the FOREST receiver and it successfully detected the ^ { 12 } CO ( J = 1–0 ) molecular line toward IRC+10216 with the Nobeyama 45-m telescope .