Features of the spectrum of a niobium Josephson junction array measured with the use of high-Tc Josephson mixer

Spectral measurements of a niobium Josephson junction arrays with overwavelength dimensions have been performed using a heterodyne receiver with a mixer based on a high-Tc bicrystal transition. The array is fabricated on a silicon substrate and is composed from 9996 series connected Nb/NbSi/Nb junctions embedded to parallel single-strip lines with the length 7.1 mm. The mixer in the receiver is a Josephson junction formed by a YBaCuO film on a bicrystal boundary of Zr1−xYxO2 substrate. Spectral measurements have been performed in the frequency ranges 150-180 GHz and 225-250 GHz. The aim of the work was the investigation of the influence to the spectral line from some factors of external electrical circuit: the type of power supply and the level of technical noises. Significant decrease of the linewidth from ~ 10 MHz to 0.5-0.8 MHz has been found at the connection of the array to voltage source instead of current source and at the minimizing of technical noises. The shift of spectral frequency from the Josephson one averaged over the array has been observed that is explained by the peculiarities of synchronization of Josephson junctions in large arrays.