Integrated microwave circuits for digital-to-analogue converters based on high-temperature superconducting Josephson junction arrays

A digital-to-analogue converter based on series arrays of high-temperature superconducting Josephson junctions can work properly when the microwave bias current distribution in large arrays is uniform. Unfortunately, the considerable microwave attenuation per overdamped junction with normal resistances R(N) approximate to 0.1 Omega results in a strong total attenuation of the microwave signal in a traditional layout. Also, the meander geometry of the array, necessary when using bicrystal junctions, disturbs the uniformity of the rf current, especially in the millimetre waveband. To overcome these drawbacks we coupled the meander array inductively to a parallel transmission line. The mechanism of microwave coupling in which both external and mutual frequency locking takes place was studied numerically. A circuit with a coplanar waveguide feed line was fabricated and successfully tested at an operating frequency of 30 GHz. A microstrip line with a small attenuation For pumping of junction arrays at frequencies up to 120 GHz was demonstrated.