Tapered Transmission Lines With Dissipative Junctions

NIST is optimizing the design of a 10 V programmable Josephson voltage standard so that it uses less microwave power by employing fewer parallel-biased arrays with higher voltage per array. Increasing the voltage per array by adding more junctions is challenging because the dissipation of the over-damped Josephson junctions limits the total number that may be located in each array. If there is too much dissipation in the array, the junctions at the end receive too little microwave power compared with the junctions at the beginning of the array. To compensate for the junction attenuation, tapered impedance transmission lines were used to maintain a nearly constant microwave current along the lossy transmission line. Simulation and testing have improved the microwave uniformity of our designs for tapered impedances from 85 ohms to 5 ohms. Low-leakage bias tees for various characteristic impedances were designed so that sub-arrays could be measured within long arrays. These tapered arrays have improved the bias current margins, junction number, and bandwidth of NIST junction arrays. By measuring the microwave power from the output of these long arrays, harmonic generation and the nonlinear properties of dissipative junction arrays are studied.