Impedance Matching of Microstrip Inductors in Digital Superconductive Electronics

State-of-the-art RSFQ circuits consist of Josephson junctions, inductances and bias current sources, where the inductances are practically used as functional elements. Even more, their value defines the functionality and therefore the conventional design process requires strict conditions for the wiring between Josephson junctions. The present design and optimization process does not take into account their parasitic capacitances to ground. This becomes an important issue for ultra fast applications above 100 GHz, but also for circuits dedicated to control quantum electronics. We report on a theoretical and experimental investigation of the influence of ground capacitances. The impedance matching between the connecting microstrip lines and the Josephson junction is a key issue for the correct operation of these circuits. We performed circuit simulations and collected experimental data of correct and incorrect operating circuits to derive design criteria. The critical reflection coefficient is found to be 0.35 and the particular design must use a smaller value for transfer and a larger value for storage of SFQ pulses, respectively.