Josephson junction arrays as a variable inductor in RF circuits and tunable filters

Abstract

We report on the investigation of the effects of the nonlinear inductance of hugeJosephson junction arrays on high-quality RF circuits. The inductance of a singlejunction is far too small for use in many RF applications; however, Josephsonjunction stacks and especially intrinsic Josephson junctions in crystalline-layeredsuperconductors open new opportunities. The stack inductance can be alteredby changing a DC Josephson current either using an external bias current oran external magnetic field applied to a closed superconducting loop containingtwo stacks connected in parallel. Using stacks of Josephson junctions in the LCresonators, a tunable 4-pole, 15 MHz band-pass filter with a centre frequency at 1.1 GHzwas designed. The filter was simulated in the time domain and the response wasconverted to the frequency domain. It was found that using an input power level of−62 dBm, corresponding to 0.5% of the junctions’ critical current, a linear approximationfor the stack nonlinear inductance can be used. By changing the appliedmagnetic field, a tuning range of 20% was demonstrated with a tuning speed of2 µs or better. An estimate of the losses is also given. In addition, power handling up to−26 dBm is reported using increasing circuit complexity.