A Josephson analog limiter circuit

The authors have designed, fabricated and successfully tested a Josephson integrated circuit that is used to limit the range of an analog input signal. The circuit consists of a single Josephson junction with a suppressed critical current. The impedance of this suppressed junction consists of the nonlinear quasiparticle resistance in parallel with the capacitance of the junction. The junction capacitance is used to advantage as one pole of a second-order low-pass filter, the other element of which is a thin-film inductor. This filter can perform the function of a slew-rate-limiting filter, at the input to a high-speed comparator. The authors fabricated individual junctions and measured their suppression characteristics and have found that a 2-μm×4.5 μm junction with a nominal critical current of 170 μA can be suppressed to less than 4 μA of critical current with a 14-mA control current. Complete limiter circuits have been fabricated and tested for both their DC and transient characteristics. Measured DC response is in good agreement with simulation, but parasitic capacitance present in the fabricated devices limited the -3 dB bandwidth to about 3.6 GHz