Influence of parasitic capacitance on the oxide SFQ circuit operation

Characteristics of SFQ (single flux quantum) circuits have been found to much change when parasitic capacitances are increased beyond a critical value. The characteristic change of the SFQ operation is especially enhanced when junction the IcRn (Ic being the critical current and Rn being the normal resistance) value is much more than 1 mV. Details of this characteristic change were studied by using a ring oscillator. In the normal operation mode, an oscillation frequency gradually increased with a supply current. On the other hand, the oscillation frequency is almost independent of the supply current and jumps at a certain supply current in the anomalous operation mode. The ring oscillator circuit was fabricated by using YBa2Cu3O7-x ramp edge junctions. Such a ring oscillator behavior was experimentally shown by measuring the supply current dependence of the oscillation frequency