Double relaxation oscillation SQUID with a 4JL on-chip digital flux locked-loop circuit

Double relaxation oscillation SQUID (DROS) combined with an on-chip superconducting digital flux locked-loop (FLL) circuit has been proposed. In this study, we described superconducting digital FLL circuit which consists of an 8-bit up/down counter and an 8-bit R-2R ladder D/A converter. The circuit was designed using 4-Junction Logic (4JL)-gates which are driven by a two phase power supply. In the up/down counter, we employed binary carry lookahead (BCL) circuits for high-speed operation. Logic simulations for the BCL 8-bit up/down counter showed correct operation up to 4 GHz, assuming a 2.5 kA/cm² Nb/Al-AlO_x/Nb junction technology. From simulation results high slew rate of 10⁷Φ₀/s and dynamic range of 2.5Φ₀ can be expected.