Bit error rate experiments in ring-shaped RSFQ circuits

Ring-shaped rapid single flux quantum (RSFQ) circuits composed of segments of Josephson transmission lines (JTLs) and other RSFQ circuits enable permanent SFQ pulse circulation. New ring structures of different designs have been realized which comprise T-flipflop (TFF) and multiplier (MULT) circuits. Reliability in circuit operation has been proven experimentally by a bit error rate BER≅10⁻¹⁶. The fabrication process has been optimized by using PTB-4 μm Nb/Al₂O₃–Al/Nb trilayer technology with externally shunted tunnel junctions of critical current densities of j_c=≅1 kA/cm². Characteristic voltage is V_c=250 μV and Steward–McCumber parameter β_c≤1. A linear dependence of pulse circulation frequency on JTL bias currents has been measured within a bias current interval of 20%.