Performance issues in single flux quantum shift registers

Testing on a single-flux-quantum (SFQ) shift register fabricated with a SFQ Nb-AlO/sub x/Nb Josephson junction process indicates that a stable flux quantum can be stored in a rapid SFQ (RSFQ) shift register stage and be shifted by SFQ pulses. Design simulations showed that logic errors can occur in SFQ shift registers even though the circuit passes the average voltage test. Three-junction-per-stage shift registers had an advantage over the two-junction-per-stage shift register in handling data with successive 1's and in incorporating resistors to increase tolerance of trapped magnetic flux. Input and output buffers for data and clock pulses were added to get an RSFQ shift register to obtain correct performance. The highest error-free operation speed for the data with successive 1's is about half of the speed for propagation of a lone data 1. Error-free operation of SFQ shift registers with three junctions per stage could be obtained up to speeds of 30-60 GHz with Josephson junctions having I/sub c/R/sub n/ (critical-current-normal-resistance product) values of 600 mu V.>