A three-phase single flux quantum shift register with serial biasing

A three-phase single-flux-quantum (SFQ) shift register has been designed and tested in niobium technology. Serial biasing provides ultralow power dissipation, while allowing operating rates simulated at tens of gigabits per second. This circuit should be compatible with a YBCO weak-link junction process. The authors describe the circuit and its advantages and disadvantages relative to other shift register circuits. They also describe the design constraints, which serve to illustrate some of the issues in general SFQ logic design. Preliminary experimental results from the niobium version are presented. Although the authors are confident that the design works as predicted by simulations they were not able to demonstrate a level of functionality of the pseudorandom generator commensurate with expectations. They believe that this is due to a combination of the usual factors attendant in SFQ circuit testing, i.e. susceptibility to noise and trapped flux, and to correctable conditions related to the treatment of input/output lines on-chip.<
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