Extraction of impacts of fabrication spread and thermal noise on operation of superconducting digital circuits

We have developed a procedure to investigate and distinguish several disturbing influences to an RSFQ circuit such as thermal noise, flux trapping and fabrication spread. The procedure is applied to a slightly modified RSFQ shift register with: 3 independent dc biases, which are common for all cells. One of the biases performs a special function. Nominally it is equal to zero and used only to create a controllable effect on the operation of the circuit. The impact of different factors is extracted from their interference with the special bias. It has been found that the influence of thermal noise in the best samples fabricated at HYPRES, Inc. is comparable with that of fabrication spread even for low (~4 K) operational temperature.