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

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