RSFQ logic devices; non-linear properties and experimental investigations

Abstract

Rapid Single Flux Quantum (RSFQ) logic has a great potential as fast digital and high frequency analog electronics. Several Logic/Memory base elements and integrated sub-systems in the RSFQ family have been devised and tested since the pioneering work in the mid 1980s by K. K. Likharev’s group at Moscow State University [1,2]. It is argumented why the RSFQ digital circuits are superior to the voltage state family circuits, which were utilised in the first development of Josephson logic. Also the parameter space for operation of the 1-D RSFQ transmission line is discussed. Presently most RSFQ circuits are made with low-Tc superconductors using the now mature whole-wafer NbAlOxNb technology, which allows for large and densely packed integrated circuits. Recently, a few operational high-Tc RSFQ circuits have been reported. An important development within the last two years is the advent of general-purpose on-chip bit-by-bit verification test systems. Timing of RSFQ circuits and a few recent RSFQ “highlights” are briefly mentioned. Basically the RSFQ technology appears “ready” for widespread industrial use. One of the key components is the RSFQ transmission line, which can both generate and transmit SFQ pulses. In order to demonstrate the importance of the fluxon dynamics we discuss a new phenomenon observed in a parallel array of identical junctions. Steps with extremely low differential resistance in the I–V characteristic are found to be due to the self-induced magnetic field produced by the edge current fed to the array. The underlying mechanism is that the nonuniform field divides the moving fluxon into “domains” covering several (unit) cells. The experimental/numerical results illustrate practical and may be more fundamental limits to RSFQ electronics.