Josephson junctions in SPICE3

SPICE3 is the most recent version of the circuit simulator SPICE from the University of California, Berkeley. Unlike its predecessors, SPICE3 is written in the C programming language, and is designed for interactive use under a modern multitasking operating system. The Berkeley distribution of SPICE3 lacks support for Josephson junctions. As a consequence, the author has developed a customized version of SPICE3b.1 which incorporates a Josephson junction model. The model supports control current modulation, as well as fifth-order polynomial description of the quasiparticle current suitable for NbN junctions. In addition, a margin analysis command has been added, allowing operating ranges to be determined for an arbitrary circuit with a minimum of simulation. Further enhancements include the addition of a second graphics post-processor with new features and display, as well as debugging and streamlining of the original code. The program is currently configured to run on an IBM/Intel 386 compatible computer with Weitek coprocessor, under DOS extender software. The modifications to SPICE3 are described, and some of the issues involved in simulating Josephson circuits are also addressed. The features and philosophy of the SPICE3 program are discussed, and sample results presented.