Investigation of dc-SQUID dynamics by multiple flux entry

Digital Josephson circuits that do not switch into the voltage state provide an opportunity for high speed signal processing. Fast SFQ (Single Flux Quantum) logic devices must record and process small fast pulses generated by single fluxoid events. An useful application in cryoelectronics is an analog-to-digital converter using the intrinsic flux quantization of closed superconducting structures. The analog signal is quantized and converted into a pulse train resulting from fluxoid transitions. The dynamics of multiple flux entry from these transitions was analyzed for a dc-SQUID structure as quantizer proposed by Phillips in view of high-Tc applications(1,2). The device performance of a incremental quantizer depending on various circuit parameters was explored by numerical simulations based on the Stewart-McCumber model of the Josephson junction. We found several conditions which must be satisfied for an unambiguously pulse train, from which the analog signal can be faithfully reconstructed. The results of the simulations are presented illustrating the various ways in which the pulse trains are affected. (orig.)