Investigation of the relationship between the gray zone and the clock frequency of a Josephson comparator

The Josephson comparator is one of the fundamental building blocks of Rapid Single Flux Quantum (RSFQ) electronics. Within this electronic family it is the exclusive device which provides logical data processing. The Josephson comparator is also the basic decision element for very fast analog-to-digital converters and sampler circuits for low input power and high-bandwidth signals based on RSFQ-technique. The performance of those devices is fundamentally determined by the characteristics of the Josephson comparator. In this study the gray zone dependency on the clock frequency of a Josephson comparator is investigated by simulations including the influence of thermal noise. This investigation is performed for a series of operating points defined by the bias current and different noise levels defined by the operating temperature. We identified a characteristic maximum frequency fc for a comparator topology suitable fo a 1kA/cm2 niobium fabrication technology. The grey zone is constant for clock frequencies below fc and starts to increase for larger frequencies. We also found that this charcteristic frequency fc is independent of the intensity of thermal noise and therefore independent of temperature.