Design and Testing of Quasi-One Junction SQUID-Based Comparators at Low and High Speed for Superconductive Flash A/D Converters

We have fabricated and tested single comparators at low and high speeds (up to 10-Ghz input bandwidth) to determine basic functionality, dynamic range, linearity, noise levels, and dynamic distortions. A 4-bit converter was tested at low speed without errors. The results indicate that a 6-bit converter may be constructed of these comparators with very good linearity and 8 bits should be achievable with slight design modification. The comparator was demonstrated to have an aperture uncertainty <= 15 ps limited by the jitter in the test setup. Simulation of the fabricated circuits with estimated parasitics show about six times smaller aperture time, which is better than the best semiconductor converters of which we are aware. Simulations performed assuming a higher current density are five times better than this, or about 6 bits at 10-Ghz.