Superconducting second-order sigma-delta modulators utilizing multi-flux-quantum generators

We have been developing a superconducting second-order sigma-delta analog-t o-digital converter with a large bandwidth and a large dynamic range for so ftware radio applications. In a superconducting second-order sigma-delta mo dulator utilizing a passive integrator and single-flux-quantum (SFQ) feedba ck, gain to compensate the large attenuation in the passive integrator of t he modulator is required to obtain a large dynamic range, In this paper, a second-order modulator utilizing multi-flux-quantum (MFQ) generators creating the needed gain is reported and the design and experime ntal test results of the elementary circuits including a comparator, a firs t-order modulator, and the MFQ generator are described, The digital outputs of the comparator and the first-order modulator were experimentally evalua ted, where the input dependence of the comparator switching probability and the noise-shaping properties of the modulator were clearly observed. The l ow speed operation of the MFQ generator creating three SFQ pulses was exper imentally confirmed, The other circuits were also tested at low frequencies .