Noise-shaping characteristics of superconducting band-pass sigma-delta modulators

The noise-shaping characteristics of superconducting band-pass single-loop sigma-delta modulators were clearly observed for the first time at 4 K usin g a 2.86 GHz sampling clock and a 0.69 GHz signal. The modulator consisted of a Josephson comparator, a driver, and a lumped-element-circuit supercond ucting resonator. The modulator chips were fabricated using NEC's Nb standa rd process. Digital data from the modulator were consecutively acquired usi ng a semiconductor data acquisition system. By using a fast Fourier transfo rm (FFT) software, the output spectrum of the noise-shaping characteristics was calculated from the measured data. Also, the input-output characterist ics were measured experimentally, and the modulator's linearity was confirm ed.