Design of superconducting band-pass sigma–delta modulators

Abstract

We have been developing a superconducting band-pass sigma–delta modulator, which is the elementary circuit of the superconducting A/D converter. We have already obtained the noise-shaping characteristic of a superconducting band-pass modulator experimentally. In this paper we investigate the noise-shaping characteristic of the band-pass sigma–delta modulator from a circuit simulation, and compare it with the experimental results. We confirmed by the simulation that the designed band-pass modulator operated properly. The signal-to-noise ratio (SNR) of the superconducting band-pass modulator was improved by increasing the oversampling ratio. The SNR of 55 dB was obtained at the sampling frequency of 20 GHz and the bandwidth of 100 MHz from the simulation. It was found that the resonant frequency of the modulator decreased with the increase of the sampling frequency. For the design of the superconducting band-pass modulator, it is necessary to take account of this resonant frequency deviation.