Development of superconducting bandpass delta-sigma analog-to-digital converter

Abstract

This paper recounts the development of a superconducting bandpass delta-sigma (ΔΣ) modulator for direct analog-to-digital conversion of radio frequency signals in the GHz range. The modulator design benefits from several advantages of superconducting electronics: high-Q resonators, fast Josephson comparators, naturally quantized single flux quantum pulses, and high circuit sensitivity. The measured center frequency (2.23 GHz), sampling rate (up to 45 GHz), dynamic range (greater than 57 dB over a 19.6 MHz bandwidth), and input sensitivity (−17.4 dBm full-scale) of the bandpass modulator are the highest reported to date in any technology. The SNR (49 dB over a 20.8 MHz bandwidth) is limited by the frequency resolution of the measurement but still exceeds the SNRs of semiconductor modulators with comparable center frequencies. The design of the modulator test chip and the high speed testing methodology are reviewed. Finally, the paper examines the prospects for improved performance with more advanced modulator architectures.