Josephson sigma-delta modulator for high dynamic range A/D conversion

A Josephson sigma-delta modulator suitable for use in high dynamic range conversion of MHz bandwidth signals was designed and simulated. Input signal voltages were integrated as current in a superconducting inductor. A single junction quantizer provided analog-to-digital (AID) conversion at 40 GHz and fed back single-flux-quantum (SFQ) voltage pulses to balance the input. A JSIM calculation for a 13,110,000-ps interval indicated ideal first-order noise shaping of a 2.44-MHz voltage-source signal. Spur free dynamic range was 102 dB and signal-to-noise-and-distortion was 98 dB on the DC 10-MHz signal band, equivalent to a 16-b, 20-MSample/s, Nyquist-rate A/D. The modulator transfer function was derived for a current-source signal applied to an input resistor. First-order quantization noise suppression was still observed for signal bands wider than the 2 pi R/L modulator cutoff frequency. The superconductive sigma-delta modulator combines the speed of Josephson junctions for GHz sampling and digital filtering with the quantum mechanical accuracy of SFQ feedback to obtain superior A/D performance.<>