SUPERCONDUCTOR ANALOG TO DIGITAL CONVERTER FOR SIGINT APPLICATIONS

Superconductor analog-to-digital converters (ADC) offer high sensitivity and large dynamic range. We report recent advances in the low-pass phase-modulation-demodulation (PMD) oversampled analog-to-digital converter (ADC). The oversampled data from the PMD front-end are passed along with sampling clock to an on-chip digital filter. In the past, the maximum frequency of operation of such chips were limited by that of the digital decimation filter (DDF) to about 30 GHz. Recently, we have designed a chip that allows the PMD front-end to be clocked twice as fast as the DDF, resulting in sampling rates of 4608 GHz and 29.44 GHz for chips with critical current densities of 4.5 and 1 kA/cm(2) respectively. We have also designed a new PAM front-end with a quarter-rate quantizer (QRQ) that allows 18-20 dB extension of dynamic range. We are also pursuing a subranging ADC design with two PAID front-ends connected with fluxon amplifier and subtractor circuitry that promises a dynamic range extension by 25-30 dB. We report signal-to-noise ratio and spur-free dynamic range measurement results for various ADC chips, and project performance for future improvements.