Quarter-Rate Superconducting Modulator for Improved High Resolution Analog-to-Digital Converter

We describe the design of a new oversampled analog-to-digital converter (ADC) based on phase modulation-demodulation (PMD) architecture. In a PMD ADC, the analog input signal modulates the phase of a periodic stream of fluxons applied to a modulator circuit for subsequent demodulation in a clocked synchronizer circuit to produce a digital code. The new modulator provides a way to quadruple the average fluxon transport rate, and hence the input dynamic range, by replacing the single junction interferometer with a high-speed symmetric divide-by-4 circuit. The divider acts as a 1:4 asynchronous demultiplexer which distributes incoming fluxons among its four quarter-rate outputs. This four-fold rate reduction, at the modulator output, allows one to increase the ADC maximum input slew rate to 2 fluxons per clock period, achieving 2 additional bits of resolution at the same sampling clock frequency. We have designed and fabricated a quarter-rate ADC front-end and present low frequency test results for the same. The ADC comprises a quarter-rate quantizer which has been successfully tested at an input frequency of 81.92 GHz.