Microwave receivers with direct digitization

Superconductor analog-to-digital converters (ADCs) and ultrafast digital circuitry enable processing of microwave signals entirely in the digital domain. We have designed and demonstrated a wide variety of continuous-time bandpass delta-sigma modulators using Josephson junction comparators. Featuring sampling frequencies up to 30 GHz, single-chip digital receivers have been demonstrated by connecting a rapid single flux quantum (RSFQ) digital circuitry with these ADCs. These receiver chips, cooled to 4 K by cryogen-free refrigerators, have been used with room-temperature digital processors to demonstrate reception of microwave signals for X-band satellite communications and Link-16 data links. To date, the highest frequency of direct digitization is 21 GHz for satellite communication. We report recent advances in ADC design to obtain higher dynamic range.