Design of Digital DROS for Wide Dynamic Operation Range with Sub-Single Flux Quantum Resolution

We designed a digital double relaxation oscillation SQUID (DROS) with an single flux quantum (SFQ) feedback circuit for digital SQUID magnetometers. The DROS is magneticall coupled to a feedback SQUID loop and a signal magnetic flux is input to the feedback SQUID loop. The DROS generates two kinds of voltage pulses corresponding to the total magnetic flux in the feedback SQUID loop. Those voltage pulses are converted to SFQ and anti-SFQ pulses and input to the feedback SQUID loop to keep the magnetic flux in the feedback SQUID loop constant. A read-out dc SQUID (R/O SQUID) is also magnetically coupled to the feedback SQUID loop for monitoring the actual magnetic flux in the feedback SQUID loop. The number of the generated SFQ pulses is estimated by SFQ feedback steps on output voltage curves generated from the R/O SQUID. Wide dynamic operation range is realized by the SFQ feedback and sub-Φ0 magnetic flux resolution is realized by the R/O SQUID, where Φ0(= h/2e) is the single magnetic flux quantum. Also one digital DROS magnetometer can be operated with less than 10 mA. We fabricated a designed digital DROS using ISTEC SRL 2.5 kA/cm2 Nb standard process II and are trying to confirm the correct operation of the digital DROS.