Comparison of Double Relaxation Oscillation SQUIDs and DC-SQUIDs of Large Stewart-McCumber Parameter

Increasing the flux-to-voltage transfer coefficient of the SQUID output voltage is necessary to release the constraint on the preamplifier input noise. As one of the approaches to increase the transfer coefficient, we have been using a double relaxation oscillation SQUID (DROS) having a transfer coefficient of about 10 times larger than those of DC-SQUIDs. Traditionally, DC-SQUIDs were operated in the non-hysteretic condition with the Stewart-McCumber parameter (beta(c)) less than 1. In this study, we fabricated hysteretic DC-SQUIDs with the beta(c) ranging from 2.5 and compared the transfer coefficient and the system noise of DC-SQUIDs and DROSs. Both DROS and DC-SQUID have the same structure and design parameters for the SQUID loop inductance, resonance-damping circuits in the SQUID loop, and input coil. All the SQUIDs were fabricated in the same wafer using the Nb/AlOx/Nb junction process. The transfer coefficients of DROSs were about 1mV/Phi(0), regardless of SQUID parameters. For DC-SQUIDs, the transfer coefficients were sensitive to the beta(c) and the operating margin was narrower than that of DROS. The white noise of both SQUIDs in the flux-locked loop mode was around 1.5 mu Phi(0)/root Hz at 100 Hz, measured with a preamplifier of input voltage noise of 1 nV/root Hz. DC-SQUIDs had mostly a narrower operating margin against the bias current, and the modulation voltage was smaller than DROS. In a near-optimized operating condition, both DROS and hysteretic DC-SQUID showed comparable noise performance.