(Double) relaxation oscillation SQUIDs with high flux-to-voltage transfer: Simulations and experiments

Abstract

Relaxation oscillation superconducting quantum interference devices (SQUIDs) (ROSs) and double relaxation oscillation SQUIDs (DROSs) have been fabricated and characterized. The SQUIDs are based on hysteretic Nb/Al,AlOx/Al/Nb Josephson tunnel junctions. The relaxation oscillations were simulated by computer. By these simulations the minimum and maximum values of the shunt inductance at which the relaxation oscillations persist have been determined for different values of the shunt resistance and of the critical current and capacitance of the junctions. The transfer from flux to dc voltage in ROSs increases linearly with the shunt resistance. The operation range of the shunt inductance, however, becomes smaller with increasing shunt resistance. In ROSs with a SQUID inductance of 20 pH, a maximum voltage modulation width of 400 μV and a flux-to-voltage transfer δV/δφ of 4 mV/φ0 have been obtained. Two ROSs have been operated in a simple flux-locked loop with direct voltage readout. The experimental white flux noise spectral density of about 3μφ0/√Hz is in good agreement with the theoretical model. The sensitivity of these SQUIDs is limited by so-called switching noise, which is noise due to the interaction between the relaxation oscillations and the Josephson frequency. In DROSs typical flux-to-voltage transfer values of 10–30 mV/φ0 and maximum values up to 80 mV/φ0 have been obtained. This transfer is determined by thermal noise on the critical currents of the SQUIDs. The effective flux noise density of DROSs in flux-locked-loop operation with direct voltage readout is in good agreement with the theoretical noise level which is mainly determined by thermal noise on the critical currents. In one of the DROSs with a SQUID inductance of 20 pH, a white flux noise of 1μφ0/√Hz has been obtained at an estimated relaxation frequency of about 80 MHz. This corresponds to an energy sensitivity of about 160 h.

Low-noise magnetoencephalography system cooled by a continuously operating reliquefier

“…To ensure sufficient cooling of the gradiometer bobbins, an insulated helmet-shaped copper mesh was used to cover the bobbin array, and then superinsulation layers and a thermal shield layer were installed. A The SQUID sensor is a double-relaxation-oscillation SQUID having a large flux-to-voltage transfer of typically about 1 mV/Φ 0 , with Φ 0 being the flux quantum [16,17]. Thus, the contribution of the preamplifier input noise can be negligible in direct voltage readout mode.…”

Simplified analysis of direct SQUID readout schemes

“…In a simplified view, the asymmetry in the junction shunts acts as a virt ual fe edback resista nce R A and the SQU ID inductance L is used for the feedback coil L A . Note that there are other alternatives to o btain a large voltage transfer coefficient, e.g., the use of se ries SQUID arrays [16], SQUIDs with weakly shunted Josephson junctions operat ed n ear t he hy steresis l imit [14,17] or SQUIDs with unshunted junctions based on rela xation oscillations [2,18]. The V- a characteristic remains symmetric in these cases.…”

Compact Readout Electronics for 62-Channel DROS Magnetocardiogram System

“…In order to simplify the readout electronics of the SQUID system, various SQUID schemes such as additional positive feedback (APF), series array of SQUIDs, Double relaxation oscillation SQUID (DROS), and digital SQUID were developed [2]- [5].…”