Search

A planar superconducting quantum interference device (SQUID) magnetometer consisting of a parallel gradiometer SQUID with integrated input coils connected to an on-chip pickup loop was designed and fabricated in conventional niobium technology. SQUID bootstrap circuitry (SBC) incorporating suitable current and voltage feedbacks was also integrated into the design. For a SQUID inductance of Ls = 350 pH and a chip size of 5 × 5 mm2, the field resolution of the voltage-biased SQUID magnetometer reached <5 fT Hz-1/2 with the bootstrap circuit and an ordinary preamplifier. We also observed that the effective McCumber parameter βc of the junctions is influenced by the bias mode. Indeed, when the nominal junction βc was larger than unity, our SQUID magnetometer operated stably in the voltage bias mode. The device exhibited low noise even without SBC. [ABSTRACT FROM AUTHOR]

The SQUID Bootstrap Circuit (SBC) for direct-coupled readout of SQUID signals in voltage bias mode was recently demonstrated. In addition to the conventional dc SQUID, the SBC incorporates a shunt resistor Rs, and two coils coupled to the SQUID via mutual inductances M1 and M2. In this paper, basic equations of SBC are formulated based on its equivalent circuit model. The expression of equivalent flux noise from the preamplifier is also given. The effect of the three adjustable parameters (M1, M2 and Rs) on the characteristics of SBC and the preamplifier noise suppression are numerically simulated. The SBC combines current and voltage feedbacks in one circuit, allowing for an effective suppression of the preamplifier voltage noise through increased flux-current transfer coefficient and dynamic resistance. In contrast to other direct-coupled schemes, it offers not only a good noise performance, but also tolerance to a wide range of adjustable parameters. [ABSTRACT FROM AUTHOR]

We recently presented a direct readout technique for the dc Superconducting QUantum Interference Device (SQUID) without flux modulation (FM), operated in voltage bias mode, and named it the SQUID Bootstrap Circuit (SBC). The SBC combines additional voltage and current feedbacks to minimize the room-temperature preamplifier noise. The main point of this paper is to compare the flux noise performance of the SBC readout with that of the FM scheme using a sine wave modulation signal. Several liquid-helium-cooled SQUID magnetometers with different layouts and loop inductances were characterized using these two readout schemes. Measured noise was comparable to or even lower than that measured by FM electronics. Furthermore, the SBC noise performance was evaluated as function of resistance which, when properly adjusted, permits us to nearly fulfill the critical noise suppression condition. We believe SBC to be a promising candidate for multi-channel SQUID systems. [ABSTRACT FROM AUTHOR]