Design, optimization, and construction of a dc SQUID with complete flux transformer circuits

Abstract

The design of a complete dc .SQUID with a flux transformer input circuit is discussed. The flux coupling circuits introduce a substantial capacitance across the SQUID and give rise to many resonances which may couple strongly to the SQUID dynamics. Both effects lead to multiple modes in the SQUID dynamics and consequently to excess noise. For a low-noise SQUID with smooth characteristics, our analysis and practical considerations suggest signal coupling via an intermediary transformer. This method allows simultaneous optimization of the SQUID parameters, minimizing the parasitic capacitance, control over the resonances, and good inductance matching to practical magnetometer coils. A model is developed to optimize the structure: it describes the whole circuit with the help of a suitably modified autonomous SQUID, provided that the system is free from multiple modes due to resonances or large parasitic capacitance. Following these design principles, we have built a dc SQUID, primarily for use in biomagnetic research, but also well suited for other applications. The fabrication of the SQUID and the high-quality electronics especially suitable for multiple-SQUID devices is presented. The SQUIDs showed smooth characteristics, and the lowest measured noise of our complete SQUID is 1.3 x 10-6tT])o//x/-~, indicating the success of the design.

SQUID Concepts and Systems

“…A different approach is to couple the SQUID to the signal via an intermediary superconducting transformer 4 , so that the number of turns on the SQUID washer and the parasitic capacitance are reduced. Knuutila et al 25 successfully damped the resonances in the input coil by terminating the stripline with a matched resistor. An alternate coupling scheme has been adopted by Carelli and Foglietti 26 , who fabricated thin-film SQUIDs with many loops in parallel.…”

SQUIDs: Principles, Noise, and Applications

“…An alternative approach is to couple the SQUID to the signal via an intermediary superconducting transforme~4, so that the number of turns on the SQUID washer and the parasitic capacitance are reduced. Knuutila et al 25 successfully damped the resonances in the input coil by terminating the stripline with a matched resistor. An alternative coupling scheme has been adopted by Carelli and Fogliette 6 , who fabricated thin-film SQUIDs with many loops in parallel.…”

Strongly coupled, low noise dc-SQUID amplifiers

“…If the parameters are chosen well, this leads to an overdamped resonance and low reachable noise due to the faster decay of the energy stored in the resonant mode τ ∝ Q/ f R,C [82,109,112,117]. In a similar approach, damping resistors are directly connected to each of the windings of the input coil [113,115].…”

“…One design approach for SQUIDs with a high input inductance is the SQUID with an integrated flux transformer [111,117]. A photograph of a fabricated device as well as a simplified schematic are shown in Fig.…”

“…5.1(b). The disadvantage of this design is that in practice one ends up with a lowered total coupling efficiency k 2 0.5 [117]. Naturally, the effective inductances are altered by the intermediate loop containing L 2 and L 3 .…”