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 % MathType!MTEF!2!1!+-% feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaac6% cacaaIZaGaey41aqRaaGymaiaaicdadaahaaWcbeqaaiabgkHiTiaa% iAdaaaGccqqHMoGrdaWgaaWcbaGaaGimaiaac+caaeqaaOWaaOaaae% aacaWGibGaamOEaaWcbeaaaaa!428B!$1.3 \times 10^{ - 6} \Phi _{0/} \sqrt {Hz} $, indicating the success of the design.
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Knuutila, J., Kajola, M., Seppä, H. et al. Design, optimization, and construction of a dc SQUID with complete flux transformer circuits. J Low Temp Phys 71, 369–392 (1988). https://doi.org/10.1007/BF00116869
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DOI: https://doi.org/10.1007/BF00116869