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Effects on DC SQUID characteristics of damping of input coil resonances

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Abstract

The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally. A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID. Ten differentRC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances. It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise. At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID. The noise level is a function of the shunt resistanceR s only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency. At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies asR /−0.7 s . The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account.

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Authors and Affiliations

  1. Low Temperature Laboratory, Helsinki University of Technology, Espoo, Finland

    Jukka Knuutila & Antti Ahonen

  2. IBM Thomas J. Watson Research Center, Yorktown Heights, New York

    Claudia Tesche

Authors
  1. Jukka Knuutila
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  2. Antti Ahonen
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  3. Claudia Tesche
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Knuutila, J., Ahonen, A. & Tesche, C. Effects on DC SQUID characteristics of damping of input coil resonances. J Low Temp Phys 68, 269–284 (1987). https://doi.org/10.1007/BF00683902

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  • DOI: https://doi.org/10.1007/BF00683902

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