Journal of Low Temperature Physics

Effects on DC SQUID characteristics of damping of input coil resonances

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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 different RC 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 resistance Rsonly, 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 as Rs/-0.7. 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. © 1987 Plenum Publishing Corporation.


01 Aug 1987


Journal of Low Temperature Physics