Weakly Damped SQUID Gradiometer With Low Crosstalk for Magnetocardiography Measurement

In a multichannel magnetocardiography system, the problem of crosstalk is serious due to mutual inductances between the pickup coils of adjacent channels. In this paper, a superconducting quantum interference device gradiometer module based on weakly damped Josephson junctions with large β
_c
(≈5) is presented. An external feedback design is also adopted to suppress the crosstalk between different channels. With a special superconducting connection between input coil and pickup coil, an axial first-order gradiometer with baseline of 70 mm and diameter of 18 mm is fabricated. Using a direct readout circuit with equivalent flux noise of 1.8 μΦ
₀
/√Hz, the flux noise of this gradiometer is measured to be 6.5 μΦ
₀
/√Hz in a white noise range, corresponding to a field sensitivity of 0.5 fT/(cm · √Hz). The crosstalk rates in external feedback mode and internal feedback mode are both successfully simulated with MAXWELL software. The results by experimental measurements using these gradiometers are in coincidence with the simulated ones; both simulation and experimental results show a substantial suppression of crosstalk in the external feedback design.