High-Tc SQUID sensors with integrated Earth field compensation

An encapsulation with integrated field compensation coils for superconducting quantum interference device (SQUID) magnetometers is presented. The capsule is fabricated from printed-circuit board material. In spite of the small capsule size of 18 mm×16 mm×4.8 mm, the compensation field at nominal capsule dimensions is homogeneous to about ±2% over the area of the SQUID magnetometer pickup coil of 9 mm×9 mm. The field homogeneity degrades by about a factor of two for worst-case tolerances in the thickness of the board material or in the positions of the field coils. A low-noise current source was integrated into our microcontroller-based SQUID readout electronics in order to supply the field coils. Compensation fields of up to ±71 μT can be produced with only 19 fT/√Hz excess noise above the 1/f corner at about 2 Hz. The current source has a 10%-90% settling time of 2.2 ms allowing one to compensate the background field dynamically in environments with very large low-frequency field fluctuations or even in slowly moved sensor systems.