Fundamental Mode Orthogonal Fluxgate Gradiometer

A new method based on the fundamental mode orthogonal fluxgate (FM-OFG) mechanism is proposed to build a gradiometer of flexible base line and a minimum set of electronics. A pair of thin sensor heads and an electronic circuit designed for a magnetometer can be used, in which pickup windings of a sensor head pair are connected in counter series to respond to the field gradient between two locations of the sensor heads. Furthermore, a fine tuning to get a good balance in sensitivity between two sensor heads can be made by adjusting a dc bias current to one of the sensor heads. Developed gradiometers show a high suppression performance to the uniform magnetic field, where the suppression ratio (gradiometer output)/(magnetometer output), reaches below 1/40 000 with the resolution of 100 pT/m/√Hz at 10 Hz.