Fluxgate and Search Coil Hybrid: A Low-Noise Wide-Band Magnetometer

We propose a new hybrid magnetometer that operates in a wide frequency range from dc to over 30 kHz and has a very high resolution. It is made in a coaxial structure by combining a pT-level fundamental mode orthogonal fluxgate and a search coil magnetometer. A search coil magnetometer consisting of a cylindrical magnetic core (5 cm long), a primary coil (800 turns) and a feedback coil (10 turns) is used. The thin fluxgate sensor head is partially inserted to the inner bore of the cylindrical magnetic core of the search coil in the coaxial structure. Magnetic interference from the fluxgate sensor head is minimized by using a hair-pin U-shaped wire core for the fluxgate sensor head. The fluxgate sensitivity is increased by a factor of 1.8 because of flux concentration effect of the cylindrical magnetic core made by amorphous tape. The cut-off frequency of the fluxgate is extended to 6 kHz and that of the search coil is beyond 30 kHz. Two outputs are combined with an R-C divider. The cross over frequency of the RC-divider is extended from our previous design, that is, 32 Hz to 640 Hz to reduce the total noises. A hybrid magnetometer so developed provided high resolution of 8 pT/√Hz at 1 Hz and 4 pT/√Hz from 10-200 Hz as same as resolution of the fluxgate used in this hybrid sensor.

I. Introduction

A low-noise wide band magnetometer that operates from dc to several tens kHz plays an important role in astrophysics, security check system and active magnetic shielding systems. For example, in case of the monitor sensor for an active magnetic shield, high resolution and wide-band magnetic sensors are required to get an accurate cancellation of disturbing magnetic fields from outside to avoid oscillation of the system and contamination of noises from a feedback system.