Apparatus for reducing noise due to sensor vibration during measurement of a weak magnetic field

An apparatus for measuring a weak, A.C. magnetic field in a background D.C. magnetic field, comprising:

• a magnetic sensor which generates a sensor signal;
• a support mechanism which supports the magnetic sensor and which does not impede minute rotation thereof in an x-axis direction and a y-axis direction of an x, y, z coordinate system, the magnetic field sensor being sensitive in a z-axis direction of the coordinate system;
• and
• means for sensing the position of the magnetic sensor, the means for sensing the position including means for generating a pair of A.C. linear gradient magnetic fields having gradient directions and having frequencies which are set higher than the frequency of the weak A.C. magnetic field that is to be measured, the gradient directions of the pair of A.C. linear gradient magnetic fields being perpendicular to the z-axis, with the gradient direction of one of the A.C. linear gradient magnetic fields also being parallel to the x-axis direction and the gradient direction of the other A.C. linear gradient magnetic field also being parallel to the y-axis direction, the means for generating a pair of A.C. linear gradient magnetic fields including two pairs of linear conductors which are perpendicular to each other and which are located adjacent the magnet sensor, A.C. currents being supplied to the linear conductors.

An apparatus for measuring an unknown magnetic field in a background magnetic field, comprising:

• means for generating an A.C. magnetic field;
• a magnetic sensor to sense the unknown magnetic field, the magnetic sensor being located in the A.C. magnetic field, the magnetic sensor generating a sensor signal having a component corresponding to the unknown magnetic field, a noise component corresponding to movement of the sensor in the background magnetic field, and an A.C. component due to the A.C. magnetic field;
• a filter to remove the A.C. component of the sensor signal, so as to provide a filtered sensor signal having the noise component and the component corresponding to the unknown magnetic field;
• means, responsive to changes in the A.C. component of the sensor signal, for generating a correction signal corresponding to the noise component;
• and
• means for subtracting the correction signal from the filtered sensor signal.