Fluxgate magnetometer circuit with earth's field elimination

A stabilized fluxgate magnetometer for use in measuring one of the three mutually onthogonal components of the earth's magnetic field taken with respect to the central axis of a tool housing the magnetometer, comprising:

• a fluxgate sensor having a ring core with a toroidal drive winding and a differential sense winding about the core for sensing the difference in saturation due to the component of earth's magnetic field normal to said differential sense winding;
• an oscillator and a driver for exciting said winding drive winding of said fluxgate sensor;
• a phase shifting circuit connected to said oscillator and driver for providing an output signal twice the frequency of the signal to said drive windings;
• a second harmonic tuning element responsive to the output of said sense winding;
• circuit means for filtering and amplifying the output signal from said second harmonic tuning element to said synchronous demodulators whereby signal offsets from the output signal of said sense winding are reduced;
• a synchronous demodulator for multiplying the second harmonic signal from said tuning element by an in phase second harmonic reference signal from said phase shifting circuit for providing an error signal having a polarity and magnitude determined by the phase and amplitude of the output signal from said sense winding as the output;
• means for integrating the error signal from said synchronous demodulator to provide the phase stability and memory for the magnitude of current to eliminate the earth's field;
• and
• a high impedance current source driven by said integrating circuit means which sums a DC current signal into the output of said fluxgate sense winding in a direction and magnitude determined by said synchronous demodulator to eliminate the effect of the earth's field in the sensor.