This sensor is not affected significantly by oscillator frequency drift.

A low-power, highly stable electronic sensor circuit for measuring a small change in the ambient magnetic field with high sensitivity exploits the giant magneto-impedance (GMI) effect, in which the radio-frequency (RF) impedance of a fiber made of a suitably formulated material varies with the externally generated magnetic field to which it is exposed. The GMI effect has been observed in fibers thinner than a human hair made of amorphous (in the sense of lacking crystalline structure) alloys of cobalt, iron, silicon, and boron.

Prior GMI-based magnetic-field sensors have been built around GMI fibers as short as a few millimeters. In a typical prior GMI-based sensor, the reactive portion of the magnetic-field-dependent impedance is used as one of the frequency- determining elements of an oscillator. Unfortunately, oscillator frequency drift attributable to causes other than a change in the magnetic field can be large enough to be significant with respect to the frequency change associated with the magnetic-field change that one seeks to measure. Hence, there is a need for a GMI-based magnetic-field sensor that is not significantly affected by oscillator frequency drift. The present sensor circuit was invented to satisfy this need.

In this Magnetometer Circuit, a stable RF oscillator is used to excite the GMI effect, which is measured at DC by an operational amplifier stabilized by an input reference voltage. Values of inductance, resistance and capacitance shown here are only typical; other values could be better suited to specific applications.