Injection Locking in Coupled Core Fluxgate Magnetometers: Exploiting Nonlinearity to Enhance Sensitivity to Weak, Low Frequency, Target Magnetic Fields

The coupled core fluxgate magnetometer (CCFG) is a nonlinear dynamic system that is underpinned by three wound ferromagnetic cores (with primary and secondary windings) electrically coupled in a unidirectional ring configuration. This device exhibits a transition to highly complex behavior when a control parameter is swept through a critical value. The oscillation frequency of CCFG is very sensitive to small changes in the external (i.e., ambient) magnetic field, e.g., those introduced by a target magnetic signal. The CCFG is known to perform quite well in the case of dc target signals. To improve the detection of ac targets, we apply the idea of injection locking (IL) to the CCFG, with a view to lowering the sensor noise-floor at low frequencies where many magnetic target signals of interest are located. An extensive simulation and experimental investigation has been performed; it shows a distinct benefit from the standpoint of reducing the noise-floor in the lower frequency range and therefore enhancing the detectability of a low frequency target magnetic signal which might, otherwise, be buried in the noise. In addition, an interesting variant of conventional IL is presented here; it affords the possibility of even better noise-floor reduction.