Excess magnetic noise in orthogonal fluxgates employing discontinuous excitation

The source of the excess magnetic noise caused by discontinuous excitation in a wire core orthogonal fluxgates is described. Discontinuous, or duty cycle, operation allows reducing the inherently high power consumption of orthogonal fluxgates and, therefore, makes possible their employment in mobile equipment. The magnetization process during an excitation cycle is highly nonlinear and inhomogeneous across the wire core. This is because the ac current component of the excitation electrical current causes a skin effect, which is strongly affected by the nonlinearity of the B-H curve. Thus, the magnetization of the domains across the core takes the shape of a spring onion, where each magnetized shell of domains has different level of magnetization, ranging from deep saturation in outer shells to zero magnetization in the innermost ones. Such a domain structure is not stable, especially at the boundary between remnantly magnetized shell and neutral shells. In the idle excitation intervals, there is an occasion for the domains to relax. The domains relaxation is a stochastic process and therefore incurs excess magnetic noise. We investigate the excess noise by measuring the sensitivity and noise in the excitation cycles using a digital selective bandpass sampling processing technique. The attained equivalent magnetic noise is <2.5 pT/√Hz at 1 Hz. We correlate the coupling between the domain shells using the time dependent portion of the Landau-Lifshitz-Gilbert equation. A close relationship between the noise model and experimental data is achieved.