A Generalized Study of Coil-Core-Aspect Ratio Optimization for Noise Reduction and SNR Enhancement in Search Coil Magnetometers at Low Frequencies

Here, a generalized induction coil sensor model (more generalized than other models) has been considered at low frequencies (within 0.1-100 Hz), and the equivalent magnetic field of the coil's thermal noise and the sensor's signal-to-noise ratio (SNR) were calculated theoretically based on the dimensions and geometry of the coil winding and its core. In our suggested theoretical consideration, all involved parameters were considered and optimized without any assumption and constraint, while some authors in their latest reports, have been used some assumptions and constraints in their sensor calculations (such as holding constant the sensor's volume and aspect ratio). Our calculations indicated that the equivalent magnetic field of the thermal noise can be minimized by the coil-to-core weight ratio. Moreover, it was found that the sensor's SNR can be maximized with only a special value of core aspect ratio (length to diameter of core ratio). The obtained theoretical results were evaluated experimentally by fabricating a search coil magnetometer model, using the optimum parameters. The resonance frequency and the parasitic capacitance of the coil were measured. Moreover, the variations of the transfer function of the magnetometer, with respect to frequency, were studied. Thus, it was shown that, at low frequencies, our experimentally measured noise data exhibit better agreement with our suggested theoretical results with respect to the state of the art.