Optimal Design of Low-Noise Induction Magnetometer in 1 mHz-10 kHz Utilizing Paralleled Dual-JFET Differential Pre-Amplifier

The optimal design of the induction magnetometer in the frequency band from 1 mHz to 10 kHz was analytically and experimentally investigated to achieve a suitable sensor configuration under a given noise equivalent magnetic induction (NEMI) and volume limitation. The transducer model is first analyzed, and the parameters R, L, C, and μ app are calculated. The equivalent input voltage noise that utilizes an n-paralleled dualJunction Field-Effect Transistor (JFET) differential preamplifier, was decreased by √n times, and the equivalent input current noise was increased by √n times. The noise calculation formulas are derived based on the mathematical principle of the flux-feedback induction magnetometer, and the NEMI was immediately calculated. A set of sensor configurations, which reached the given NEMI, were achieved by solving the NEMI equation. The optimal sensor configuration was obtained, considering the smallest coil outer diameter limitation. To verify the theoretical analysis, the optimal induction magnetometer was manufactured, and its NEMI was measured in an electromagnetic shielding room. The experimental results show good agreement with the theoretical analysis. Furthermore, the optimal induction magnetometer had a smaller volume with a similar NEMI level compared with the well-known sensor MFS-06.