A 2 nV/root Hz and 10 fA/root Hz at 1-Hz ultralow noise preamplifier is designed to improve the sensitivity of the induction magnetometer (IM) from 0.1 Hz to 10 kHz. Meanwhile, the noise characteristics of IM are analyzed and distinguished. A novel optimization method to find the minimum length of the core under the required noise level and the limited IM weight is proposed. Furthermore, an optimal IM prototype from 0.1 Hz to 10 kHz is developed with the noise level of 1 pT/root Hz at 1 Hz and 1.5 fT/root Hz at 2 kHz. The length is only 360 mm and the weight is 1.5 kg. The measurement results of the IM also confirm the theoretical noise model and the optimization method.
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