Magnetic Sensor Design for Femtotesla Low-Frequency Signals

A system for detecting magnetic fields in the femtotesla range at low frequencies is developed, including the antenna, transformer, and amplifier. Each component is described with relevant tradeoffs which allow a large variety of receivers to be easily designed for any magnetic field sensing in the very low frequency range. A system using a 1-??-1-mH impedance antenna is developed further as an example that has been used extensively in measurements all over the world in the frequency range of 50 Hz-30 kHz. It has a gain of 0.58 V/nT and a sensitivity of below 1 fT/Hz^1/2 above 250 Hz.