Single-lockin detection of AC magnetic fields by fluxgate sensor

We present a simple narrowband method to detect the amplitude of alternating magnetic fields by a fluxgate sensor using a single lock-in amplifier, improving the common method of using two phase sensitive demodulators (lock-in amplifiers). We eliminated the need for two phase-sensitivite demodulators by processing the reference signals of lock-in amplifier and of the fluxgate excitation unit. This setup can be used as a high-resolution, narrowband spectral analyzer of magnetic fields or as a system for measuring the AC response of magnetic markers. Two methods were introduced and tested; we achieved 2 kHz detecting noise spectral density of 80 pT/√Hz in the laboratory environment. The method is directly usable for any fluxgate sensor or magnetometer, where the reference signals are available. Using a slow feedback compensating-loop further improves the dynamic range of the instrument.