Noise spectrum of pulse excited fluxgate sensor

Abstract

A low-cost flat miniature fluxgate magnetic field sensor is excited by short current pulses in order to reduce the power consumption. In this mode, the output signal contains very rich spectrum of even harmonics, each of them being sensitive to the measured field. The noise spectrum of individual harmonics, both in the untuned and tuned modes, was measured. The output tuning substantially improved the noise properties of the sensor: the noise power spectral density (PSD) at the second harmonic was 320 p T RMS / Hz @ 1 Hz and 67 p T RMS / Hz @ 1 Hz at the untuned and tuned sensors, respectively. The noise levels on individual harmonics differ substantially—it is the designer's task to select the tuning frequency to the harmonics with low noise. The minimum noise of 17 p T RMS / Hz @ 1 Hz was achieved for the sensor tuned at the eighth harmonic frequency. The previously observed fact, that the noise of the fluxgate sensor dramatically worsens once the pulse excitation is applied, has to be revised: the sensor pick-up coil tuning to certain harmonics can improve the noise to a level comparable to sine wave excitation. This allows use of a power-efficient pulse excitation while keeping the noise comparable to sine wave excitation.