Processing of the fluxgate output signal P. Ripka, S. Kawahito* Dept. of Measurement, Faculty of Electrical Engineering CTU, Technicka 2, 166 27 Praha 6, Czech Republic, ripka@feld.cvut.cz Dept. of Information and Computer Sciences, Tyohashi University of Technology, Tempaku-cho, 441, Japan kawahito@signal.tutics.tut.ac.jp Abstract Fluxgate sensors measure magnetic field with a resolution up to 10 pT. New methods of the output signal processing allow to decrease the sensor size, lower the energy consumption and increase the working frequency. Tuning the voltage output may substantially increase the sensitivity, but in certain cases it may cause unstability. Fluxgate in current-output mode requires lower number of turns of the pick-up coil; its output may be processed by gated integrator rather than by the classical synchronous detector. The output of the short-circuited fluxgate can be tuned by serial capacitor. The use of synchronous filters (revolving and SC) brought no practical advantages. The digital magnetometers performing the detection in numerical form in the DSP have appeared. The methods of3 increasing the frequency range of the fluxgate magnetometer, the crossfield effect and the problems associated with the use of fluxgates in multi-sensor systems are also discussed. Keywords: magnetic sensor, fluxgate, magnetometer, gradiometer 1. Introduction Fluxgates are the most suitable sensors for precise measurement of the DC and low-frequency AC magnetic field vector in the range of 1 nT ..1 mT [1]. They are solid-state sensors resistant to rough environment and they are stable over the wide temperature range. In many applications such as magnetic ink reading, security and navigation it is necessary to lower the sensor size, increase the frequency range and lower the power consumption. Other applications such as ultrastable gradiometers require lowering of the sensor noise and increasing of the offset stability. Such demands call for new methods of the processing of the fluxgate output signal. We review the presently used techniques and propose the new solutions suitable for integrated fluxgate sensors. 2. Tuned voltage output Tuning the voltage-output pick-up coil by parallel capacitor increases the sensitivity at second harmonic frequency [2]. This effect of parametric amplification was theoretically analyzed in [3]. Increase of the sensitivity by the factor of 10 to 100 is easily achievable. For high quality factor (low pick-up coil resistance) the circuit may become unstable [4]. This situation is usual for very sensitive fluxgates, for example race-track core sensors. Fig. 1 shows the unloaded output voltage of the race-track fluxgate sensor [17] for the measured field of 5μT. Fig. 2 shows the same sensor tuned by parallel capacitor C 2 = 11 nF. The sensor is in the unstable mode: due to the high quality factor of the nonlinear resonant circuit the output oscillates even for zero measured field. The sensor may be stabilized by damping resistor either in series or parallel to the tuning capacitor. Fig. 3 shows the same