Orthogonal Fluxgate Gradiometer With Multiple Coil Pairs

In this paper, we study the possibility to exploit the noise correlation in a fundamental mode orthogonal fluxgate used as a gradiometer in order to maximize the noise suppression. We study the correlation of the noise along a single core fluxgate using two short coils at a variable distance, and we derive how it drops as the distance increases. Furthermore, we observed this phenomenon with the central point of the coils located at different parts of the wire, obtaining a similar behavior. This implies that the noise propagates in the core in a similar fashion, and-most important-that any mechanism taking advantage of the noise correlation to suppress noise can be replicated along the whole length of the core. We have also investigated the dependence of the noise correlation on the ac component of the excitation current, finding out that in the range where the 1/f noise of the magnetometer does not significantly change larger excitation current increases the correlation thus must be preferred as it allows better noise suppression when used as a gradiometer. Next, we verified that the noise suppression mechanism based on correlation does in fact return lower noise at higher correlation verifying that it matches with the expected values. Finally, we proposed a new configuration of the gradiometer where instead of two long coils we use multiple short coils at close distance calculating that the maximum length of such coils in order to take advantage of this configuration. We also explain that the response of such configuration of the gradiometer to the homogeneous field is zero even if the single coils are not symmetric with the respect of the center of the core if the pairs of coils are all symmetric.