Spatial frequency response of conventional and non-conventional SQUID gradiometers

The gradiometer spatial frequency response is an important performance feature, mainly if one wants to tailor a gradiometer design to specific applications. Because many nonconventional planar gradiometers have been recently built, we extend a two-dimensional filtering model that predicts the spatial frequency response of ordinary planar gradiometers to incorporate those designs, such as directly coupled and flip-chip gradiometers. We also expand the model to obtain the three-dimensional spatial frequency response of conventional wire-wound axial gradiometers. An assessment of a nonconventional planar design with actual parameters is made and the principal features of its spatial frequency response are compared with the ones of typical conventional gradiometer designs.