A compact planar gradiometer system for measuring tangential components of biomagnetic fields
Presents a compact and reliable planar gradiometer for a multichannel system measuring tangential components of biomagnetic fields. The readout electronics was simplified by using a double relaxation oscillation SQUID (DROS) which provides very large flux-to-voltage transfer of typically 3 mV//spl Phi//sub 0/, enabling direct readout by a DC preamplifier with modest voltage noise. The DROS was made from hysteretic Nb/AlO/sub x//Nb junctions. To realize a compact and reliable gradiometer, the planar pickup coil is integrated on the same wafer with the SQUID. The planar pickup coil consists of two 10 mm/spl times/10 mm coils connected in series to form a first-order gradiometer and has a baseline of 30 mm. The field gradient noise is 9 fT/cm/spl radic/Hz at 1 Hz inside a magnetically shielded room. The gradiometers were arranged to measure simultaneously x and y components of biomagnetic fields.