Performance of an Electronic Gradiometer in Noisy Environments

An electronic first-order gradiometer consisting of two fully integrated dc SQUID magnetometers has been operated successfully in different noisy environments: at the PTB Berlin inside and outside the heavily shielded room, at a Berlin clinic without a shielded room, and at Siemens Erlangen in a moderately shielded room. In the Siemens room, a maximum feedback range of ±3 for each magnetometer (10% of the standard magnetometer feedback range) has been sufficient for safe operation, and noise levels of \(14fT/\sqrt {Hz} {\mkern 1mu} \) at 1 Hz and \(540fT/\sqrt {Hz} {\mkern 1mu} \) 0.1 Hz have been achieved with a baseline of 20 cm. Based on the noise measurements presented, a novel modular 55-channel system is proposed.