Integrated gradiometer-SQUID system for fetal magneto-cardiography without magnetic shielding

First-order integrated planar SQUID gradiometers fabricated on the basis of the Nb/AlO(X)/Nb technology have pick-up loops with dimensions of 2 cm x 2 cm and a baseline of 4 cm. Their intrinsic noise corresponding to a field resolution in one loop is better than 3 fT Hz(-1/2) and balance better than > 10(4). Two of the gradiometers were used for measurements of a fetal heart signal. The third one was placed over the first two as a reference to build second-order gradiometers electronically. In addition, three SQUID magnetometers arranged orthogonally to each other were used to improve the disturbance suppression. The system shows stable operation in the laboratory without magnetic shielding. The SQUID signals were digitized using a 16-bit ADC and digitally filtered. A post-processing filter algorithm for enhancing the signal-to-noise ratio is discussed. A fetal cardiogram could be recorded in real time with a signal-to-noise ratio better than 2 for a test person in the 36th week of pregnancy.