Electronic gradiometer using HTc SQUIDS with fast feedback electronics

An electronic gradiometer was built using a HTS SQUID magnetometer array. A SQUID magnetometer in the center of the array was used to pick up backgrou nd noise, and the output signal fed back to other magnetometers to cancel b ackground noise. Fast feedback electronics were built for the background ch annel with a slew rate about 10(7) phi (0)/sec and 10 MHz small signal band width, Two other magnetometers of the array were connected to pcSQUID (TM) electronics with 5x10(4) phi (0)/sec slew rate using the AC bias mode to de crease l/f noise. The output signals from these two magnetometers were inpu t to a summing amplifier resulting in a gradiometric output signal without background channel noise. Constructing the gradiometer with different magne tometers on the array enables us to vary the baseline from 0.75 mm to 7.5 m m with 2x10(-12) T/root Hz field resolution in an unshielded laboratory env ironment, This variable-baseline gradiometer can be used for NDE, biomagnet ism and other applications.