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 background noise, and the output signal fed back to other magnetometers to cancel background noise. Fast feedback electronics were built for the background channel with a slew rate about 10(7) phi (0)/sec and 10 MHz small signal bandwidth, 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 decrease l/f noise. The output signals from these two magnetometers were input to a summing amplifier resulting in a gradiometric output signal without background channel noise. Constructing the gradiometer with different magnetometers on the array enables us to vary the baseline from 0.75 mm to 7.5 mm with 2x10(-12) T/root Hz field resolution in an unshielded laboratory environment, This variable-baseline gradiometer can be used for NDE, biomagnetism and other applications.