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.