2 METHODS FOR A FIRST-ORDER HARDWARE GRADIOMETER USING 2 HIGH-TEMPERATURE SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

Two different systems for noise cancellation (first order gradiometers ) have been developed using two similar high temperature superconducti ng quantum interference devices (SQUIDs). ''Analog'' gradiometry is ac complished in hardware by either (1) subtracting the signals from the sensor and background SQUIDs at a summing amplifier (parallel techniqu e) or (2) converting the inverted background SQUID signal to a magneti c field at the sensor SQUID (series technique). Balance levels (abilit y to reject a uniform background magnetic field) achieved are 2 x 10(3 ) and 1 x 10(3) at 20 Hz for the parallel and series methods, respecti vely. The balance level as a function of frequency is also presented. The effects which time delays (phase differences) in the two sets of S QUID electronics have on these balance levels are presented and discus sed. It is shown that these delays, along with geometrical considerati ons, are the limiting factor for balance level for any electronic grad iometer system using two (or more) SQUIDs, a very different situation from the case with wire-wound gradiometers. Results using a dipole fie ld to study the performance of both the parallel. and series devices f unctioning as gradiometers in an unshielded laboratory are presented a nd compared with theory. (C) 1998 American Institute of Physics.