Two methods for a first order hardware gradiometer using two HTS SQUIDs

Two types of first order electronic gradiometers have been developed using high temperature superconducting (HTS) SQUIDs. Gradiometry is accomplished in hardware by either 1) subtracting the output of the signal and background SQUIDs in a summing amplifier (parallel technique) or 2) converting the inverted background SQUID output to a magnetic field at the sensor SQUID (series technique). Balance levels achieved are 2000 and 1000 at 20 Hz for the parallel and series methods respectively. Balance level as a function of frequency is presented. Balance level for hardware gradiometry is limited by time delays from the electronics and how well the signal amplitudes are matched. A simple algorithm that allows one to estimate the limit on balance level from these factors is presented and compared with data.