HIGH-PERFORMANCE SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE FEEDBACKELECTRONICS

A de superconducting quantum interference device (SQUID) modulation an d feedback circuit operating at a modulation frequency of 16 MHz has b een constructed. Using a novel wide band superconducting thin film tra nsformer to impedance match the SQUID to a rf amplifier allows the sys tem to operate at the SQUID noise level for most types of low-T-C SQUI Ds. This system has a closed loop bandwidth exceeding 2.5 MHz and a sl ew rate greater than 1x10(6) Phi(0)/s at frequencies up to 1 MHz. This greatly improved performance compared to existing modulation methods can be obtained without enhancing the transfer function of the SQUID. The system allows low- and high-T-C SQUID magnetometers and gradiomete rs to be operated totally unshielded without unlocking in the de, 60 H z, and radio frequency electromagnetic fields present in most SQUID ap plications. (C) 1996 American Institute of Physics.