High performance superconducting quantum interference device feedback electronics

A dc superconducting quantum interference device (SQUID) modulation and feedback circuit operating at a modulation frequency of 16 MHz has been constructed. Using a novel wide band superconducting thin film transformer to impedance match the SQUID to a rf amplifier allows the system to operate at the SQUID noise level for most types of low-Tc SQUIDs. This system has a closed loop bandwidth exceeding 2.5 MHz and a slew rate greater than 1 X 106Φ0/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-Tc SQUID magnetometers and gradiometers to be operated totally unshielded without unlocking in the dc, 60 Hz, and radio frequency electromagnetic fields present in most SQUID applications. © 1996 American Institute of Physics.