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 1×106Φ0/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.