High frequency ac bias for direct-coupled dc superconducting quantum interference device readout electronics

A high frequency ac-bias technique for direct-coupled readout electronics, for high temperature dc superconducting quantum interference devices (HTc dc SQUIDs), is presented. The method was tested for ac-bias frequencies between 2 and 10 MHz on different HTc dc SQUIDs. The minimum white flux-noise level of 9.5 μΦ0/Hz1/2, corresponding to a noise limited magnetic field resolution of 24 fT/Hz1/2, was achieved with a HTc dc SQUID with a maximum voltage swing of 64 μVpp and an ac-bias frequency of about 7.7 MHz. A maximum system dynamic range of 156 dB (±600 Φ0) in the white noise frequency region was achieved for this sensor. For various HTc dc SQUID magnetometers a bandwidth of between 1.6 and 4 MHz was measured. The slew rates were in the range of 1–2 MΦ0/s for several HTc dc SQUIDs.