High-performance DC SQUID read-out electronics

The dynamic behavior of superconducting quantum interference devices (SQUIDs) operated in a flux-locked loop (FLL) is discussed using a simple mathematical description. It is shown that the slew rate of any FLL is limited by the linear flux range of the SQUID Φ lin and the effective loop delay t d to approximately Φ ̇ f, max =Φ lin /(4t d ) if a one-pole integrator is used. This allows one to estimate the dynamic limits of both analog SQUIDs with flux modulation or direct read-out and digital SQUIDs with on-chip read-out. In addition to theoretical limits, practically achievable performance is presented using our latest direct-coupled FLL electronics as an example of state-of-the-art SQUID read-out. This electronics is designed for both low-critical temperature (low- T c) and high- T c SQUIDs. It combines low noise with high bandwidth and slew rate. All functions are computer controlled using a LabVIEW ® program. An automatic bias voltage tuning circuit increases the system stability in case of large cryogenic temperature fluctuations. Furthermore, an ultra-low-noise current source is integrated into the FLL board which can be used to supply a coil system compensating the Earth's magnetic field of about 60 μT while producing only 17 fT/ Hz extra noise at 1 kHz and 41 fT/ Hz at 1 Hz, respectively.