High-Tc and low-Tc dc SQUID electronics

摘要: Superconducting quantum interference devices (SQUIDs) are commonly operated in a flux-locked loop (FLL). The SQUID electronics amplifies the small signal to an acceptable level without adding noise, and it linearizes transfer function of order provide sufficient dynamic range. In this paper, fundamentals readout reviewed including discussion preamplifier noise. basic FLL concepts, direct flux modulation readout, discussed both with dc bias reversal. Alternative concepts such as additional positive feedback (APF), two-stage SQUIDs, series arrays, relaxation oscillation SQUIDs digital briefly described. dynamics on basis simple model finite delay. It is shown that optimized system bandwidth ≈18 MHz corresponding slew rate ≈8 Φ0 µs−1 possible. A novel scheme involving Smith predictor presented which allows one increase about 100 MHz. theoretical predictions experimentally checked using high-speed prototype small-signal 300 Methods for increasing range systems described: flux-quanta counting field compensation (DFC). With DFC, residual magnetic at can be kept close zero even if device moved Earth's field. Therefore, noise high-Tc magnetometer measured inside magnetically shielded room (60 fT Hz−1/2 1/f corner 2 Hz) remained unchanged after moving outside µT plus 0.8 peak-to-peak power line interference).