Evaluation of High-T_c Superconducting Quantum Inter-ference Device with Alternating Current Bias DOIT and Additional Positive Feedback (Special Issue on Basic Properties and Applications of Superconductive Electron Devices)

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This study shows the results of evaluating the flux noises at low frequency when the alternating current (AC) bias direct offset integrated technique (DOIT) with additional positive feedback (APF) is used in a high-T_c dc superconducting quantum interference device (SQUID). The AC-bias DOIT can reduce low-frequency noise without increasing the level of white noise because each operating point in the two voltage-flux characteristics with AC bias can always be optimum on the magnetometer in the high-T_c dc-SQUID. APF can improve the effective flux-to-voltage transfer function so that it can reduce the equivalent flux noise due to the voltage noise of the preamplifier in the magnetometer. The use of APF combined with the AC-bias DOIT reduced the noise of the magnetometer by factors of 1.5(33μΦ_0/√<Hz> vs. 50μΦ_0/√<Hz>) at 100 Hz, 3.5(43μΦ_0/√<Hz> vs. 150μΦ_0/√<Hz>) at 10 Hz, and 5.2(67μΦ_0/√<Hz> vs. 351μΦ_0/√<Hz>) at 1 Hz as compared with the noise levels that were obtained with the static-current-bias DOIT. The contribution of the factors at 1 Hz is about 2 by APF and 2.6 by AC bias. The performance of improving the flux noise in the AC-bias DOIT with APF is almost equal to that of the flux locked loop (FLL) circuits in which the flux modulation uses a coupling system with a transformer and with the AC bias.
一般社団法人電子情報通信学会の論文
1997-10-25