AN AC BIAS READOUT SCHEME FOR HIGH-TC DC-SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

We propose a readout scheme for high-T(c) dc-SQUIDs (superconducting q uantum interference devices) with alternating current bias and synchro nized off set flux. By use of this scheme, low-frequency noise due to critical current fluctuations, which exist in grain boundary junctions , can be canceled. The readout electronics are simple and operation pa rameters requiring adjustment are minimized. The ac bias scheme has be en applied to YBCO step-junction SQUIDs, where partial reduction of th e equivalent flux noise below 100 Hz is observed.