Effect of capacitive feedback on the characteristics of direct current superconducting quantum interference device coupled to a multiturn input coil

Distortion of voltage versus flux (V-Φ) relation of a dc superconducting quantum interference device (SQUID) coupled to a multiturn input coil is studied. First, resonant behavior of the coupled SQUID due to the so-called input coil resonance is clarified. It is shown that large rf noise flux is produced by the input coil resonance. This rf flux is added to the SQUID, and results in large rf voltage across the SQUID. In the case where parasitic capacitance exists between the input coil and the ground of the SQUID, this rf voltage produces the rf flux again, i.e., a feedback loop for the rf flux is formed. Taking into account this capacitive feedback, we study the V- Φ relation of the coupled SQUID. Numerical simulation shows that the V-Φ relation is distorted considerably by the feedback mechanism. The simulation result explains well the experimental V-Φ relation of the coupled SQUID. The combination of the input coil resonance with the capacitive feedback is the most likely mechanism for the distorted V-Φ curve of the coupled SQUID. The condition for occurrence of the distorted V-Φ curve due to the capacitive feedback is also obtained, and methods to prevent degradation are discussed.