Distortion of voltage vs flux relation of dc SQUID coupled to multiturn input coil due to input coil resonance combined with capacitive-feedback effect

Distortion of voltage vs flux (V-Φ) relation of a dc SQUID coupled to a multiturn input coil is studied. It is shown that the so-called input coil resonance by itself does not result in the distorted V-Φ curve. Therefore, we study one more mechanism, i.e. feedback effect due to a parasitic capacitance between the input coil and the ground of the SQUID. In this case, it is shown that a feedback loop is formed for the magnetic flux, and that the distortion of the V- curve is strongly related to the stability of the feedback system. Good agreement between theory and experiment suggests that the input coil resonance combined with the capacitive feedback is the most likely mechanism for the distorted V-Φ curve of the coupled SQUID. Methods to obtain the smooth V-Φ curve are also discussed.