Spurious inductive coupling of a dc-SQUID to a resonant input circuit

A mutual inductance different from zero exists in most real dc-SQUIDs between the feedback/modulation coil and the input coil. This spurious coupling causes many effects when the input circuit of the SQUID is a high Q resonant circuit. In particular, it affects the Q of the input circuit, decreasing it or making the system unstable. A series of measurements showing these effects were made on the dc-SQUID coupled to the gravitational radiation detector of the Rome group operating at CERN. A model is then presented to explain, in detail, these spurious effects and a comparison is made with the experimental measurements. A way to overcome these unwanted effects by modifying the feedback loop is also presented.