MODELING THE DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE COUPLED TO THE MULTITURN INPUT COIL .3.

The radio frequency (rf) properties of a dc superconducting quantum interference device (SQUID) coupled to a multiturn input coil have been studied both experimentally and theoretically using an expanded model of the coupled SQUID. The input coil and washer resonances observed in the measured impedance of the expanded model are shown to be in quantitative agreement with the theory presented previously. When the input coil resonances are suppressed by resistive damping, it is shown that the impedance of the coupled SQUID at low frequencies is determined solely by an effective inductance that is much lower than the SQUID inductance. A simple equivalent circuit of the coupled SQUID for this case is proposed. The rf properties of SQUIDs with gradiometric washer configurations are also discussed. It is shown that the rf properties of these devices can be predicted from the properties of the individual washers.