A double DC-SQUID device for flux locked loop operation

A novel double DC SQUID device has been developed. It consists of two SQUIDs in a cascade configuration with the same electrical parameters but with a different geometry. This structure makes the device particularly suitable for flux-locked-loop operation without need for modulation flux. Consequently the readout electronic scheme is greatly simplified without degrading the noise performance of the first stage. The first DC SQUID has a gradiometer configuration with extra damping resistors shunting the inductance. The technology used is similar to the seven-layer IBM process with planar Nb-Nb/sub 2/O/sub 5/-PbAuIn junctions and 2.5- mu m minimum size lithography.