Double relaxation oscillation superconducting quantum interference devices (SQUIDs) (DROSs) have been fabricated with estimated relaxation frequencies up to 14 GHz. Both the intrinsic flux noise and the performance in a flux locked loop with direct voltage readout have been studied. In flux locked loop, a noise level of 0.55 mgr;fgr;0/sqrt;Hz corresponding to an energy sensitivity of 34 h has been obtained for a DROS with a SQUID inductance of 29pH. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13 h at relaxation frequencies higher than about 3 GHz. copy;1994 American Institute of Physics.
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