Non-constant bias current for dc SQUID operation

Abstract

The in-phase current noise of the two junctions leads to additional 1/ f noise in dc SQUIDs, which is reduced by the well-known bias reversal technique (ac bias reversal). In this paper, a new idea for dc SQUID operation with ac bias reversal non-constant bias current is described. In most conventional readout electronics with constant dc bias, the dc SQUID is connected to the preamplifier by means of a transformer in order to enhance the SQUID voltage signal and thus reduce the preamplifier noise contribution. We analyse this case in ac bias condition and find that the current through the SQUID is not constant after the bias reversal. In order to solve this problem, we derived the differential equation governing the network and solved it for a constant current through the SQUID. The transient of the applied bias current is chosen to satisfy this differential equation. The time constant of the transient does not depend on the dynamic resistance of the SQUID. The bias reversal frequency could be increased without introducing additional noise to the SQUID.