Flux Modulation Scheme (FMS)

Summary

This chapter introduces the “mixed bias mode” and discusses the flux modulation scheme (FMS). It revisits the FMS by analyzing the bias mode and the transfer characteristics of a step‐up transformer. The readout schemes share the common idea of modifying the Superconducting QUantum Interference Device (SQUID) input circuit to enhance the SQUID's apparent transfer coefficient at the input terminals of the preamplifier. The SQUID is matched to a preamplifier via a four‐terminal step‐up transformer. The “head stage” of the FMS is essentially an AC circuit and is calibrated by the amplitude‐ and phase‐to‐frequency transfer characteristics of the transformer. The two imperfections for SQUID operation in FMS, one is the shunt effect of primary winding and the other is nonlinear transformation from primary winding to secondary winding, to reduce benefit of the transformer.