Squid Fundamentals

Abstract

Superconducting quantum interference devices (SQUIDs) are the most sensitive devices for measuring weak magnetic fields. Beside the SQUID itself, the design of the read-out electronics decisively determines the performance of the whole sensor. In standard read-out electronics, a cooled impedance-matching transformer between SQUID and preamplifier, and a flux modulation technique are employed. Recently, several novel SQUID read-out concepts without flux modulation have been developed, mainly for biomagnetic multichannel systems. This chapter gives a description and comparison of the most important ones: concepts using multiple SQUIDs, SQUIDs with additional positive feedback, relaxation oscillation SQUIDs, and digital SQUIDs. Both the noise and the dynamic behavior are discussed, and a simple model for the achievable speed of a directly-coupled feedback loop without flux modulation is presented, which allows one to estimate the dynamic merits of the various SQUID concepts