Most biomagnetic studies depend on the SQUID magnetic field sensor to provide the sensitivity and bandwidth required for meaningful measurements. Fluxgates and induction coils are appropriate for certain limited applications, but for all-around versatility and ultimate sensitivity the SQUID offers significant advantages, despite the inconvenience and cost of the liquid helium that is presently required for cooling. This chapter describes the operating principles of the SQUID to make clear its capabilities and limitations. We shall also point out the direction in which the developmentment of improved SQUIDS is going. Useful articles on the principlesles of the rf- and dc-SQUID have been written by Clarke (1977) and an up-to-date assessment of SQUID performance can be found in the proceedings of a recent conference (Hahlbohm and Lübbig 1980).
KeywordsJosephson Junction Parametric Amplifier Josephson Effect Tank Circuit Output Voltage Versus
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