Input SQUID-magnetometer circuits: matching and optimization

The mutual dynamic influence bem een a quantum interferometer (QI) and an input circuit (IC) is described. It is shown that the matching problem of SQUID IC is the inverse problem. The QI-flux transformer (FT) matching problem is solved in a general case with and without the negative feedback (NF). It is found that one do not need to perform the matching for an FT with a strong NF and for a tuned IC at resonance measurements. Expression for energy sensitivity in an IC pick-up coil as the function of six variables is obtained, and the DC SQUID noise back reaction is taken into account. A detailed analyses of the conditions is performed for the noise optimisation in two limiting cases concerned with a high-quality. tuned IC and with a broadband FT.