Performance analysis of modified flux-locked-loop additional positive feedback SQUID

An analysis of the additional positive feedback (APF) effect on SQUID performance in terms of frequency response is presented. The effect of negative feedback with an external flux-locked-loop amplifier on overall performance in terms of immunity to electromagnetic interference (EMI) is discussed. Disadvantages and limitations of APF are pointed out. An improved APF circuit with local negative feedback at high frequencies which improves the SQUID´s overall EMI robustness is introduced. A 1/f noise reduction technique which can be used with APF while improving EMI rejection is also introduced. Simulations and a comparison between the two alternative designs are performed.