Bias reversal technique in SQUID Bootstrap Circuit (SBC) scheme

Abstract

Recently, a SQUID direct readout scheme called voltage-biased SQUID Bootstrap Circuit (SBC) is introduced to reduce preamplifier noise contribution. In this paper, we describe a concept of SBC with bias reversal technique which can suppress SQUID intrinsic 1/f noise. When applying a symmetrically rectangular voltage across SBC, two I-Φcharacteristics appear at the amplifier output. In order to return to one I -Φcurve, a demodulation technique is required. Because of the asymmetry of typical SBC I-Φcurve, the demodulation method is realized by using a flux compensation of one half Φ0 flux shift. The output signal is then filtered and returned to one I-Φcurve for ordinary FLL readout. It was found, the reversal frequency fR can be dramatically enhanced when using a preamplifier consisting of two operational amplifiers. A planar Nb SQUID magnetometer with a loop-inductance of 350 pH, fR =50kHz and a second order low pass filter with 10kHz cut off frequency was employed in our experiment. Results prove the feasibility of SBC bias reversal method. Comparative experiment on noise performance will be carried out in further studies.