An HTS SQUID picovoltmeter with a flip-chip flux transformer

An HTS SQUID picovoltmeter was constructed, which in two different configurations gave a voltage noise of e/sub n/=3.2pV//spl radic/Hz and a current noise of i/sub n/=310 pA//spl radic/Hz, and a voltage noise of e/sub n/=75 pV//spl radic/Hz and a current noise of i/sub n/=10 pA//spl radic/Hz, respectively. The low frequency noise was reduced to below 1 Hz using bias current reversal. The SQUID is coupled to the input coil via an intermediate flip-chip flux transformer. Measurements were performed to determine the input coil to SQUID mutual inductance for different input coil configurations.