A SQUID picovoltmeter working at 77 K

A SQUID picovoltmeter working at 77 K has been constructed with a bandwidth of 37 kHz. The white noise of the picovoltmeter was 32 pV//spl radic/Hz with the input shorted. For an optimal source resistance of 0.8 /spl Omega/ the noise temperature was 46 K. The picovoltmeter was constructed from a multilayer high-T/sub c/ SQUID-magnetometer coupled to the signal source via a 10-turn copper coil. The mutual inductance between the coil and the SQUID washer was 0.7 nH. To reduce the noise contributions of the first-stage room temperature amplifier, additional positive feedback (APF) was used together with direct read-out electronics. As a demonstration, the current-voltage characteristics of another SQUID was measured with the picovoltmeter.