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/root Hz with the input shorted. For an optimal source resistance of 0.8 Ohm the noise tempe rature was 46 K. The picovoltmeter was constructed from a multilayer high-T -c SQUID-magnetometer coupled to the signal source via a 10-turn copper coi l. 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 ampli fier, additional positive feedback (APF) was used together with direct read -out electronics. As a demonstration, the current-voltage characteristics o f another SQUID was measured with the picovoltmeter.