Integrated high-transition temperature SQUID-based voltmeters operated at 77 K

The voltage noise levels of high-T/sub c/ based YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal SQUIDs and SQUID voltmeters were measured to investigate their noise characteristics. The SQUIDs are of washer type with outer dimension of 90 /spl mu/m/spl times/90 /spl mu/m and inner dimension of 20 /spl mu/m/spl times/20 /spl mu/m. The SQUIDs have a normal inductance of about 60 pH. The integrated modulation and input coils were circular in shape. The modulation coil has a radius of r/sub 2/=80 /spl mu/m and r/sub 1/=60 /spl mu/m while the input coil for voltage measurement has radius r/sub 2/=50 /spl mu/m and r/sub 1/=30 /spl mu/m. Using a dc bins and standard flux locked loop, the spectral density of the voltage noise, S/sub v/, was observed to behave as 1/f at low frequencies and the root mean square flux noise, S/sub /spl Phi///sup 1/2/, is white and has a value of /spl sim/15 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ at 1 kHz. The flux noise levels were reduced to /spl sim/4 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ using the ac bias reversing technique. These data suggest that the dominant 1/f voltage noise was caused by critical current fluctuations. We further demonstrate that the 1/f can be induced by flux motion. The voltage noise levels of the SQUID voltmeters are also reported.