Noise analysis of DC SQUIDs with damped superconducting flux transformers

An analysis was performed of intrinsic noise for high-T-c DC SQUID with superconducting flux transformer (FT) containing resistive elements. For a SQUID with a loop inductance of about 40 pH we observed voltage swings of similar to 55 mu V and a flux noise of similar to 4 mu Phi(0)/root Hz at 77 K. Inductive coupling of an 8-mm multilayer superconducting FT to the SQUID increased voltage swings to similar to 70 mu V due to effective reduction of the SQUID loop inductance. This also increased the flux noise to similar to 6 mu Phi(0)/root Hz, corresponding to a field resolution of similar to 18 fT/root Hz at 77 K with a white noise spectrum down to frequency similar to 10 Hz. The main sources of white flux noise were the Nyquist noise in the Josephson junctions and the FT, as well as the suppression of the DC SQUID voltage swings caused by parasitic capacitance between the FT and the SQUID. An ultra-low-ohmic resistor with resistance value between the flux-creep-induced resistances of superconductors (below similar to 0.1 n Omega) and resistances of conventional resistors (above similar to 0.1 m Omega) was developed. An RL-circuit based high-pass filter (HPF) with time constant similar to 7 sec was realized and integrated in the superconducting FT. The contribution of the HPF to the noise of the sensors was measured and compared with calculated values.