A Coupled DC SQUID with Low 1/f Noise

A low-noise coupled dc SQUID especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the centre of the washer close to the SQUID loop. A scaled up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The dc SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The white flux noise varies from 1.10(-7) Phi 0/root Hz to 3.10(-7) Phi(0)/root Hz depending on the critical current. The contribution of the 1/f noise at 1 Hz is found to be about 5.10(-7) Phi(0)/root Hz. The measured noise characteristics are very close to those predicted by the analytic model which was employed in the SQUID design.