A computer model for noise in the DC SQUID

A computer model for the dc SQUID is described which predicts signal and noise as a function of various SQUID parameters. Differential equations for the voltage across the SQUID including the Johnson noise in the shunted junctions are integrated stepwise in time. Noise-rounded I-V characteristics are computed as a function of applied flux, Φa, and ring inductance, L. A measure of the SQUID response, dV/dΦa, is calculated as a function of bias current. Low frequency voltage power spectral densities computed for various Φaand L show considerable variation from the corresponding single junction Values. The flux resolution as a function of bias current is computed for several values of L and Φa. The results are in good agreement with experiment.