Signal Enhancement in a Nonlinear Transfer Characteristic

We study nonlinear behavior in a model of a periodically modulated, overdamped rf SQUID loop operating in the dispersive (i.e., nonhysteretic) mode. In the presence of correlated noise we find an enhancement of the output signal-to-noise ratio (SNR) as a function of the nonlinearity parameter of the device. The calculation involves knowledge only of the input-output transfer characteristic of the device. These signal enhancement properties appear to be generic to devices characterized by nonlinear transfer characteristics. We also use our transfer characteristic approach to explain recent experimental results showing SNR enhancement in dc SQUIDs as a function of dc bias current and flux.