Characterization of the input noise sources of a dc SQUID

From the the theory of the intrinsic noise in a de SQUID by Tesche and Clar ke, we derive the expressions of the current and voltage input noise spectr al densities in a de SQUID current aw amplifier operated in a flux locked m ode. The expected current and voltage noises are compared at audio frequenc ies, with the experimental results obtained with a low noise dc SQUID in wh ich the input load (resonant and not) and rite operating temperature (1-4 K ) are changed In order to evaluate the input voltage noise, which is direct ly I elated to the curl ent noise around the SQUID loop and is usually negl ected, we have used as the input circuit a LC resonator with a very high qu ality factor (approximate to 10(6)). Both the voltage and current input noi ses exceed the expected values by the same factor of about 8. This means th at the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximate ly given by the product of the input coil inductance and the angular freque ncy. To explain the excess noise results, we propose a model in which the v oltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.