A new method for measurements of ultralow currents in the audio-frequency range (from dc up to over 104 Hz) has been developed. The combined use of a ferromagnetic current transformer and of a superconducting flux transformer together with a rf-biased SQUID allows measurements of currents as low as 10−14 A. A detailed circuital analysis and considerations on the input impedance of the device are presented. The agreement between predictions and experiments is quite good. The measured noise level beyond the low-frequency region keeps at values typical of commercial SQUID’s.
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Barbanera et al. (1978) studied this question.
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