Practical dc SQUID system: Devices and electronics

• The current noise contribution in direct readout scheme for dc SQUID was analyzed.
• A current feedback circuit for suppressing the current noise was introduced.
• Two criteria to measure the suppression factor were introduced.
• A practical SQUID system with acceptable noise performance was suggested.

In order to set up a liquid helium-cooled practical dc SQUID system with acceptable noise figure, we employ a weakly damped dc SQUID with a large flux-to-voltage transfer coefficient ∂V/∂Ф in a direct readout scheme (DRS) without flux modulation. Two preamplifiers are utilized: (1) AD 797 for a so-called “single chip readout electronics” (SCRE); (2) 6 parallel-connected bipolar transistors (PCBT). The latter reduces the preamplifier voltage noise Vn but increases its current noise In, which plays a leading role in the system low-frequency noise. We introduce a current feedback circuit (CFC) consisting of an inductor Li coupled to the SQUID with mutual inductance Mi to improve the noise performance. In this work, the preamplifier In contribution and CFC are analyzed. To evaluate the In suppression with CFC, two criteria are presented. Furthermore, we establish a dimensionless parameter ξ to describe CFC quantitatively. The system noise is compared with and without CFC using the two preamplifiers. For a dc SQUID with a loop inductance of 350 pH, an intrinsic noise of about 5 μФ0/√Hz and a corner frequency at 2 Hz are measured using PCBT with CFC.