Analysis of a dc SQUID readout scheme with voltage feedback circuit and low-noise preamplifier

摘要

We analyzed the dc SQUID with voltage feedback circuit (VFC) and a low-noise roomtemperature preamplifier to evaluate the feasibility of a low-noise SQUID direct-coupled readout scheme (DRS), possibly eliminating the need for a two-stage scheme employing a SQUID preamplifier. The passive VFC, connected in parallel to the SQUID, consists of a resistor R_s in series with an inductor L_s. This inductor is coupled to the SQUID by a mutual inductance M_s. The purpose of the VFC is to increase the SQUID's flux-to-voltage transfer coefficient ?V/?Φ, thus reducing the preamplifier noise contribution δΦ_(preamp). However, at the same time, VFC introduces the thermal noise of Rs, δΦ_R, which may not be negligible. Generally, the noise of the readout scheme, δΦ_(readout), may thus include both δΦ_(preamp) and δΦ_R, i.e., δΦ_(readout)~2 = δΦ_(preamp)~2 + δΦ_R~2.To characterize the SQUID operation with VFC we introduced two dimensionless parameters, r = R_s/R_d and Δ = (M_s/M_(dyn)) - (R_s/R_d), where Rd and Mdyn = 1/(?i/?Φ) are dynamic properties of the SQUID itself. For assumed intrinsic SQUID parameters, we then numerically analyzed the dependence of δΦ_(readout) noise components on r and Δ to determine their suitable ranges and the minimum of δΦ_(readout). To verify our analysis, we experimentally characterized, in liquid helium, three niobium SQUIDs with VFC, having suitably chosen r and Δ. The measured SQUID system flux noise was on the order of 1 μΦ_0/√Hz, comparable to the intrinsic noise of the SQUID itself. The deduced equivalent voltage noise was comparable to that of a SQUID preamplifier in the twostage readout. Simple single-stage ultra-low-noise SQUID DRS readout was thus demonstrated.