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IEEE Transactions on Applied Superconductivity

A Practical Two-Stage SQUID Readout Circuit Improved With Proportional Feedback Schemes

Yongliang Wang, Guofeng Zhang, Shulin Zhang, Hui Dong, Liangliang Rong, Yong Wang, Xiaoming Xie

Published Jan 29, 2020• 2 citations
Publisher

Abstract

Nonlinearity with multiple working points limits the practical applications of a two-stage superconducting quantum interference device (SQUID) readout circuit, which is promising for low-noise performance. To solve this problem, we proposed a simple two-stage SQUID readout circuit with two conventional dc SQUIDs improved with proportional feedback schemes, where transfer coefficient of the first-stage SQUID circuit is enhanced by additional positive feedback circuit; transfer characteristics of the second-stage SQUID circuit are linearized by an additional negative feedback circuit. Two proportional feedback schemes are cooperated together based on matching conditions to achieve both noise suppression and working point matching. Working principles were verified in experiment results. The overall transfer characteristics achieved have only one working point and are simply adjusted according to the voltage swing of an amplifier. Transfer coefficient of the first-stage SQUID circuit was raised up to 10 (20 dB), and the total flux noise of two-stage SQUID readout circuit was reduced below 1 μΦ0/√Hz, where Φ0= 2.07×10-15Wb. Our two-stage SQUID readout circuit is easy-to-operate and suited for high-performance practical SQUID systems.

DOI
10.1109/tasc.2020.2970230

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Nonlinearity with multiple working points limits the practical applications of a two-stage superconducting quantum interference device (SQUID) readout circuit, which is promising for low-noise performance. To solve this problem, we proposed a simple two-stage SQUID readout circuit with two conventional dc SQUIDs improved with proportional feedback schemes, where transfer coefficient of the first-stage SQUID circuit is enhanced by additional positive feedback circuit; transfer characteristics of the second-stage SQUID circuit are linearized by an additional negative feedback circuit. Two proportional feedback schemes are cooperated together based on matching conditions to achieve both noise suppression and working point matching. Working principles were verified in experiment results. The overall transfer characteristics achieved have only one working point and are simply adjusted according to the voltage swing of an amplifier. Transfer coefficient of the first-stage SQUID circuit was raised up to 10 (20 dB), and the total flux noise of two-stage SQUID readout circuit was reduced below 1 μΦ0/√Hz, where Φ0= 2.07×10-15Wb. Our two-stage SQUID readout circuit is easy-to-operate and suited for high-performance practical SQUID systems.
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Feedback circuits, which act as functional units in superconducting quantum interference device (SQUID) readout circuits, such as positive feedback circuits for suppressing the preamplifier noise contribution or negative feedback circuits for increasing the linear flux range of SQUID, enable SQUIDs to achieve high performance in practical applications. Integrating feedback circuits with different functions into a single SQUID chip contributes to the construction of highly compact SQUID electronics. Here, we propose a SQUID readout circuit with tunable feedback polarity (TFP). The switching of the feedback polarity is easily realized by applying a control current to the superconducting switches integrated into the SQUID chip. The enhancement of the flux-to-voltage transfer coefficient or the linear flux range depends on the choice of feedback polarity. In addition, we introduce a two-stage scheme to address the degraded noise performance induced by negative feedback. This work attempts to develop a compact and versatile architecture of SQUID readout by using a set of compatible superconducting technologies.
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A low-noise single-chip two-stage superconducting quantum interference device (SQUID) system with a double relaxation oscillation SQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was 1.3 μΦ0/√Hz, corresponding to an energy sensitivity of ε≈27h. Owing to the large flux-to-voltage transfer of up to 3.6 mV/Φ0, the room-temperature preamplifier noise did not dominate the overall flux noise.
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User-friendly ultra-high-speed readout electronics for dc superconducting quantum interference devices (SQUIDs) are presented. To maximize the system bandwidth, the SQUID is directly read out without flux modulation. A composite preamplifier is used consisting of a slow dc amplifier in parallel with a fast ac amplifier. In this way, excellent dc precision and a high amplifier bandwidth of 50 MHz are achieved, simultaneously. A virtual 50 Ω amplifier input resistance with negligible excess noise is realized by active shunting, i.e., by applying feedback from preamplifier output to input via a high resistance. The white voltage and current noise levels are 0.33 nV Hz −1 and 2.6 pA Hz −1/2 , respectively. The electronics is fully computer controlled via a microcontroller integrated into the flux-locked loop (FLL) board. Easy-to-use software makes the various electronic settings accessible. A wide bias voltage range of 1.3 mV enables the readout of series SQUID arrays. Furthermore, additional current sources allow the operation of two-stage SQUIDs or transition edge sensors. The electronics was tested using various SQUIDs with input inductances between 30 nH and 1.5 µH. Typically, the maximum FLL bandwidth was 20 MHz, which is close to the theoretical limit given by transmission line delay within the FLL. Slew rates of up to 4.6 Φ 0 µs −1 were achieved with series SQUID arrays. Current noise levels as low as 0.47 pA Hz −1/2 and coupled energy sensitivities between 90 h and 500 h were measured at 4.2 K, where h is the Planck constant. The noise did not degrade when the system bandwidth was increased to the maximum value of about 20 MHz. With a two-stage set-up, intrinsic white energy sensitivities of 30 h and 2.3 h were measured at 4.2 and 0.3 K, respectively.