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Superconductor Science and Technology

Simplified analysis of direct SQUID readout schemes

D. Drung

Published Apr 21, 2010• 25 citations
Publisher

Abstract

A simple approach to understanding and analyzing direct readout schemes for superconducting quantum interference devices (SQUIDs) is presented. It is shown that the existing methods for suppression of room temperature amplifier noise are based on voltage feedback and current feedback in the SQUID, that were introduced in the original schemes of additional positive feedback and bias current feedback. It is also shown that the way the SQUID is biased (at constant current or voltage) does not affect the noise suppression.

DOI
10.1088/0953-2048/23/6/065006

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A simple approach to understanding and analyzing direct readout schemes for superconducting quantum interference devices (SQUIDs) is presented. It is shown that the existing methods for suppression of room temperature amplifier noise are based on voltage feedback and current feedback in the SQUID, that were introduced in the original schemes of additional positive feedback and bias current feedback. It is also shown that the way the SQUID is biased (at constant current or voltage) does not affect the noise suppression.
We experimentally studied two important parameters of helium-cooled superconducting quantum interference devices (SQUIDs) in the voltage bias mode: the dynamic resistance Rd and the flux-to-current transfer coefficient ∂i/∂Φ, with different junction shunt resistors RJ. We investigated a voltage-biased SQUID using the direct readout current-to-voltage converter scheme involving an operational amplifier. At higher RJ, the flux-to-voltage conversion coefficient ∂V/∂Φ becomes sufficiently large to effectively suppress the room-temperature amplifier's noise without any need for additional feedback circuits. The McCumber parameter limits the rise of ∂V/∂Φ. We discuss the performance of voltage-biased SQUIDs at different effective McCumber parameters.
The ac bias method is used in the readout system of dc-superconducting quantum interference devices (SQUIDs) to suppress the low-frequency noise that originates in the fluctuation of parameters of two junctions in the SQUID. From the analysis of various schemes of the ac bias method, we propose a simple form which expresses the alternation of the operation variables, i.e., the bias current, modulation flux, and bias flux. We considered the modification of the previous schemes in the context of this form and have found a possible method to directly detect the noise component due to the parameter fluctuation. In the modified ac bias scheme, the low-frequency noise of the parameter fluctuation can either be detected or suppressed by manipulating a single variable. We fabricated the electronics to verify this function, and confirmed the versatility of this method from the measurements of the high-T C SQUID.
Recently, we demonstrated and analysed the superconducting quantum interference device (SQUID) bootstrap circuit (SBC). It is a direct readout scheme for dc SQUID in the voltage bias mode, permitting one to suppress the preamplifier noise. The SBC enables us to control the two key parameters of a voltage-biased SQUID: the flux-to-current transfer coefficient and the dynamic resistance. The flux-to-current, I??, characteristics of SBC are made asymmetric by introducing the additional current feedback. Depending upon the choice of the working point, this feedback can be positive (working point W2 on the steeper I?? slope) or negative (W1 on the less steep slope). The dynamic resistance is controlled by the additional voltage feedback. In our publications to date we presented only the SBC operation at W2, while in this paper we demonstrate operation at W1 and show that also in this regime the preamplifier noise suppression is possible. We used a liquid-helium-cooled Nb SQUID with a loop inductance of 350 pH and attained white flux noise of 2.5? ??0?Hz ? 1/2 both at W2 and at W1. In the latter case, the linear flux range exceeded one half-flux quantum ?0. This large linear range should lead to a significantly improved stability and slew rate of the system and also make the tolerable spread in circuit parameters much wider than in all SQUID direct readout schemes known to date. Consequently, operation in this regime opens a new path to possible SBC optimization.
There are two bias schemes, namely current-bias and voltage-bias, for the directly coupled readout circuit of direct current superconducting quantum interference device (dc SQUID). In order to find out the difference between two bias schemes, we present the theoretical analyses on their equivalent circuits, working point locations, and noise performances. The theoretical analysis and experimental validation prove that the voltage-bias scheme achieves a larger transfer coefficient and exhibits a better noise performance than the current-bias scheme in the reading of conventional dc SQUID.
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.
We have designed and tested a superconducting quantum interference device (SQUID) readout circuit, which can be operated at liquid helium temperatures. Although room-temperature SQUID electronics perform well, it is sometimes desirable to keep the wires between SQUID, readout electronics, and feedback coil as short as possible to minimize phase shifts and time delays. Cooling the readout circuit to low temperatures can also decrease its thermal noise. Our readout circuit uses conventional ac-flux modulation, which significantly reduces low frequency excess noise and drift in the preamplifier. In this case, simple complementary metal–oxide–semiconductor circuits with low power dissipation can be used as amplifier, phase-sensitive detector and integrator. The power dissipation of the complete readout is less than 15mW at 5V supply voltage.