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Physica C: Superconductivity and its applications
journal homepage: www.elsevier.com/locate/physc
Study on bias reversal readout working at suppressing low frequency noise
of dc SQUID with different β
c
Hua Chen
a,b,c
, Tiantian Liang
a,b,c
, Xiangyan Kong
⁎
,a,b,c
, Yongliang Wang
a,b
, Hai Wang
a,b,c
,
Guofeng Zhang
a,b
, Xiaoming Xie
a,b
a
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences
(CAS), Shanghai, 200050, China
b
CAS Center for Excellence in Superconducting Electronics (CENSE), Shanghai, 200050, China
c
University of Chinese Academy of Sciences, Beijing, 100049, China
ARTICLE INFO
Keywords:
SQUID
Low frequency noise
Bias reversal
Readout
PACS:
85.25.Dq
43.50.+y
87.57.cm
ABSTRACT
How to reduce of low frequency noise of low Tc DC superconducting quantum interference device (DC SQUID)
has been an important problem and bias reversal readout scheme is an effective method to tackle it. However,
the effect varies with the change of the parameter of sensors. Wang et al. found that DC SQUID with different
ranges of β
c
matches different readout circuits to gain the lowest white noise. It is regrettable that this study did
not involve the discussion of low frequency. In this work, we compare the noise of SQUID with a large range of β
c
measured by static bias and bias reversal. The preamplifier used is six parallel connected bipolar transistors
(6 × SSM2220). Finally, we found that the bias reversal scheme is suitable to match SQUIDs with β
c
<1.
1. Introduction
The source and suppression scheme of low frequency noise (1/f
noise) of DC Superconducting Quantum Interference Device (DC
SQUID) has been an important research subject as its low frequency
application in biomagnetism and long-wave communication. Several
studies suggest that fluctuation of the junction critical current causes
the low frequency noise of DC SQUID [1,2] and has been approved [3].
To suppress the low frequency noise of DC SQUID, R H Koch et al.
[4] proposed bias reversal scheme and achieved some good results. As
reported, bias reversal scheme has been used to measure different types
of DC SQUIDs like high-Tc DC SQUID, SNS junction based DC SQUID
and nano-SQUID, and the low frequency noise has been successfully
reduced [5–9]. However, the result of bias reversal scheme presented
little difference to that measured by static bias sometimes [10], which
suggested that the effect of bias reversal is affected by the property of
devices.
Conventionally, in order to avoid the hysteresis, SQUIDs should be
operated in the range of β
c
< 1, where β
c
is the Stewart-McCumber
parameter. Recently, Liu et al. extended the SQUID operating area to β
c
> 1 due to a large noise parameter Γ* [11]. Therefore, noise matching
between sensors with different β
c
and readout electronics becomes an
important problem. White noise matching between SQUID sensor and
readout electronics with increasing β
c
has been studied [12], however,
low frequency is not involved in this study. In this work, we examine
the low frequency noise matching between SQUID sensor and bias re-
versal readout scheme for SQUID with β
c
varying in a large range. The
noise spectral density measured by bias reversal is compared to that
measured by static bias to verify the effect of bias reversal.
2. Electronic principle and realization
In the bias reversal readout technique, the SQUID is biased with a
rectangular current which is switched between +I
B
and -I
B
at a fre-
quency of f
b
. The voltage-flux characteristics (VFC) of the SQUID are
changed with the bias current as shown in Fig. 1. The position of the
working point, where the flux-lock-loop (FLL) circuit works stably, is
also changed (points A and B in Fig. 1). In order to provide proper
operation of the FLL circuit, the VFC must be shifted in both directions
(along V and Φ axis) until the points A and B coincide on the Φ axis.
Therefore, an additional voltage V
B
must be supplied to FLL circuit (bias
compensation) and an additional flux (Φ
B
) must be introduced to
SQUID (bias flux). While the voltage amplitudes at both polarities are
different, an offset voltage V
O
should be supplied to circuit. As a result,
https://doi.org/10.1016/j.physc.2018.08.007
Received 22 March 2018; Accepted 10 August 2018
⁎
Corresponding author.
E-mail address: xykong@mail.sim.ac.cn (X. Kong).
Physica C: Superconductivity and its applications 553 (2018) 61–64
Available online 11 August 2018
0921-4534/ © 2018 Elsevier B.V. All rights reserved.
T