IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 7, OCTOBER 2016 1501805
Compact Dual-Band HTS Bandpass Filter Using
Spirally Asymmetric Stepped-Impedance Resonators
Haiwen Liu, Senior Member, IEEE, Fan Liu, Feng Qin, Baoping Ren, Pin Wen, and XueHui Guan, Member, IEEE
Abstract—In this paper, a compact dual-band high-temperature
superconducting (HTS) bandpass filter (BPF) is constructed by
spirally asymmetric stepped-impedance resonators (SIRs) and a
pair of parallel-coupled feed lines. The resonant characteristics
of spirally asymmetric SIRs with one step discontinuity are in-
vestigated in the dual-band BPF design. Owing to its intrinsic
characteristics and the multipath propagation effect, multiple
transmission zeros are created to improve the selectivity of the
passband and the rejection level of stopband. For demonstration
purposes, a compact HTS filter operating at 2.4/5.8 GHz for
wireless local area networks is fabricated. Measured insertion
losses at each passband are 0.12 and 0.18 dB, respectively. A
good agreement between the measurements and the simulations
is observed, which verifies the circuit design methodology. Finally,
the temperature dependence and the input-power dependence of
microwave nonlinear properties of the dual-band HTS filter at
each passband are also measured and analyzed. Measured results
have a good agreement with simulations.
Index Terms—Bandpass filter (BPF), dual-band, high-
temperature superconducting (HTS), nonlinearity, spirally
asymmetric stepped-impedance resonator (SIR).
I. INTRODUCTION
I
N MODERN wireless and mobile communication systems,
dual-band microwave components have been gaining wide
attention in recent years. To meet various application require-
ments, some methods have been proposed for dual-band BPF
design [1]–[5]. In [1], the dual-band filter was realized by
directly combining two sets of different resonators with com-
mon input and output, which led to a relatively large circuit
size. To design miniaturized dual-band BPF, various multi-
mode resonators (MMRs) cell, such as stub-loaded resonators
(SLRs) [2], split-ring resonators (SRRs) [3], split-ring res-
onators (SRRs) [4], and square ring loaded resonators (SRLRs)
[5] have been presented. However, the flexibility in the choice
of band allocation and passband selectivity still need to be im-
Manuscript received March 9, 2016; accepted July 19, 2016. Date of
publication June 22, 2016; date of current version August 12, 2016. This
work was supported in part by the National Science Foundation of China
under Grants 61461020, U1431110, and 61261041; by the 555 Talent Program
of Jiangxi Province and Technology Innovation Team of Jiangxi Province of
China under Grant 20142BCB24004; and by the Co Innovation Center of the
Intelligent Management Technology and Equipment.
The authors are with the School of Information Engineering, East China
Jiaotong University, Nanchang 330013, China (e-mail: haiwen_liu@hotmail.
com; newfans@foxmail.com).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2016.2593950
Fig. 1. Traditional asymmetric SIR.
proved. Recently, asymmetric SIRs with only one discontinuity
can also be used to construct multi-band BPFs with the merit
of more design feasibility [6]–[8]. However, it is still a great
challenge to design filters with compact circuit size, low insert
loss, and high band-to-band isolation level.
On the other hand, it is well known that the extremely low
surface resistance of high-temperature-superconductors (HTS)
thin films at microwave frequencies allows its application to
fabricate high performance RF/ microwave devices, especially
in the aspect of multi-band microstrip BPFs with low insertion
loss and high out-of-band rejections [9], [10]. Nevertheless,
only few attempts have so far been proposed to design a high-
performance HTS filter using asymmetric SIRs [11].
In this work, a compact dual-band HTS BPF constructed by
using spirally asymmetric SIR is reported. The resonant char-
acteristics of the asymmetric SIR can be easily controlled and
the first two resonant modes are utilized to realize a dual-band
performance at 2.4/5.8 GHz for WLAN applications. In order
to achieve the compact size and high selectivity, some improve-
ments are conducted on the asymmetric SIR with spiral high-
impedance transmission-line. Besides, multiple transmission
zeros (TZs) for high selectivity and band-to-band isolation are
generated. What’s more, a prototype of the proposed dual-band
HTS filter is fabricated and measured. Experimental verifica-
tion is provided to approve the design method and the predicted
results. Moreover, measured temperature dependence and
input-power dependence of microwave nonlinear behaviors for
the dual-band HTS filter are also presented and discussed. Fi-
nally, a brief conclusion is given for this dual-band filter design.
II. R
ESONANT CHARACTERISTICS OF
THE
ASYMMETRIC SIR
Fig. 1 shows a configuration of the traditional asymmetric
SIR. It is composed by a high-impedance transmission-line sec-
tion and a low-impedance one with characteristic impedances
Z
1
and Z
2
, physical lengths L
1
and L
2
, physical widths W
1
and W
2
, and electrical lengths θ
1
and θ
2
, respectively. The input
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