IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 2, APRIL 2014 1500308
Compact High-Temperature Superconducting Filter
Using Multimode Stub-Loaded Resonator
Haiwen Liu, Senior Member, IEEE, Jiuhuai Lei, Xuehui Guan, Member, IEEE,
Yulong Zhao, Liang Sun, and Yusheng He
Abstract—A simple multimode resonator loaded by open- and
short-end stubs is proposed in this paper. Its transmission char-
acteristics and equivalent circuit models are investigated by
even–odd mode analysis. In addition, the multiband filter design
theory using the proposed resonator is explained, and the desired
passbands can be conveniently allocated by properly choosing the
dimension parameters of the stub-loaded resonator. For demon-
stration purposes, a compact high-temperature superconducting
(HTS) filter operating at 1.57, 2.6, and 3.50 GHz is designed
and fabricated. To sharpen the passband skirts of the filter, a
source–load coupling configuration with interdigital structure is
arranged to produce several transmission zeros. Then, multipath
coupling diagrams are constructed, and the relative phase shifts of
each path are studied to explain the responses of the triple-band
filter. Furthermore, a grounding technique of the short-end stub
is realized by subtly connecting with the metal wall packaging
instead of via holes in the substrate. In consideration of the
inaccuracy in HTS fabricating and packaging, an optimization
scheme is proposed. The results of the sensitivity analysis revealed
that the optimization scheme is viable. Finally, measurements are
in good agreement with the electromagnetic simulations and verify
the circuit design methodology.
Index Terms—Even–odd mode, grounding technique, high-
temperature superconducting (HTS), multimode resonator,
multiple-band filter.
I. INTRODUCTION
T
HE RAPID development of multiband wireless com-
munication systems has been gaining much interest in
recent years, and microwave filters, antennas, couplers, and
other devices also are developed in the direction of miniature
and multiband design. For example, multiband filters utilizing
multifrequency planar resonators offer the possibility of reduc-
ing circuit sizes without significant attenuation performance.
Recently, microwave bandpass filters (BPFs) with a triband
Manuscript received October 2, 2013; revised December 21, 2013; accepted
January 17, 2014. Date of publication January 21, 2014; date of current version
February 14, 2014. This work was supported in part by the National Science
Foundation of China under Grant 61061001 and Grant 61161005 and in part
by the 555 Talent Program of Jiangxi Province of China. This paper was
recommended by Associate Editor J. E. Mazierska.
H. Liu, J. Lei, and X. Guan are with East China Jiaotong University,
Nanchang 330013, China (e-mail: liuhaiwen@gmail.com; xuehuiguan@yahoo.
com.cn).
Y. Zhao was with East China Jiaotong University, Nanchang 330013, China.
He is now with FORCONN, Shenzhen 518000, China.
L. Sun and Y. He are with the Beijing National Laboratory for Condensed
Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing
100190, China (e-mail: yshe@aphy.iphy.ac).
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.2014.2301753
characteristic have been designed through various approaches.
In [1], several composite split-ring resonators are used to design
a triple-band filter with flexible passbands. In [2], a square ring
loaded resonator has been proven in the compact design of
a triple-band filter. In addition, square ring short stub-loaded
resonators have been introduced for the design of a triband filter
in [3]. The passband frequencies can be conveniently tuned to
the desired values by controlling the corresponding resonator
dimensions. Moreover, multiband BPFs can be constructed
using stepped-impedance resonators (SIRs), such as the ones in
[4] and [5]. However, the dependence on the resonant frequen-
cies of the SIR complicates the filter design. Thus, more flexible
design in the choice of band allocation and high selectivity is
needed.
On the other hand, adopting a single multimode resonator
to design a compact multiband BPF becomes an interesting
research for its small size, simple structure, and excellent
multimode performances [6]. In [7], three pairs of degenerate
modes in a ring resonator are excited by four open-circuited
stubs, whereas the other method of exciting multimode charac-
teristics uses a short-end stub [3]. Those stubs are easily applied
to independently tune the center frequency of the passband
and produce transmission zeros for high selectivity. Thus, this
simple method can reduce the filter design difficulty and obtain
excellent transmission results.
Nowadays, high-temperature superconducting (HTS) thin
films with extremely low microwave surface resistance are able
to produce high-performance microwave devices with both low
insertion loss and sharp rejection. In [8], spiral resonators are
used to design contiguous diplexer. In [9], an ultrawideband
BPF was constructed using SIRs. In [10], a dual-band super-
conducting BPF was designed by using embedded split ring
resonators. In addition, a ten-pole HTS filter with triple-band
performance was designed by using ten dual spiral resonators
[11]. To achieve a good triple-band performance, an optimiza-
tion algorithm is required. Nevertheless, only few attempts have
so far been proposed for triband filter design and wireless
applications using HTS technology.
In this paper, a compact multiple-band BPF is proposed by
using a multimode stub-loaded resonator. The characteristics
of the multimode resonator are explained by even–odd mode
analysis in Section II. In Section III, a triband HTS filter
operating at 1.57, 2.6, and 3.50 GHz is designed using the
proposed multimode stub-loaded resonator. Its three desired
passbands can be conveniently allocated by properly choosing
the dimension parameters of the resonator. In addition, several
extra transmission zeros are realized by using a source–load
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