selectivity tunable dual-band

1. 736 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 11, NOVEMBER 2014
A High-Selectivity Tunable Dual-Band
Bandpass Filter Using Stub-Loaded
Stepped-Impedance Resonators
Bin You, Long Chen, Yaping Liang, and Xuan Wen
Abstract—A novel varactor tunable dual-band bandpass filter
(BPF) using stub-loaded stepped-impedance resonators is pro-
posed in this letter. Compared with the traditional tunable
filters, the source-load coupling and T-shape stub-loaded lines
are employed in this design. The proposed BPF architecture has
the advantages of high selectivity and less control voltages. In
the overall tuning range, the proposed filter is designed with
5–6 transmission zeros and more than 30 dB rejection between
the two passbands. Meanwhile, only one control voltage is needed
for each passband. A prototype of this filter is fabricated and
measured. The measurement results show great agreement with
simulated results, which show that the first passband can be
tuned in a frequency range from 0.8 to 1.02 GHz, and the second
passband varies from 2.02 to 2.48 GHz.
Index Terms—Dual-band, high selectivity, stub-loaded res-
onators, tunable bandpass filter (BPF).
I. INTRODUCTION
ELECTRONICALLY tunable/reconfigurable filters are
one of the most essential microwave components for
multi-band communication systems due to their potential for
size and complexity reduction. Recently, in exploring the
advanced multi-band wireless systems, tunable bandpass filters
(BPF) for RF devices have become quite popular [1]–[3].
However, there still exist many unsolved problems for these
reported tunable BPFs. The tunable BPFs in the past publica-
tions usually do not have enough signal selectivity for modern
communication systems. Meanwhile, dual-band tunable filters
require multiple dc bias voltages [4]–[6], which increase the
complexity of the circuits and the difficulty of tuning. Based on
the above issues, a high-selectivity tunable dual-band bandpass
BPF using stub-loaded stepped-impedance resonators (SL-SIR)
is firstly proposed, as shown in Fig. 1. The proposed filter struc-
ture offers independent dual-passband characteristics with
improved stopband characteristics and less control voltages.
Manuscript received June 04, 2014; accepted July 31, 2014. Date of publica-
tion August 29, 2014; date of current version November 04, 2014. This work
was supported by Zhejiang Province Natural Science Foundation of China under
grant LY14F010020 and the Oversea Returnee Research Project Fund.
B. You, L. Chen, and Y. Liang are with the Circuits and Systems Key Lab-
oratory of the Ministry of Education, Hangzhou Dianzi University, Hangzhou,
Zhejiang, China.
X. Wen is with the Nokia Siemens Networks Technol. (Beijing) Co. Ltd.,
Hangzhou Zhejiang, China.
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2014.2348322
Fig. 1. Structure of the proposed tunable dual-band BPF.
II. DESIGN OF A MULTI-TRANSMISSION ZEROS SL-SIR FILTER
The passive microstrip structure of the proposed tunable filter
can be regard as a fixed multi-transmission zeros (TZ) SL-SIR
filter. SL-SIRs have been used to design the multi-band BPFs or
the tunable BPFs for their advantages of small size and multi-
mode characteristics [6], [7]. In this letter, we proposed a novel
improved SL-SIR and its application in high-selectivity tun-
able dual-band BPF design. As shown in Fig. 2 compared with
the traditional tri-mode SL-SIR, the source-load coupling and
T-shape stub-loaded lines are employed in this design. The tri-
mode SL-SIRs with one open stub and one short stub have an in-
herent TZ in the upper stopband, while the source-load coupling
are adopted to generate two more TZs, therefore, total six TZs
are realized in this dual-band BPF design to improve the selec-
tivity. The strength of the source-load coupling can be used to
tuned the frequency of TZs. The T-shape stub-loaded lines are
utilized to reduce the number of control voltages which will be
introduced in the next section. Similar to the stub-loaded res-
onators in [8], odd- and even-mode analysis can be used for
characterizing the resonator. The proposed SL-SIR has three
resonant modes: , , and . can be totally
controlled by , , and . When is fixed,
and are mainly controlled by the short stub and the
open stub, respectively. In practice, the short stub realized by
a through hole in this filter. The three resonant modes have to
satisfy the size condition as . The ad-
mittance ratios, which can be expressed as , are used in all
three resonant modes to reduce the resonator’s size and widen
the upper stopband bandwidth. The parasitic passband would be
moved more than three times away from the original center fre-
quency by designing [9].
To prove the above characteristics, a 1.05 GHz/2.7 GHz
high-selectivity dual-band BPFs with absolute-bandwidths of
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2. YOU et al.: HIGH-SELECTIVITY TUNABLE DUAL-BAND BPF USING SL SIRS 737
Fig. 2. Microstrip circuit models (without lumped elements).
TABLE I
THE DIMENSIONS OF THE PROPOSED FILTER (UNIT: mm)
Fig. 3. Simulated and measured results of the multi-TZs SL-SIR filter.
170 MHz/240 MHz is designed and fabricated. The substrate
we used in this work is the Rogers RT/Duriod 6010 with
, and . The
dimensions are listed in Table I. The S-parameter simulation
and measurement results are shown in Fig. 3. The first passband
with the center frequency of 1.05 GHz has less than 0.9 dB
insertion loss and greater than 20 dB return loss. The second
passband with the center frequency of 2.7 GHz has less than
1.1 dB insertion loss and greater than 18 dB return loss. In ad-
dition, the six TZs provide a better cutoff rate in the stopband.
III. DESIGN OF A TUNABLE DUAL-BAND FILTER
For this tunable filter design, total eight identical varactors
are loaded onto the open-ends, as shown in Fig. 1. The tradi-
tional stub-loaded tunable filters usually need two types of con-
trol voltages for each passband [4]–[6]. In this tunable filter de-
sign, the T-shape open stubs are utilized, which makes the pro-
posed filter only need one control voltage for each passband. To
simplify the analysis, is assumed and filter B, as shown
in Fig. 1, is chosen as an example. , are the total capac-
itance for series connection of dc block and , , respec-
tively. For the odd- and even-mode resonant condition, and
can be expressed as
(1)
Fig. 4. Calculation results of against B and R.
Fig. 5. Simulated results of SL-SIR against C. (a) With T-shape open stub.
(b) Traditional design.
(2)
where is the phase velocity, and
. If we remove all varactors, the proposed
tunable filter become a fixed filter which mentioned above. We
assume the fixed center frequency is and the fractional band-
width (FBW) is B. When we add the varators, we define
, which is associated with the tunable elements. Then
, , , can be expressed as , ,
, , respectively. When the center frequency
is tuned, the ratio of to can be derived as
(3)
According to (3), the ratio of to is only influ-
enced by B and R. In Fig. 4, based on (3), the results shows
in narrow-band filters. It proves that the pro-
posed tunable filter only need one control voltage for each
passband if the FBW is narrow enough. Simulated results in
Fig. 5 show that the proposed SL-SIR is tuned well with only
one control voltage, while the traditional one is not.
A high-selectivity tunable dual-band BPF using the SL-SIRs
is fabricated and measured. The passive microstrip structure
is as same as the one mentioned above. The variable capaci-
tances are implemented by the JDV2S71E varactors. As shown

3. 738 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 11, NOVEMBER 2014
Fig. 6. Simulated and measured results of the proposed tunable dual-band filter.
Lower passband frequency is unchanged. , , 7 V, 20 V.
Fig. 7. Simulated, measured results and the photograph of the proposed tunable
dual-band filter. Higher passband frequency is unchanged. , 4.4 V, 20 V,
.
in Fig. 1, in filter A, the RF chokes are 22 nH. In filter B, the
RF chokes are 100 nH. All dc blocks are 1 pF. Fig. 6 and Fig. 7
plot the simulated and measured results of the tunable dual-band
BPF. In Fig. 6, it can be seen the second band frequency varying
from 2.02 to 2.48 GHz while the first band frequency is un-
changed and fixed at 1.02 GHz. In Fig. 7, it can be seen the
first band frequency varying from 0.80 to 1.02 GHz while the
second band frequency is unchanged and fixed at 2.48 GHz. In
TABLE II
COMPARISON WITH PRIOR DUAL-BAND TUNABLE BPFS(FT DENOTES
FREQUENCY TUNING RANGE, CV DENOTES TYPES OF CONTROL VOLTAGES,
A DENOTES INDEPENDENTLY TUNABLE PASSBAND
the overall tuning range, the measured passband return losses
are all better than 15 dB and the passband insertion losses are
1.12–2.93 dB and 1.45–4.89 dB, respectively. Moreover, the
measured results show that the maximum passband center fre-
quency ratio is about 3.1. We have not seen a larger center fre-
quency ratio in the previous publications. It shows a large fre-
quency shift of the harmonic bands of the first passband is ob-
tained. The performance comparisons of the proposed tunable
BPF with other tunable BPFs are summarized in Table II. Com-
pared to previously filter designs, this filter design can provide
independently tunable dual-band passbands characteristics with
high selectivity and less control voltages.
IV. CONCLUSION
This letter presented a novel varactor tunable dual-band BPF
using SL-SIRs. The source-load coupling are employed to im-
prove the selectivity. In the overall tuning range, the proposed
filter is designed with 5–6 transmission zeros and more than
30 dB rejection between two passbands. This letter presents the
brief analysis of a novel T-shape open stub, which can reduce
the types of control voltages in a narrow band tunable filter de-
sign. Both the simulated and measured results indicate that the
proposed tunable dual-band bandpass filter has the advantages
of high selectivity and less control voltages.
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