616IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 11, NOVEMBER 2010
Dual-Band Bandpass Filter With Controllable
Bandwidths Using Two Coupling Paths
Xiu Yin Zhang, Member, IEEE, Chi Hou Chan, Fellow, IEEE, Quan Xue, Senior Member, IEEE, and
Bin-Jie Hu, Member, IEEE
filters using two coupling paths to control the bandwidth of each
passband. One coupling path only delivers signals at the upper
passband frequency whereas the other one couple signals at both
passbands. Utilizing this method, the two bandwidths can be ad-
justed. The filters make use of stub-loaded resonators and it is con-
venient to tune the passband frequencies. Therefore, both the fre-
quency and bandwidth of each passband can be easily controlled.
For demonstration purpose, two example filters are implemented
with different bandwidths. The experimental results are presented
to verify the proposed method.
Index Terms—Bandpass filter (BPF), bandwidth control, cou-
pling path, dual band.
work was conducted, and various design approaches were
proposed. Among them, two kinds of methods are popular. The
first category is to combine two sets of resonators with common
input and output ports , . The passband frequencies and
bandwidths can be independently controlled by using proper
filter configurations. However,thestructures are relativelycom-
plex. The second category is to use dual-band resonators such
as stepped-impedance resonators (SIRs) to generate two pass-
bands –. Although the passband frequencies can be tuned
to desirable values, it is difficult to control the bandwidths.
To solve this problem, dual-band coupling and feed structures
were proposed to fulfill the requirements of both passband
frequencies and bandwidths . Also, dual-band inverters were
utilized to obtain desirable frequency and bandwidth at each
passband , .
In this letter, a novel method for designing dual-band BPFs
with controllable bandwidths is proposed. The filters utilize
stub-loaded resonators  and the passband frequencies can be
easily tuned. Two coupling paths are utilized. One path delivers
UAL-BAND bandpass filters (BPFs) are heavily de-
manded in wireless systems. In the past, much research
Manuscript received April 06, 2010; revised June 25, 2010; accepted July
24, 2010. Date of publication September 07, 2010; date of current version
November 05, 2010. This work was supported by the Research Grants Council
of Hong Kong SAR, under grant CityU110808, and the NSFC under grant
X. Y. Zhang and B.-J. Hu are with the School of Electronic and Informa-
tion Engineering, South China University of Technology, Guangzhou, 510641,
China. (e-mail: firstname.lastname@example.org).
C. H. Chan and Q. Xue are with the State Key Laboratory of Millimeter
Waves, Department of Electronic Engineering, City University of Hong Kong,
Hong Kong, China.
Color versions of one or more of the figures in this letter are available online
Digital Object Identifier 10.1109/LMWC.2010.2066553
Fig. 1. (a) Filter configuration. (b) The stub-loaded resonator involved in the
only the signals at the upper passband frequency and the other
path is able to couple signals at both passbands. By properly
controlling the coupling strength at each path, the desirable
coupling coefficients at both passbands can be obtained, and
thus the bandwidths can be controlled. As a result, both the fre-
quency and bandwidth of each passband can be easily adjusted.
Based on the idea, two BPFs are implemented. The design
methodology and experimental results are presented.
II. FILTER DESIGN
A. Filter Topology and Mechanism
Fig. 1(a) shows the configuration of the proposed microstrip
BPFs. The filter utilizes two stub-loaded resonators  as illus-
trated in Fig. 1(b). The open stub is loaded at the center of the
stepped-impedance line. The resonator is symmetrical and thus
odd- and even-mode analysis can be used to characterize it, as
detailed in . The two open stubs are coupled to each other,
forming a coupling path denoted as Path 1. The other coupling
path, Path 2, is near the open ends of the lines. The input and
output are tapped at the resonators.
The first two resonant frequencies of the stub-loaded res-
onators are used as the lower and upper passband frequencies,
and . Following the analysis in , it is found that
is only determined by the stepped-impedance line and the
open stub only affects
. The ratio of ranges from 1 to 3.
1531-1309/$26.00 © 2010 IEEE
ZHANG et al.: DUAL-BAND BPF WITH CONTROLLABLE BANDWIDTHS USING TWO COUPLING PATHS617
Fig. 2. Coupling mechanism. (a) At ? . (b) At ? .
Fig. 3. Current distribution. (a) At ? . (b) At ? .
Fig. 4. Bandwidths versus coupling length ? .
The bandwidths of the two passbands are mainly affected by
the two coupling paths. The coupling structure is illustrated in
Fig. 2. As analyzed in , the voltage at the stepped-impedance
line center is zero at
. Hence, no signals at
ered to the open stub and the Path 1 does not affect the coupling
. This is indicated by the simulated current distri-
2 can couple signals at
, as illustrated in Fig. 3(b). Hence the
the coupling dimensions of the two paths and can be expressed
can be deliv-
at and are determined by
pling Path 1 does not affect
When the length
can be adjusted, whereas the lower passband re-
mains unaltered. Besides the coupling coefficients, the external
means the function of . It can be seen that the cou-
and this is indicated by Fig. 4.
of coupling Path 1 is tuned, the band-
s at the two passbands are determined by the tap position of
the ports and the line widths of various resonator sections .
within a certain range. Therefore, there are sufficient degrees of
freedom to control the bandwidths at the two passbands.
, also affects the bandwidths. In this design,
B. Design Methodology
To design the proposed filter, the first step is to obtain desir-
able passband frequencies. As stated above, the open stub will
. Therefore, we first tune
changing the dimensions of the stepped-impedance line. The
overall electrical length of the line is a half guided-wavelength
. After that, theopen-stublength is adjusted to obtaindesir-
without affecting. The overall electrical length of the
open stub plus half the stepped-impedance line is half guided-
The second step is to obtain the required bandwidths at
and, denoted asand . As stated previously, the cou-
pling Path 1 has no impact on the lower passband. Hence, we
first meet the requirement of
Path 2 and then obtain desirable
of Path 1. It is experimentally found that when
should try to utilize wider coupling gap
to obtain desirable
andare different and thus the same coupling region has
various impacts on
and . After the dimensions of Path 2
are determined, the dimensions of coupling Path 1 are tuned to
. As for thes we can tune the tap position and the line
widths of different resonator sections to control them. Finally, a
fine tuning is performed to fulfill the requirements of both pass-
to desirable values by
by changing the dimensions of
by altering the dimensions
and longer coupling
should be nar-; otherwise,
III. RESULTS AND DISCUSSIONS
Following the design methodology, two dual-band BPFs, de-
noted as Filters I and II, are implemented on a substrate with a
relative dielectric constant of 6.15 and thickness of 0.635 mm.
The simulation and measurement are accomplished using the
IE3D and 8753ES network analyzer, respectively.
FilterI is designed with a narrowerlower passband and wider
upper passband, i.e.,
sions are tabulated in Table I, and the overall size is
, whereis the guided wavelength at the lower pass-
and upper passbands are centered at 1.6 GHz and 2.45 GHz,
with the 3 dB fractional bandwidth (FBW) of 4.5% and 5.6%,
respectively. The insertion loss is measured at 1.46 dB and 1.16
dB, and the return loss within the two passbands is greater than
12 dB. Two transmission zeros are generated near the passband
edges, resulting in high selectivity. There are some slight dis-
crepancies between simulated and measured results, which are
primarily due to the fabrication tolerance.
Filter II is designed with the bandwidths:
. The dimensions are listed in Table I. Fig. 6 illus-
trates the filter responses. Similar to Filter I, this filter has the
two passbands centered at 1.6 and 2.45 GHz. The lower and
and. The dimen-
618IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 11, NOVEMBER 2010 Download full-text
Fig. 5. Simulated and measured responses of Filter I.
DIMENSIONS (IN MILLIMETERS) OF THE FILTERS
Fig. 6. Simulated and measured responses of Filter II.
upper passbands have the 3 dB FBW of 5.6% and 4.6%, respec-
tively. The measured insertion loss of both passbands is around
1.3 dB and the in-band return loss is greater than 12 dB.
For comparison, the bandwidths of the two filters are tabu-
lated in Table II. It is seen that various FBWs have been ob-
tained, indicating that the bandwidths can be controlled. The
terms of the flexibility of frequency and bandwidth adjustment
BANDWIDTH COMPARISON OF THE IMPLEMENTED FILTERS
as well as structure complexity. First, the proposed BPFs are
BPFs are more flexible in terms of frequency and bandwidth
adjustment. For the proposed BPFs, tuning the upper passband
will not affect the lower passband. However, this is not true for
the SIR filters. Secondly, the proposed BPFs are compared to
the dual-band BPFs using two sets of resonators , . The
proposed filters have simple structure and compact size but less
flexibility. This is because that the bandwidths and frequencies
of the two passbands can be independently controlled when two
sets of resonators are involved.
This letter has presented a novel method for designing dual-
band filters with controllable bandwidths. Two coupling paths
are adopted between the two resonators. One path is for con-
trolling the coupling strengths at both passbands, whereas the
other path is only for the upper passbands. Using this config-
uration, the bandwidths and frequencies of both passbands can
be tuned to desirable values. The design methodology has been
described, and two example filters have been implemented. The
results have been presented to verify the proposed method.
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