1. Compact dual-band BPF with wide stopband
using stub-loaded spiral stepped-impedance
resonator
V. Singh✉
, V.K. Killamsetty and B. Mukherjee
A compact dual-band bandpass filter (BPF) with wide stopband per-
formance using stub-loaded spiral short-circuit λ/4 stepped-impedance
resonator is proposed. Spiral configuration has been used for compact-
ness of filter. Both passbands can be controlled individually by chan-
ging the geometric parameters of resonator. Multiple transmission
zeros provide high selectivity to both passband and extend stopband
up to 3 GHz. The filter has compact size of 0.06λg × 0.09λg. A dual-
band BPF has been designed and fabricated for terrestrial trunked
radio (TETRA) band and global system for mobile communication
applications.
Introduction: Owing to increasing demand for dual-band operation in
wireless communication, dual-band filters having compact size, good
isolation between passbands, high selectivity, and wide stopband are
required. Several methods are investigated to design dual-band bandpass
filter (BPF) [1–6]. In [1], series and parallel open stubs are used as reso-
nators to design dual-band BPF. The filter has good selectivity, but the
stopband rejection needs improvement and also has large size. In [2], a
split ring λ/4 resonator and a stepped-impedance resonator (SIR) were
used to design dual-band BPF. However, the selectivity and stopband
have to be improved. Novel stub-loaded (SL) theory [3] was used to
design balanced dual-band BPF with independently controlled passband
frequencies and bandwidths. Still selectivity of the second passband
needs improvement and also size has to be miniaturised. In [4], dual-
band performance was achieved without increasing the overall circuit
size. Here, first passband generated by ring resonator and second pass-
band is introduced because of tightly coupled input and output struc-
tures. In [5], open-/short-circuited SL resonators were used to build
dual-band BPFs. Here, selectivity of passbands needs improvement
and design should be compact.
In this Letter, two SL spiral short-circuit λ/4 SIR (SLS-SIR) are used
to design a dual-band BPF at central frequencies of f1 = 0.350 GHz and
f2 = 0.900 GHz. Spiral configuration helps for the miniaturisation of
filter. SIR is used for pushing the harmonics away up to 8.57f1
(3.33f2). Two passbands are generated and controlled individually.
Eight transmission zeros (TZs) offer high selectivity and wide stopband.
Filter design: Fig. 1 shows the configuration of the proposed dual-band
BPF. The filter consists of two SL short-circuit quarter wavelength SIRs
(SL-SIRs), and for the miniaturisation of filter spiral configuration of the
proposed resonator is used.
L2
G1
G4
G6
L8
L7
G8
K2
K1
K5
W2
W1
W0
L4
L6
G2
G3
G7
G5
K4
K3
L5
L3
port 1 port 2
D
L1
Fig. 1 Configuration of proposed dual-band BPF
From [2], a short-circuit λ/4 SIR is designed at the centred frequency
of f1 (0.350 GHz) and then for dual-band operation of filter, a stub is
loaded on an SIR as shown in Fig. 2. The input admittance for the pro-
posed resonator can be calculated as
Yin =
1
Z2
Z1 K1 − tan u1 tan u2( ) + jZL K1 tan u1 + tan u2( )
ZL 1 − K1 tan u1 tan u2( ) + jZ1 tan u1 + K1 tan u2( )
(1)
where
ZL =
jK2 tan u3 cot u4
K2 cot u4 − tan u3
(2)
and K1 = Z2/Z1 and K2 = Z4/Z1 are impedance ratios. Resonant frequency
of the proposed resonator can be calculated by setting Yin = 0. Therefore,
resonant condition are given by (3a) and (3b)
Z1 K1 − tan u1 tan u2( )(K2 cot u4 − tan u3) − (K2 tan u3 cot u4)
× K1 tan u1 + tan u2( ) = 0 (3a)
tan u3 tan u4 = K2 (3b)
Equations (3a) and (3b) show that the proposed resonator gives two
resonating frequencies. Therefore, the proposed resonator is a dual-
mode resonator and each mode decides one passband of dual-band
BPF separately.
ground
loaded
stub
Z1,q3 Z1,q1
Z2,q2 Yin
Z4,
q4
Fig. 2 Schematic of proposed SL-SIR
Fig. 3 shows simulated results of single-band filters operating at
different centre frequencies and dual-band BPF. Passband-1 is created
when both resonators without loaded stubs are resonating at 0.35 GHz
and passband-2 is created when loaded stub is behaving as λ/4 SIR at
f2 (0.90 GHz) with shared path as shown in Fig. 4. The proposed dual-
band BPF’ characteristics is the combination of each passband.
S21
–20
0
–40
–60
magnitude,dB
–80
–100
–120
0.5 1.0 1.5
0.90 GHz
0.350 GHz 0.35/0.90 GHz
frequency, GHz
2.0 2.5 3.0
0.90 GHz
0.35/0.90 GHz
0.350 GHz
Fig. 3 Simulated frequency response of filters
l/4 at f1
l/4 at f2
shared path loaded stub
port 2port 1
Fig. 4 Schematic of dual-band BPF using proposed resonator
TZs (TZ1, TZ3, and TZ6) are generated due to mixed coupling [6],
TZs (TZ2, TZ4, and TZ7) are generated because the lengths between
the tapped points and open-ends of the input/output resonators behave
as open-circuit λ/4 resonator at these frequencies and TZs (TZ5 and
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2. TZ8) are generated due to source-coupling [7] and Fig. 5 shows that TZ5
and TZ8 are varying with gap (G7).
0.4 0.8
TZ2
TZ4
TZ1
TZ6
3
2
1
frequency,GHz
0
TZ5
TZ3
TZ7
TZ8
G7, mm
1.2
Fig. 5 Variation of TZs with gap (G7)
Fig. 6 shows that both passbands of dual-band BPF can be tuned indi-
vidually by changing geometric parameters of resonator. The filter is
optimised and simulated using Computer Simulation Technology soft-
ware. The prototype is designed and fabricated on Rogers RO3010
dielectric sheet (having ɛr = 10.2, tan δ = 0.0022, height of substrate =
1.28 mm, and thickness of metal = 0.017 mm). Filter having compact
size of 20.8 × 31 mm2
(0.06λg × 0.09λg), where λg is the guided wave-
length at centre frequency of passband-1 (0.350 GHz) and the optimised
dimensions of the filter are given as: L1 = 9.0, L2 = 10.3, L3 = 22, L4 =
1.8, L5 = 3.9, L6 = 7.9, L7 = 12, L8 = 8.8, K1 = 12.2, K2 = 0.38, K3 =
7.25, K4 = 2.2, K5 = 4.4, G1 = 1.3, G2 = 0.8, G3 = 3.1, G4 = 0.5, G5 =
1.85, G6 = G8 = 0.2, G7 = 1.14, W0 = W1 = 1.2, and W2 = D = 1 (unit:
millimetres).
0
–20
–40
–60
|S21|,dB
–80
0.3 0.6 0.9
frequency, GHz
L1 = 6 mm
L1 = 9 mm
L1 = 12 mm
L7 = 11 mm
L7 = 12 mm
L7 = 13 mm
1.2
0
–20
–40
–60
|S21|,dB
–80
0.3 0.6
frequency, GHz
a b
0.9 1.2
Fig. 6 Passbands of dual-band BPF
a Independently tuning of passband-1
b Independently tuning of passband-2
0
–30
–60
TZ1
TZ2
TZ4
TZ3
TZ5
TZ7
TZ8
TZ6
S11
S21
magnitude,dB
–90
1.00.5 1.5
simulated results
measured results
frequency, GHz
2.0 2.5 3.0
Fig. 7 Simulated and measured results of proposed filter
Results and discussion: Measurement of fabricated design is done by
using Agilent E5071C Vector Network Analyser. Fig. 7 shows simu-
lated and measured results of the filter. Filter operates at 0.35/
0.90 GHz, with fractional bandwidth of 10.57%/13.67%, respectively.
Passband-1/passband-2 having a measured insertion loss of 1.6 dB/
1.4 dB and measured return loss is better than 14 dB/17 dB. Eight
TZs are generated at 0.284, 0.471, 0.657, 1.04, 1.37, 1.68, 1.91, and
2.90 GHz. These TZs provide high selectivity, good isolation between
passbands and improve the stopband performance up to 2.3 GHz with
>19 dB rejection level and from 2.3 to 3 GHz with >12 dB rejection
level. A comparison of the proposed structure with other references is
as tabulated in Table 1 and it shows that this work has compact size
and both passbands have good selectivity.
Table 1: Comparisons of proposed design with previous designs
Ref.
f0, GHz/3 dB FBW
(%)
Insertion
loss, dB
Size
(λg × λg)
Selectivity of
bands
[2] 2.49/3.85, 3.85/4.4 1.62/1.31 0.18 × 0.18 Yes/No
[3] 1.8/8.64, 5.8/5.35 1.2/2 0.37 × 0.28 Yes/No
[4] 2.4/9.2, 5.2/9.5 1.4/2.7 0.18 × 0.18 Yes/Yes
[5] 2.4/4.63, 5.8/3.6 1.35/1.97 0.39 × 0.25 No/Yes
This work 0.35/10.57, 0.90/13.67 1.6/1.4 0.06 × 0.09 Yes/Yes
Conclusion: A compact dual-band BPF with wide stopband is pre-
sented using SLS-SIR for TETRA band and global system for mobile
communication applications. The filter has compact size of 0.06λg ×
0.09λg due to spiral configuration. The filter has good selectivity and
wide stopband due to generation of eight TZs generated. It has been ver-
ified by simulated and measured results. The proposed BPF has signifi-
cant improvement in selectivity of both passband and size reduction in
comparison with the references presented in Table 1.
© The Institution of Engineering and Technology 2016
Submitted: 23 August 2016
doi: 10.1049/el.2016.2838
One or more of the Figures in this Letter are available in colour online.
V. Singh, V.K. Killamsetty and B. Mukherjee (Department of
Electronics and Communication Engineering, PDPM Indian Institute
of Information Technology, Design and Manufacturing, Jabalpur,
Madhya Pradesh, India)
✉ E-mail: vivekchauhan10@gmail.com
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