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Miniaturised microstrip lowpass filter
with sharp roll-off and ultra-wide stopband
Gh. Karimi, F. Khamin-Hamedani and H. Siahkamari
A new semi-fractal technique is applied to hairpin microstrip lowpass
filters to realise compact size and sharp roll-off. To achieve wide stopband suppression, both a semi-fractal hairpin resonator and a U-shaped
resonator are employed in the filter. A demonstration filter with 3 dB
cutoff frequency at 1.51 GHz has been designed, fabricated and
measured. The results show that a roll-off of 217.6 dB/GHz with a relative stopband bandwidth of 159.7% (referred to a suppression degree
of 32 dB) can be obtained while achieving a high figure-of-merit of
44 832.
Introduction: Miniaturised lowpass filters (LPFs) with sharp transition
and wide stopband are highly desirable in wireless communications
systems to suppress harmonic and spurious signals. However, conventional LPFs using shunt stubs or high–low impedance transmission
lines can only provide a gradual transition and a narrower stopband
[1]. Therefore, various methods have been used to design a lowpass
filter with good characteristics. In [2], a microstrip lowpass filter
based on cascading multi-resonators was introduced. This work indicated that sharp roll-off and wide stopband were achieved whereas the
size was relatively large [2]. To achieve a compact design, both radial
shape patches and a meandered main transmission line are adopted in
the structure of the lowpass filter [3]. In [4], a simple compact structure
by using a hairpin resonator with a pair of coupled resonators was
reported. However, the disadvantage of these configurations [3, 4], is
that the skirt characteristics are not sharp enough. The LPFs using a
defected ground structure can provide a sharp and extended stopband
[5]; however, these structures lead to many drawbacks such as
complex configuration and fabrication difficulties. Another approach
to miniaturise the filter structure and to achieve other good properties
is the use of fractal structures. Also, the fractal geometries have a
common property, such as self-similarity [6]. In this Letter, a novel
compact microstrip lowpass filter using a semi-fractal hairpin line is
proposed.
32 dB stopband rejection from 1.66 to 14.77 GHz, with a sharp
roll-off of 217.6 dB/GHz. Furthermore, the size of the filter is only
18.52 × 18.88 mm2, which corresponds to an electrical size of
0.156λg × 0.159λg, where λg is the guided wavelength at 1.51 GHz.
The performance of the filter, for highlighting the advantage of this
design, is presented and compared with the other work.
Filter design: Fig. 2 shows the layout of the proposed lowpass filter.
This filter is composed of semi-fractal hairpin resonators and
U-shaped resonators, which can generate multiple transmission zeros
in the stopband region.
semi-fractal
hairpin
resonators
U-shaped
resonators
Fig. 2 Layout of proposed lowpass filter
Fig. 3 shows the layout and the simulated S-parameters of the studied
resonators.
l15
l2
l3
lf1
Wf1
g1
lf2
Wf2
l1
l14
l13
W12
W11
g2
l11
l12
W1
W13
a
Wu4
lu4
Wu1
lu1
Wf
l21
l22
l23
lf
Wu2
lu2
Wu3
lu3
b
a
–20
magnitude, dB
magnitude-S21, dB
0
non-fractal
semi-fractal
0
magnitude, dB
0
–20
–40
–60
–80
–20
–40
–60
–80
0
5
10
15
20
0
5
–40
c
|S11|sim.
10
15
20
frequency, GHz
. GHz
frequency,
d
|S21|sim.
TZ: transmission zero
–60
0
1
2
3
frequency, GHz
b
4
5
Fig. 1 Layout, with comparison between S21-parameter, of simple structure
of hairpin resonator and semi-fractal hairpin resonator
a Layout
b Comparison between S21-parameter
As shown in Fig. 1, by applying a semi-fractal technique to the
hairpin microstrip resonator, the sharpness of the proposed resonator
is enhanced. The dimensions of the semi-fractal hairpin resonator in
Fig. 1a are similar to the smaller semi-fractal hairpin resonator shown
in Fig. 3a. To achieve wide stopband suppression, both semi-fractal
hairpin resonators and U-shaped resonators are adopted in the design.
The measured results indicate that the designed filter has a better than
Fig. 3 Layout with dimensions and simulated S-parameters of designed
resonators
a,b Layout with dimensions of resonators
c S-parameters of semi-fractal hairpin resonators
d S-parameters of U-shaped resonators
The dimensions of the semi-fractal hairpin resonators in Fig. 3a are as
follows: l1 = 0.24 mm, l2 = 0.48 mm, l3 = 0.7 mm, l11 = 3.67 mm, l12 =
1.48 mm, l13 = 2.34 mm, l14 = 5.21 mm, l15 = 5.86 mm, lf1 = 0.7 mm,
lf2 = 0.9 mm, W1 = 0.3 mm, W11 = 2.95 mm, W12 = 6.19 mm, W13 =
9.14 mm, Wf1 = 0.45 mm, Wf2 = 0.35 mm, g1 = 0.3 mm and g2 =
0.24 mm. Also, the dimensions of the U-shaped resonators in Fig. 3b
are: l21 = 2.14 mm, l22 = 1.96 mm, l23 = 2.83 mm, lu1 = 1.12 mm, lu2 =
3.28 mm, lu3 = 4.78 mm, lu4 = 1.12 mm, Wu1 = 1.44 mm, Wu2 =
3.25 mm, Wu3 = 6.35 mm and Wu4 = 1.44 mm. The feed line is designed
for matching 50 Ω with lf = 3 mm and Wf = 1.19 mm. To validate the
ELECTRONICS LETTERS 10th October 2013 Vol. 49 No. 21 pp. 1343–1345
design and analysis, the proposed lowpass filter has been designed and
fabricated on a 20 mm-thick RO4003 substrate with a relative dielectric
constant of 3.38 and loss tangent of 0.0021. Fig. 3c shows the frequency
response of the semi-fractal hairpin resonators, and it can be seen that
five transmission zeros are located at 3.8, 5.5, 8, 8.9 and 13.6 GHz
with attenuation levels equal to −54, −64, −60, −61 and −53 dB,
respectively. As seen in this Figure, a very sharp cutoff characteristic
is achieved. As shown in Fig. 3d, the U-shaped resonator can create
two transmission zeros at about 5.8 and 11 GHz with attenuation
levels near −72 and −46 dB, respectively. By adding the U-shaped
resonator, the attenuation level in the stopband region is improved.
Fig. 4a is the photograph of the proposed lowpass filter. As seen from
Fig. 4b, enhanced stopband performance and sharp roll-off are finally
achieved.
the measured and simulated performances are in good agreement. The
fabricated filter has a 3dB cutoff frequency at about 1.51 GHz. Inside
the passband, the insertion loss is < 0.3 dB from DC to 1 GHz. The
proposed filter exhibits a wide stopband suppression higher than 32 dB
from 1.66 to 14.77 GHz. For comparison, Table 1 gives the performance
of some lowpass filters. As seen from the Table, the designed filter exhibits a high figure-of-merit (FOM) (44 832) among the quoted filters.
Conclusion: A novel and compact lowpass filter based on symmetrically loaded semi-fractal hairpin resonators and U-shaped resonators is
proposed. With this structure, a prototype filter with a cutoff frequency of 1.51 GHz has been designed, fabricated and measured.
The results indicated that the proposed filter has many desirable features, such as compact size, low passband insertion loss, wide stopband and a very high FOM of 44 832. With all these good
properties, the proposed lowpace filter is applicable for modern communications systems.
© The Institution of Engineering and Technology 2013
12 July 2013
doi: 10.1049/el.2013.2321
One or more of the Figures in this Letter are available in colour online.
a
Gh. Karimi and H. Siahkamari (Electrical Engineering Department,
Razi University, Kermanshah 67149, Iran)
magnitude-S21, dB
0
|S11|meas.
|S11|sim.
–20
|S21|meas.
|S21|sim.
E-mail: [email protected]
–32 dB
F. Khamin-Hamedani (Department of Electrical Engineering,
Kermanshah Science and Research Branch, Islamic Azad University,
Kermanshah, Iran)
–40
–60
References
–80
0
5
10
frequency, GHz
15
20
b
Fig. 4 Fabricated filter, and simulated and measurement results
a Photograph of fabricated lowpass filter
b Simulated and measurement results of proposed filter
Table 1: Performance comparisons among published filters and
proposed one
Suppression
factor (SF)c
Normalised
circuit size
(NCS)d
Architecture
factor (AF)e
FOMf
92.5
36.3
Relative
stopband
bandwidth
(RSB)b
1.355
1.323
3
1.5
0.351 × 0.106
0.079 × 0.079
1
1
10 106
11 543
30.8
130
217.6
1.636
0.933
1.597
1
2
3.2
0.037 × 0.093
0.227 × 0.089
0.156 × 0.159
1
2
1
14 644
6004
44 832
Refs.
Roll-off
rate ζ a
[2]
[3]
[4]
[5]
This
work
1 Hong, J.S., and Lancaster, M.J.: ‘Microstrip filters for RF/microwave
applications’ (Wiley, New York, 2001)
2 Li, J.-L., Qu, S.-W., and Xue, Q.: ‘Compact microstrip lowpass filter with
sharp roll-off and wide stopband’, Electron. Lett., 2009, 45, (2), pp. 110–111
3 Wang, J., Xu, L.-J., Zhao, S., Gua, Y.-X., and Wu, W.: ‘Compact
quasi-elliptic microstrip lowpass filter with wide stopband’, Electron.
Lett., 2010, 46, (20), pp. 1384–1385
4 Yang, M.H., and Xu, J.: ‘Design of compact, broad-stopband lowpass
filter using modified stepped impedance hairpin resonators’, Electron.
Lett., 2008, 44, (20), pp. 1198–1200
5 Mandal, M.K., Mondal, P., Sanyal, S., and Chakrabarty, A.: ‘Low
insertion-loss, sharp-rejection and compact microstrip lowpass filter’,
IEEE Microw. Wirel. Compon. Lett., 2006, 16, (11), pp. 600–602
6 Eccleston, K.W.: ‘Shunt-loaded fractal meandered microstrip’. IEEE MTT-S
Int. Microwave Workshop series on Art of Miniaturizing RF and Microwave
Passive Components, Chengdu, China, December 2008, pp. 67–70
a
Roll-off rate is defined as: ξ = (αmax − αmin)/( fs − fc)(dB/GHz), according to 3 and
40 dB attenuation points
b
RSB is calculated by: RSB = (stopband bandwidth/stopband centre frequency)
c
SF is based on stopband suppression
d
NCS can be derived as follows: NCS = physical size (length × width)/l2g , where
λg is the guided wavelength at 3 dB cutoff frequency
e
AF can be recognised as the circuit complexity factor
f
Finally, the FOM is the overall index of a proposed filter and is given by: FOM =
(RSB × ξ × SF)/(AF × NCS)
Simulation and measurement results: The design is simulated by the
full-wave electromagnetic (EM)-simulator (ADS). Fig. 4b shows that
ELECTRONICS LETTERS 10th October 2013 Vol. 49 No. 21 pp. 1343–1345