353
Broad-Band Band-Pass and Band-Stop Filters with Sharp Cut-off
Frequencies Based on Series CPW Stubs
A. Q. Liu1, A. B. Yu1 and Q. X. Zhang2
1
School of Electrical & Electronic Engineering Nanyang Technological University, Singapore, 639798
2
Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore, 117685
Abstract — Broad-band band-pass filters(BPFs) and bandstop filters (BSFs) constructed with coplanar waveguide’s series
open and short stubs are reported in this paper. The insertion
loss is less than 1.6 dB for the BPF, and it has 60% relative 3-dB
bandwidth and 77% 30-dB bandwidth. For the BSF, the 20 dB
stop band is 20.5 GHz, with return loss is only 0.3 dB. The lower
pass band insertion loss is less than 0.8 dB and higher pass band
insertion loss is less than 1.5 dB.
Index Terms — Bandpass filter (BPF), bandstop filter (BSF),
CPW, insertion loss, band rejection.
I. INTRODUCTION
Broad-band bandpass filters (BPF) with sharp rejection are
the fundamental building blocks for modern wideband
wireless communication systems. Alternatively, a broad-band
BPF can be constructed in coplanar stripline (CPS) [1],
microstrip structure [2] and coplanar waveguide (CPW)
transmission lines [3]-[5] etc. The most important issue in
designing a broad-band BPF is to implement strong coupling
structure. In [3], Nguyen suggested the broadside-coupled
CPW to achieve wide-bandwidth, which will destroy the uniplanar feature of CPW transmission line. A ribbon-of-brickwall CPW BPF is presented in [5], however, the lower
frequency side response is not sharp enough.
Besides the BPF, broad-band band-stop filter (BSF) is also
highly desirable for modern communication systems. In [6], a
broad-band BSF is developed using double-plane
superposition, the obtained frequency response has big ripples
in the higher pass-band. In [7], a photonic bandgap (PBG)
structure constructed in CPW transmission line is used for
broad-band BSF, but it occupies larger area because its PBG
structure is implemented in the ground planes of the CPW
transmission line,
The objective of this paper is to investigate in detail the
application of series open and short stubs in broad-band BPF
and BSF construction. In section II, unit-cell BPF and BSF
consist of two series open and short stubs are discussed. In
section III, broad-band BPF and BSF with sharp cut-off
frequencies are presented. Each of them consists of three unitcell. Section IV describes the fabrication process. The
measurement results for the BPF and BSF are presented in
Section V. Finally, some brief conclusions are drawn in
section VI.
0-7803-9542-5/06/$20.00 ©2006 IEEE
II. UNIT-CELL BANDPASS AND BANDSTOP FILTER
A. Unit-Cell Bandpass Filter
Figure 1 shows a schematic diaphragm of a unit-cell
bandpass filter, which consists of two series open-stub
connected reversely. The length of the series open-stub is
close to a quarter wavelength of the center frequency, i.e., l =
λeff/4. The feeding and output CPW has characteristic
impedance of Z0 = 50 Ω with G/S/G = 30/250/30 µm, the
substrate used is glass with thickness of 500 µm and dielectric
constant of 4.6. In [8], series open-end and short-end stubs
within the center conductor are discussed. This geometry
leads to greater field confinement and thus lower radiation
loss than that of straight gap-coupled resonators or stubs with
lateral extension. Because of this advantage, these stubs are
used to construct broad-band bandpass filter at 250 GHz
frequency [9], but no detail discussion of design parameters.
Figure 2 is the simulation result for this unit-cell BPF by
using the commercial EM simulator [10]. It is seen that the
center frequency is at 22 GHz with 3-dB bandwidth about
60%.
From the simulation results, lumped element equivalent
circuit is derived to model this unit-cell BPF, as given in Fig.
3. Each open-stub is represented by three series LC tanks
instead of only one as that in [8]. Therefore, this model can
predict the unit-cell BPF over the relatively wider frequency
range from DC to 40 GHz, as shown in Fig. 2, in which the
circuit simulation results and the EM simulation results show
good agreement. The capacitances and inductances are
evaluated from the resonant frequency of the open-stub [7]
and listed in Table I.
g
ls
ws
d
wc
l
s
Fig. 1 Schematic diaphragm of a unit-cell BPF (not to scale)
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0
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0
10
g
ls′
ws
20
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40
30
Fig. 2 Simulated frequency response of the unit-cell (g = 50
µm; ls = 1900 µm; l = 1950 µm; d = 50 µm; ws = 50 µm; wc =
50 µm; s = 50 µm)
C3
L3
L2
Cp1
C0
L4
Cp2
C2
C3
L4
L3
Cp2
Fig. 4 Schematic diaphragm of a unit-cell BSF (not to scale)
10
0
C1
L2
L0 (nH)
0.6
0.05
L1 (nH)
0.05
0.63
L2 (nH)
0.1
0.042
L3 (nH)
0.09
0.08
L4 (nH)
1.04
0.17
C0 (fF)
47
-
C1 (fF)
296
85
C2 (fF)
385
60
C3 (fF)
160
80
Cp1 (fF)
28
4
Cp2 (fF)
66
150
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TABLE I SUMMARY OF TYPOGRAPHICAL SETTINGS
BSF
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Cp1
Fig. 3 Equivalent circuit of the unit-cell BPF
BPF
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L1 L0
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L0 L1
C2
s
l′
Frequency (GHz)
C1
d
wc
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Circuit simulation
EM simulation
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S11 (dB)
modeled by cascading three parallel LC tanks and all the
inductances and capacitances are deduced from the EM
simulation results and are also listed in Table I. Again the
response predicted by the circuit and the EM simulation
results can match well.
S21 (dB)
0
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Circuit simulation
EM simulation
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10
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20
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40
30
Frequency (GHz)
Fig. 5 Simulation frequency response of the unit-cell BSF (s =
50 µm; g = 50 µm; ls′ = 1900 µm; l′ = 1950 µm; d = 50 µm; ws
= 50 µm; wc = 50 µm)
L0
C3
C2
C1
L1
L2
L3
Cp1
Cp2
C3
L4
L3
C2
L2
Cp2
C1
L1
Cp1
Fig. 6 Equivalent circuit of the unit-cell BSF
III. BPF AND BSF WITH SHARP CUT-OFF
FREQUENCIES
B. Unit-Cell Bandstop Filter
Figure 4 shows a schematic diaphragm of a unit-cell BSF, it
is made up of two series short-end stub connected reversely.
Similar structures are reported in [8],[11], but they are mainly
used for low-pass filter. Fig. 4 gives the simulation result of
this unit-cell BSF. It is seen that there is a band-stop
frequency range with 20-dB rejection from 17 GHz to 25 GHz.
Similar to the unit-cell BPF, a lumped equivalent circuit is
proposed to model this unit-cell BSF. Each short-end stub is
A. Bandpass Filter Consists of Three Unit-Cell
In order to improve the sharpness of the cut-off frequencies,
more unit-cell can be cascaded together. Fig. 7 is a schematic
diaphragm of a BPF cascaded by three unit-cell BPF and its
simulation results is shown in Fig. 8. As expected, much
sharper cut-off frequencies are obtained. The 3-dB relative
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bandwidth is 58% and the insertion loss in the passband is less
than 1.5 dB, the 30-dB rejection is at 14.5 GHz and 30 GHz.
Fig. 8 also gives the circuit simulation results by cascading
three circuits given in Fig. 3. The EM simulation results and
the circuit simulation results show good match.
can precisely control small dimensions and mass production
can be obtained. Fig. 11 shows the fabricated unit-cell BPF
and BSF.
Fig. 9 Schematic diaphragm of a BSF consists of three unitcell BSF (not to scale. size: 13 mm × 2.3 mm)
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S11 (dB)
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S21 (dB)
S11 (dB)
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0
10
20
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EM simulation
Circuit simulation
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30
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EM simulation
Circuit simulation
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10
20
S21 (dB)
Fig. 7 Schematic diaphragm of a BPF consists of three unitcell BPF (not to scale. size: 13 mm × 2.3 mm)
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30
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Frequency (GHz)
Fig. 10 Simulation results of the three unit-cell BSF
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Frequency (GHz)
Fig. 8 Frequency response of the BPF with three unit-cell
B. Bandstop Filter Consists of Three Unit-Cell
Similarly, a three unit-cell BSF can also be cascaded to
make up of a BSF to improve the cut-off frequencies, as
shown in Fig. 9 whereas Fig. 10 shows its simulation results.
It is seen that the 20 dB stop band width is 19 GHz (from 12
GHz to 31 GHz). The return loss in the passband is higher
than 9 dB at both lower and higher frequencies. The lower
passband insertion loss is better than 1 dB and higher
passband insertion loss is less than 4 dB. The circuit
simulation results by cascading three equivalent circuits of Fig.
5 are also shown in Fig. 10 and it can match the EM
simulation results well.
(a)
(b)
Fig. 11 Optical photos of the fabricated (a) unit-cell BPF (b)
unit-cell BSF
IV. FABRICATION
V. MEASUREMENT RESULTS AND DISCUSSION
The designed CPW structure are fabricated on a glass
substrate (εr = 4.6, tanδ = 0.01 with thickness h = 0.5 mm) by
using surface micromachining fabrication technology. The
metal layer is 3 µm Cu with 50 nm Ti as adhesion layer. The
Cu thin film is deposited onto the glass substrate by sputtering
with careful stress control. The finial CPW patterns are
defined by lithography and wet etch. Compared to normal
MMIC fabrication, the surface micromachining technology
The RF performance of the switch is characterized using an
HP8510C vector network analyzer and a RF probe station
with 150 µm probes. A full Thru-Reflect-line (TRL) routine is
used to calibrate with the NIST software MULTICAL.
A. Bandpass Filters
Figure 12 presents the measurement results for the BPFs.
The center frequency is at 22 GHz with 60 % relative 3-dB
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bandwidth. For the unit-cell BPF, the insertion loss in passband is less than 0.3 dB and the band rejection is high than 30
dB. For the BPF with three unit-cell, the band rejection is
higher that 12 dB and the insertion loss in the passband is less
than 1.6 dB. Good agreement between the previous simulation
results and the measurement results can be obtained.
VI. CONCLUSION
In conclusion, this paper presents broad-band BPF and BSF
with very sharp cut-off frequencies by implementing series
open and short stubs in the center conductor of the CPW
transmission line. The structure is physically compact and
easy to fabricate. Low pass-band insertion loss, high rejection
in the stop-band and flat response at the stop-band and the
pass-band are achieved for both the BPF and the BSF.
0
0
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Unit-cell BPF
Three unit-cell BPF
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20
30
ACKNOWLEDGEMENT
S21 (dB)
S11 (dB)
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The authors wish to acknowledge the assistance and support
of Associate Professor Radhakrishnan K., Mrs. Zeng Rong
and all technicians of photonics lab for RF characterization,.
REFERENCES
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40
Frequency (GHz)
Fig. 12 Measurement results of the BPFs
B. Bandstop Filters
Figure 13 shows the measurement results of the BSFs. The
return loss in the stop-band is less than 0.3 dB. Broad 20-dB
stop band, from 12.5 GHz to 33 GHz can be obtained for the
BSF with three unit-cell. For the pass band, the return loss is
higher than 10 dB at both lower and higher frequencies. The
lower pass band insertion loss is less than 0.8 dB and higher
pass band insertion loss is less than 1.5 dB. The small
difference between the simulation results and the
measurement results can be attributed to the fabrication error.
Compared to the BSF reported in [7], the 20 dB stop band
width is wider and the insertion loss is lower in both the lower
and higher frequency band.
0
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Unit-Cell BSF
Three Unit-Cell BSF
0
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20
30
S21 (dB)
S11 (dB)
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40
Frequency (GHz)
Fig. 13 Measurement results of the BSFs
4