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Current Developments of Microwave Filters for Wideband Applications

Authors:
  • Universiti Teknikal Malaysia Melaka (UTeM), Malaysia

Abstract and Figures

This paper presents a compilation of important review in the development of microwave filters for wideband technology used in previous years. The major research work for each year is reviewed. Several wideband filters based on the planar and non-planar circuits are compared and examined in order to propose a new topology of wideband filter using Suspended Stripline Structure (SSS). More importantly, this new proposed structure will be integrated with the Defects Ground Structure (DGS) to form an advanced hybrid system for wideband applications. This system will produce the band-pass and band-stop responses simultaneously in order to discriminate between the desired and undesired signals in the wideband spectrum. The proposed system outlined in this paper, featuring new innovation in hybrid structure as well as providing an insight of the direction of future research works. The contribution of this study is useful for applications where the reduction of physical volume is very important, while maintaining its good performance such as Ultra-Wideband (UWB), commercial radar as well as the wideband warfare receiver. As for future works, integration techniques between the UWB filter and DGS will be designed and analyzed to form an advanced new microwave device in order to produce bandpass and bandreject response in the same structure simultaneously.
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World Applied Sciences Journal 21(Special Issue of Engineering and Technology): 31-40, 2013
ISSN 1818-4952
© IDOSI Publications, 2013
DOI: 10.5829/idosi.wasj.2013.21.1004
Corresponding Author: Zahriladha Zakaria, Centre for Telecommunication Research and Innovation (CeTRI), Faculty of
Electronic and Computer Engineering, Universiti Teknikal Malaysia Melaka (UTeM), Hang Tuah Jaya,
76100, Durian Tunggal, Melaka, Malaysia. Tel: +6013-6203584, E-mail: zahriladha@utem.edu.my.
31
Current Developments of Microwave Filters for Wideband Applications
Zahriladha Zakaria, Mohamad Ariffin Mutalib, Kamaruzaman Jusoff,
1 1 2
Mohd Sa’ari Mohamad Isa, Mohd Azlishah Othman, Badrul Hisham Ahmad,
1 1 1
Mohamad Zoinol Abidin Abd. Aziz and Shadia Suhaimi
1 3
Centre for Telecommunication Research and Innovation (CeTRI),
1
Faculty of Electronic and Computer Engineering, Universiti Teknikal
Malaysia Melaka (UT eM), Hang Tuah Jaya, 76100, Durian Tunggal, Melaka, Malaysia
Department of Forest Production, Faculty of Forestry,
2
Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
Faculty of Business and Law, Multimedia University,
3
Jalan Ayer Keroh Lama, 75450, Melaka, Malaysia
Abstract: This paper presents a compilation of important review in the development of microwave filters for
wideband technology used in previous years. The major research work for each year is reviewed. Several
wideband filters based on the planar and non-planar circuits are compared and examined in order to propose
a new topology of wideband filter using Suspended Stripline Structure (SSS). More importantly, this new
proposed structure will be integrated with the Defects Ground Structure (DGS) to form an advanced hybrid
system for wideband applications. This system will produce the band-pass and band-stop responses
simultaneously in order to discriminate between the desired and undesired signals in the wideband spectrum.
The proposed system outlined in this paper, featuring new innovation in hybrid structure as well as providing
an insight of the direction of future research works. The contribution of this study is useful for applications
where the reduction of physical volume is very important, while maintaining its good performance such as
Ultra-Wideband (UWB), commercial radar as well as the wideband warfare receiver. As for future works,
integration techniques between the UWB filter and DGS will be designed and analyzed to form an advanced
new microwave device in order to produce bandpass and bandreject response in the same structure
simultaneously.
Key words: Hybrid Microwave Device % Microwave Filter % Defected Ground Structure (DGS) % Suspended
Stripline Structure (SSS)
INTRODUCTION Wireless technologies such as 802.11b and short-range
In communication, the system can be termed as UWB products that would have a throughput capacity of
wideband when the message bandwidth considerably 1,000 times greater than 802.11b (11 Mbit/sec). Those
exceeds the coherence bandwidth of the channel. The numbers mean UWB systems have the potential to
wideband bandwidth is forced to use for any support larger number of users, at much higher speeds
communication link due to a high data rate. But another and lower costs than the current wireless LAN systems.
link may have slightly low data rates intentionally use Current UWB devices can transmit data-up to 100 Mbps,
a wider bandwidth in order to gain another advantage. compared to the 1 Mbps of Bluetooth and the 11 Mbps of
The UWB is another modulation technique that uses the 802.11b. Moreover, it involves a fraction of current
same purpose, follows on transmitting duration pulses. technologies like Bluetooth, WLANs and Wi-Fi [1].
Bluetooth radios eventually could be replaced by the
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
32
Fig. 1: Untethered wireless UWB tracking system architecture (more slave sensors can be connected) [1]
UWB is a technology for transmitting information In designing the microwave filter, researchers focus
which spreads over a large bandwidth (>500 MHZ) that on how to produce the wideband frequency and apply
should, in theory and under the right circumstances, be them to ultra-wideband. In order to avoid the interference
able to share spectrum with other users. Regulatory with WLAN radio signal, different methods and structures
setting of Federal Communications Commission (FCC) in have been used to develop a UWB bandpass filter.
United States is planned to offer an effective use of scarce In UWB, it has some interference with other signal
radio bandwidth while allowing both great data rate frequency. Research on how to reject the unwanted signal
“Personal Area Network” (PAN) wireless connectivity has to be developed and studied. A novel compact
and longer-range, low data rate applications as well as structure that integrates bandpass filter and DGS in a
radar and imaging systems [1]. single device to reduce the overall physical size will be
UWB devices can be used for a multiplicity of discussed. The resulting structure will exhibit bandpass
communications applications including the transmission and band reject response simultaneously. This will
of very high data rates over short distances without overcome the problem face by the UWB application in
suffering the effects of multi-path interference. UWB order to remove undesired signal, i.e. 5.2 GHz and 7.75
communication devices might be used to wirelessly GHz Wireless Local Area Network (WLAN) radio signal
allocate services such as cable, phone and computer [1].
networking through a building or home. These devices In this paper, an effort has been made to describe and
can also be used by police, fire and rescue personnel to show the exciting advances in microwave filter in term of
provide undercover, protected communications devices. their technologies. The structure to produce the wideband
Figure 1 shows the untethered wireless UWB tracking frequency signal is shown with some comparison based
system architecture. The UWB can be used in various on planar and non-planar structure of the filter. This paper
applications which widely used in the home environment. also shows the hybrid structure of integrating with DGS
There are some advantages by using UWB in order to produce bandpass and bandstop filter
frequency with the communication speed up to 1 Gbps simultaneously. From this structure, it provides small size,
can be achieved for modern living. By utilizing Time of light weight and helps to reduce the cost of the
Arrival (TOA) of UWB, the high resolution of 3-D manufacturing. It is expected that the information from
location system can be reached. The interference between this paper will help researchers to get a broader
electrical equipment can be decreased with less perspective of hybrid structure of the filter and perhaps
penetration of the human body. The power consumption can be considered as an alternative to produce other
could be minimized because less wireless communication methods.
devices is being carried. Some applications use the UWB
system like High Resolution Wireless Surveillance Camera Microwave Filters: The history of microwave filter began
and Wireless Video Adapter. This camera uses high in 1937 when Mason and Sykes initiated the development
resolution video image to transfer capability and the of waveguide filter [2]. Most researches were conducted
wireless video adapter may connect personal computer in various laboratories in the US. For example, at the
and projector in wireless. Massachusetts Institute of Technology (MIT) Radiation
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
33
Laboratory, researchers were concentrating on the Basically, researchers are focusing on analysis, synthesis
waveguide cavity filters for radar systems. In other and calculation of the microstrip circuit. It includes
laboratories, the researches were conducted with different configuration, dimension and structure of the microstrip
methods such as developed on broadband low pass, high conductor, while the ground side remains a complete
pass and band pass coaxial filters and narrow-band metallization structure. However, by disturbing the
tunable filter for receivers. The development of UWB ground plane structure, it can increase the effectiveness
microwave filters has been increasing ever since the of capacitance and inductance. Some modifications are
Federal Communications Commission (FCC) approved the needed to improve electrical performance and reduce the
commercial use of UWB in 2002 [1]. Since then, there has size of microstrip circuit. In recent years, DGS structures
been a considerable research effort geared into UWB with reconfigurable by active device have been
radio technology worldwide. Radio Frequency (RF) filter investigated. Several solid state devices such as PIN
or microwave filter is a type of passive device with two diode, Varactor or Tunnel Diode and Field Effect or
portent works because it passes the desired signal and Bipolar Transistor can be used to vary the resonant
blocks the unwanted signal. Filters are commonly frequency of DGS structure.
employed in microwave and millimeter-wave transceivers Due to the demand of wide operating frequency for
as channel separators. Basically, it has four types of the UWB bandpass filter, this study offers a synthesis
frequency responses which are lowpass, highpass, technique based on generalized Chebyshev characteristic
bandpass and bandstop characteristics. A compact which is proposed to produce bandpass filter with sharp
communication system works in this UWB frequency response and good performance. The main advantage of
band require a small bandpass filter (BPF) with a notched the generalized Chebyshev characteristic is the finite
band in the UWB passband in order to avoid being frequency can be mathematically placed at the location of
interfered by the WLAN radio signals. two transmission zeros (pole-zero plot). Hence, it
The defects in the ground plane and which have produces good selectivity and improve the filter’s
disturbances in the shield current of distribution, either in performance Moreover, the number of elements can be
etched periodic or non-periodic cascade configuration reduced by using this method and absolutely the
defect, (e.g. microstrip, coplanar and conductor backed physical size can be decreased [6]. Therefore, the UWB
coplanar waveguide) are known as DGS. This disturbance bandpass filter can be integrated with the DGS in order to
occurs in the ground plane will alter the characteristics of remove undesired signal frequency within the UWB
a transmission line such as line capacitance and frequency range. For example, in WLAN, the operated
inductance. In other words, the effects of capacitance frequency of 5.2 GHz and 5.75 GHz can be eliminated.
and inductance can be increased if the ground plane of Therefore, the integration technique can produce
microstrip has a defect etched [3]. The DGS can also bandpass and band reject with the same structure
eliminate the unwanted frequency which produces the simultaneously.
band rejection and the frequency range can be tuned by
controlling one physical dimension of DGS pattern [4]. Development of Microwave Filters for Wideband
Quasi-lumped inductive element was offered in DGS Application: The motivation of this study is driven by the
structure. It can be used to replace the high impedance fact that for the UWB technology, UWB signals must
narrow microstrip line which is normally used as an comply with the spectrum mask as required by the FCC
inductor for designing high-low impedance low-pass filter Rules and Orders. Therefore, the design of UWB
(LPF) [5]. The drawback with the very narrow width microwave bandpass filter is very critical in order to
microstrip line section is the difficulty in fabricating the achieve broad bandwidth, while has the capabilities to
LPF. In addition, the increased length of the filter will remove any undesired signal within the UWB frequency
increase the dimensions of physical layout. Therefore, range. To provide the hybrid structure of the application,
DGS acts as an inductor and can reduce the length of the the UWB can be applied at the microwave filter and DGS
LPF. A resonant gap or slot in the ground metal is the to produce bandpass and rejection frequency signal
basic element of DGS which placed directly under a simultaneously. In order to fulfill the satisfaction for the
transmission line and aligned for efficient coupling to the users, this application can be used in the modern living;
line. PAN. Therefore, the suitable method which has been used
A microstrip with a conductor trace on one side of to produce the hybrid structure and to apply at modern
the substrate and a single ground plane on the other side technology can be regarded as a hybrid bandpass filter
is the most popular microsrip line produced by DGS. with the DGS.
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
34
Fig. 2: Evolution of the proposed composite BFR[10] was discussed in [17]. However, the fractional bandwidths
Several methods and structures have been studied in implemented and produced a cascading of cascadable
order to develop new structures of these UWB filters. 180° hybrid rings by finite-ground coplanar waveguide
UWB filter was produced by mounting a microstrip line in [18]. UWB with hybrid Microstrip-Coplanar waveguide
a lossy composite substrate with the higher insertion loss was produced but the stopband frequency is still only
[7]. However, at the lower frequency, the filter lacks of 16.0 GHz [19]. The structure in Figure 4 called an
sharpness and produces a larger insertion loss which is asymmetric parallel coupled-line has been attempted in
greater than 6.0 dB with poor impedance matching at the bandpass filter design [20] but their shape of transmission
high frequency. In other studies, a microstrip ring UWB line has some difference in the shape proposed in [21].
is constructed by allocating transmission zeros below 3.2 The new method to produce UWB by using WLAN
GHz and above 10.6 GHz [8, 9]. This filter with a ring notch was introduced [22, 23]. This UWB suspended
resonator is larger in size and produces narrow, lower and stripline filter which has a single stopband by
upper stopbands. The results show that, insertion loss is incorporating a resonant slot into one of its elements.
better than 0.53 dB and return loss is greater than 10 dB in In this method, a Duroid RT 6010 substrate with a
the passband from 3.8 GHz to 9.2 GHz and indicates the dielectric constant of 10.2 and a folded slot are selected to
group delay below the 0.6 nsec within the passband ensure a resonance at a sufficiently low frequency. A slot
UWB. resonator is included into the patch to provide the
Hsu et al. [10] used a different technique to produce rejection of a small frequency band. The author connects
UWB by combining lowpass and highpass structures. a lowpass filter and highpass filter in series to improve the
Figure 2 shows the evolution of the purpose composite upper stopband performance of the filter. The notch leads
BFR. Both BPF consist of a hi-Z, low-Z LPF and an to some additional ringing of a transmitted Gaussian
HPF structure designed with shunt quarter-wave monocycle, but the distortion is not very large compared
short-circuited stubs separated by 8 /4 sections which act to a UWB filter without notch.
g
as impedance inverters. In [11-13], a broadside-coupled Shaman and Hong [24, 25] designed the compact
Microstrip-coplanar Waveguide (CPW) structure with a UWB filter by using the short stubs to create
tightened coupling degree is utilized to design an transmission zero. This paper proposed a filter with low
alternative UWB filter with one, two and three sections. insertion loss, sharp rejection and excellent performance
The two microstrip line are separated with a gap and both side and outside the band. For this method, the
broadside coupled to one open-end CPW which is a basic purpose filter with five short-circuited stubs as
section of the filter. The tight coupling will provide very demonstrated in Figure 5 has been designed for an
wide bandpass operation. optimum stub filter whose connecting lines are
The most important part of the structure in Figure 3 non-redundant. The conventional 9-pole Chebyshev filter
is a broadside-coupled microstrip-coplanar waveguide will allow the filter to exhibit high selectivity.
(CPW). This structure is fabricated on the ground of the This filter in [25] is formed by using GML 1000
microstrip line. The reasons for introducing the extra high substrate with a relative dielectric constant of 3.05 and the
rejection filter (HFR) are the BPF needs to have good thickness of 0.508 mm. This filter was fabricated on a
bandpass performance and requires not only in-band but microstrip substrate and connected using the SMA
also out-band. By using this type of structure, the connectors. The filter improved the response by
performance of out-band up to 16 GHz with attenuation producing transmission zeros at desired frequencies.
larger than 22 dB and the group delay is 0.27 ns. This was realized by placing the short-circuited stubs with
Zhu et al. [14] constructed a UWB BPF by using
single multiple-mode resonator (MMR) that is driven at
two sides by two identical parallel-coupled lines.
Stepped-Impedance Resonator (SIR) will effectively widen
the upper stopband by enlarging the space between
resonant modes (first and second order). Similarly, UWB
BPF is produced by using open stub loaded (MMR),
improved upper-stopband performance and reduced
radiation loss [15]. The basic principle of UWB filter is
originated in [16] and the compact and broadband BPF
were only about 40% to 70%. The wideband BPF was
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
35
Fig. 3: UWB BPF using broadside-coupled microstrip-coplanar waveguide structure [11]
Fig. 4: UWB BPF using Microstrip Stub-Loaded Tri-Mode Resonator [20]
Fig. 5: Circuit model for UWB BPF with Transmission and upper stopbands. This filter exhibited low insertion
Zero[24, 25] loss which includes the losses from the SMA connector
Fig. 6: Physical layout of UWB BPF with Pairs of this UWB filter. This method can increase the filter’s
Transmission Zeroes [25] selectivity.
two-section open-circuited stubs. However, in this
technique, the size of the filter is increased significantly.
The new technique is very simple for implementation
by placing two sections of the feed lines of the length,
L parallel to each other separate spacing, S as shown in
Figure 6.
The new transmission zeroes improved the filter
performance outside the passband by widening the lower
of about 1.1 dB at the midband frequency and a flat group
delay of about 0.6 ns at the midband frequency.
Overall, the new transmission zeroes enhanced the
bandwidth of the lower and upper stopbands with only a
few resonators. A UWB filter with the Low Temperature
Co-fired Ceramic (LTCC) technology has been proposed
[26]. A couple of transmission zeros can be generated
in two-side of passband’s skirt of the ultra-wideband
filter. By this method, the filter can increase the
sensitivity and linearity of the wireless communication.
The multi-layered can reduce the size of the circuit. In this
paper, the both equivalent capacitance B-network and
equivalent inductance B-network are adopted to construct
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
36
(a) (b)
Fig. 7: (a) Equivalent circuit of four-resonator BPF with inductance feedback and (b) fabricated layout of UWB BPF [26]
Table 1: Comparison of different methods of UWB frequency
Filter/ Year f (GHz) Fractional bandwidth, FBW (%) Insertion loss, S (dB) Return loss, S (dB) Size (mm )
o 21 11
2
[7] 2003 6.8 - 6.7 >10 20×50
[8] 2004 6.5 1.5 0.53 >10 -
[10] 2005 7.0 3 - >15 15×30
[11] 2005 6.83 - 0.32 >10 20×50
[13] 2005 5.63 - 0.5 >10 9.8×11.8
[14] 2005 6.85 - 0.55 >10 17×2.16
[20] 2008 7.0 90 0.5 >10 20×15
[21] 2011 6.8 - <1.0 >13 16×5
[25] 2007 6.85 110 0.3 >10 13.9×26.1
Figure 7(a) shows the circuit diagram for the four methods and design were presented in Table 1. Each
coupled transmission which is used to replace the four design of filter has its own advantage. Some modifications
resonators. The gap of transmission lines can control the can be made by improving the structure of a design in
amount of coupling. This LTCC UWB filter is designed order to produce a hybrid structure which is DGS.
using the substrate of Dupont 951, with a dielectric
constant of 7.8 and loss tangent of 0.0045. The compact Topology of Hybrid Structure: Bandpass filter is
UWB filters have been widely investigated for highly commonly used by researchers and applied for the
integrated low cost solutions by using LTCC technology wideband frequency. This study proposes a new hybrid
and semi-lumped type filter circuits [27-30]. However, in structure which can be designed by combining
the higher order, resonances were degraded and caused suspended stripline structure and DGS to produce the
by parasitic electromagnetic (EM). Although the LTCC required results. DGS can produce the outstanding
still suffers from certain temperature-related even the performance in terms of sharp selectivity at the cut-off
packaging and miniaturizing were effective. frequency, spurious free wide stopband and ripples in the
A passive technology is applied by using PCB passband [29, 30, 32-34].
embeds to produce UWB [31]. The third-order Chebyshev A simple technique to design the seven poles
and J-inverter transformation are modified to secure a lowpass filter using DGS was produced [35]. By using this
large bandwidth and avoid the unwanted EM coupling technique, the length is decreased by 42.3% and the area
between the circuit elements. Tree independence has been reduced by 44.2% compared to the conventional
transmission zeros were formed in the desired stopbands lowpass filter. Figure 8 shows the structure of DGS in
by connecting a capacitor and two inductors in series ground plane and the equivalent circuit applied at DGS.
with the parallel LC resonators in order to obtain high Lim et al. [36] used the similar method to produce lowpass
rejection characteristics. This UWB filter was fabricated filter with dumb-bell-shaped DGS. The defected area
into eight-layered PCB substrate. The second and third consists of two rectangular and one connects to slot
layers were embedded by Metal-Insulator-Metal (MIM) corresponding to the equivalent of inductance (L) and
capacitor. The review and analysis among the available capacitance (C). However, this design shows the higher
reported UWB frequency in the past few years has been N, the higher loss and the larger size. Hence, increasing N
done. The comparison and performance of the different can affect the size of the designed structure.
1
2
3
4
5
6
7
0
8
-50
-40
-30
-20
-10
0
-60
5
freq, GHz
DB (S (1,1))
DB (S (2,1))
1
2
3
4
5
6
0
7
-60
-50
-40
-30
-20
-10
0
-70
5
freq, GHz
DB (S (1,1))
DB (S (2,1))
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
2.5
7.0
-20
-10
-30
0
freq, GHz
D
B (S(1,1))
D
B (S(2,1))
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
37
(a)
(b)
Fig. 8: (a) DGS in ground plane, (b) Equivalent circuit of
DGS [35]
Fig. 9: Fabrication filter (a) Bottom view (b) Top view [37]
Fig. 10: Example of generalized Chebyshev lowpass
prototype filter (c)
Bahmani et al. [37] designed a compact slot geometry Fig. 11: Preliminary Result for wideband frequency (a)
to produce the DGS for a microstrip line. By using Lowpass filter at 6 GHz (b) Highpass filter at 3.1
U-shape slot, the capacitive of the structure and overall GHz (c) Bandpass filter at 3.1 to 6 GHz
length of the pattern can be increased. This design of LPF
produces the excellent stop band more than 30 dB from finite frequencies as shown in Figure 10. The suitable
2.8 to 10 GHz with 1.2 sharpness factor (f / f ). Figure 9 lumped element needs to be chosen based on the number
o c
shows the fabrication structure of filter by applying the of order and attenuation of this design filter.
DGS. The measured filter has 3 dB cutoff frequencies at The lowpass prototype operates in a system
2.4 GHz and the insertion loss in the passband is less than impedance of 1 S and the cutoff frequency with 1 rad/s.
0.6 dB. The transmission zeros at 2.9 GHz enhanced the To convert from 1 S to 50 S impedance, all the circuit
roll off and stopband rejection. elements must be scaled by 50 S. A lowpass filter with 6
There are some mathematical formulae that can be GHz cut-off frequency based on the generalized
used in order to prove the method of designing the Chebyshev prototype of stopband insertion loss of
bandpass filter. The design will start from the lowpass 60 dB and minimum passband return loss of 20 dB will be
prototype network which satisfies a generalized used as a specification in the synthesis and mathematical
Chebyshev response with two transmission zeroes at modelling. The lowpass filter prototype and its element
(a)
(b)
World Appl. Sci. J., 21(Special Issue of Engineering and Technology): 31-40, 2013
38
(a) to the incessant development in both theoretical filter
(b) radar as well as the wideband electronic warfare receiver.
Fig. 12: (a) Method for Measuring Filter (b) Undesired between the UWB filter and DGS can be designed and
signal at UWB spectrum analyzed to form an advanced new microwave device in
values can then be constructed and transformed to the same structure simultaneously.
highpass filter in order to produce bandpass filter.
The formula to transform this design from lumped element ACKNOWLEDGEMENTS
to distributed circuit and lastly to suspended is provided
[38]. The authors would like to thank UTeM for
A combination of the lowpass and highpass filter in sponsoring this work under the short-term grant
a cascaded method can produce a new class of bandpass UTeMPJP/2012/FKEKK(11C)/S01015.
circuit. The overall physical realization of the filter will be
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CONCLUSION
In this paper, an effort has been made to relate some
of the several advances in filter technology for modern
applications such as in the UWB spectrum. This has led
design methods and in the technology used for
realizations. The hybrid technique for designing bandpass
filter and defected ground structure is proposed in this
study. This technology can be used for emerging digital
home environment which can support the different
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only restricted to UWB applications but can also be used
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