2011 IEEE Symposium on Wireless Technology and Applications (ISWTA), September 25-28, 2011, Langkawi, Malaysia
Design of Third Order Wideband Bandpass Filter Using Dual-path Coupled Lines S. A. Nordin, M. K. Mohd Salleh Faculty of Electrical Engineering Universiti Teknologi MARA (UiTM) 40450 Shah Alam
[email protected],
[email protected]
Zoo Zoe
Abstract—A novel topology of wideband bandpass filter is proposed. The filter is composed of two identical paths of coupled lines connected in parallel. The overall frequency response of the filter can be adjusted by varying the impedance value of the filter elements leading to possibilities to obtain the desired bandwidth of the passband. The proposed topology is used to design a wideband third order filter centered at 3 GHz using microstrip technology and the measurement results are shown to be coherent with those from the simulation.
2 λ/4 Zoo Zoe
λ/4
λ/4
Zoo Zoe
λ/4
Keywords-coupled-lines bandpass filters; microstrip; wideband bandpass filters
1
I.
Zoo Zoe
INTRODUCTION
With the increase in number of applications for wireless technologies, various topologies of wideband bandpasss filter have been proposed and developed wisely [1]-[11]. The interesting design of the wideband bandpass filter requires new specifications of microwave component and devices especially in terms of selectivity, low power consumption and low cost of the filter [1]-[3] . On the other hand, required bandwidth also important issue when design a filter, for example using parallel coupled-line techniques, filter with large bandwidth could be designed [4]. Many efforts have been carried out to realize various kind of wideband filter using coupled line [5]-[7] and ring resonator [8]. In [5], a tapped wiggly coupled line with embedded narrow stub has been proposed to form a wideband with fractional bandwidth 33%. However, the narrower of the gap and width in the structure will cause the difficulties in fabrication.
Control Parameters: :center frequency in GHz c Zoe : even mode impedance of the coupled line in Ω Zoo : odd mode impedance of the coupled line in Ω Figure 1. Basic circuit of the proposed wideband bandpass ring filter with its control parameters.
as referred to the center frequency. Furthermore, recent development has emerged a new type of wideband bandpass filter which based on the classical parallel coupled lines. In this paper, a design of wideband bandpass filter which is based on the dual-path coupled lines filter is proposed. The filter is designed using four quarter-wavelength coupled lines which are combined to each other to form a ring structure as shown in Fig.1. With such configuration, third order passband responses with two transmission zeros can be obtained. Furthermore, the filter is simply controlled by varying the oddand even-mode coupled line impedances. In addition, the order of the filter can be increased by increasing the number of coupled line. Simulation design was done using electromagnetic wave (EM) simulator. Finally, an experimental result of third order wideband bandpass ring filter using dualpath coupled lines at center 3 GHz is demonstrated to validate the idea.
Various designs of new topologies for wideband filter based on asymmetrical coupled line also have been presented in [6]. It consists two asymmetrical interdigital coupled lines in order to improved out-of-band performance and results a compact wideband filter. Based on the above wideband techniques, it is interesting to explore another simple techniques using coupled line to produce such wideband applications. As a result, an interesting concept of dual-path coupled lines filter has been proposed where quarterwavelength of the coupled lines is used to feed the structure of ring filter. However, its fractional bandwidth can be varied by controlling the characteristic impedance even- and odd-mode
978-1-4577-1497-9/11/$26.00 ©2011 IEEE
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-20
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transmission zeros
50 Ω
gap=0.15 mm
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Figure 4. Layout of the proposed wideband filter
fo
0
50 Ω
2.40 mm
64.00 mm
third order passband
27.72 mm
dB(S11), dB(S12)
0
Frequency (GHz) 0
dB(S11), dB(S12)
Figure 2. Frequency response of the proposed wideband bandpass filter
0 -10
dB(S12)
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2.0
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6.0
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Figure 5. Simulated and measured frequency response of the proposed filter.
The proposed topology is connected in parallel by using four asymmetrical coupled lines. The asymmetric type of the coupled lines is chosen due to its flexibility in controlling the response of the topology. It is noted the parameters Zoo and Zoe are the characteristic impedance of the coupled lines. These parameters are used to control the response of the topology in term of the bandwidth, matching level in the passband and transmission zeros frequencies.
-20 dB(S11)
-30
Frequency (GHz)
-10
-30 -40
In Fig. 3, the parameter odd-mode impedance, Zoo is equal to 22 Ω. The bandwidth in the passband can be controlled by increasing the value of even-mode impedance, Zoe. As can be seen, larger different between both impedance value is important in order to achieve wideband filter. The variations of both impedances value also affect the filter response S11 and S12. The effect of varying the bandwidth to the coupling level of the coupled lines center at 3 GHz is depicted in the Fig. 3 On the other hand, two transmission zeros are found outside its 3rd order passband and in-band ripple is 0.05 dB.
-50 2.0
3.0
4.0
5.0
Frequency (GHz) Zoe = 53 Zoe = 74 Zoe = 38
Figure 3. Effect of varying the bandwidth to the coupling level of the coupled lines center at 3 GHz
II.
-20
0.0
Frequency (GHz)
-60 1.0
S12
-60
fo = 3 Ghz y = 0.005 dB Zoo = 22 Ω
-50
S11
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III.
BASIC CIRCUIT OF THIRD ORDER DUAL-PATH COUPLED LINES
3RD ORDER FILTER REALIZATION
Simulation for the 3rd order parallel coupled line dual-path filter design was carried out using the electromagnetic wave simulator. In simulation, the filter was designed at 3 GHz and constructed on a FR-4 microstrip laminate with relative permittivity εr = 4.1, substrate thickness, h = 1.6 mm, and loss tangent, tan δ = 0.02. The characteristic impedance of the input and output microstrip-line is set to 50 Ω. The filter layout is
The basic circuit of the proposed wideband bandpass ring filter is shown in the Fig. 1. As can be seen, two identical first orders coupled lines filters are combined in parallel thus result in a wideband bandpass response where the total circumference length of the ring are equal to one wavelength at the resonant frequency, ƒo. An example frequency response of this filter is shown in the Fig.2.
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ACKNOWLEDGMENT Financial support from Universiti Teknologi MARA and Ministry of Higher education, Malaysia for this project is gratefully acknowledged. REFERENCES [1]
G. P. Hancke and B. Allen, "Ultrawideband as an Industrial Wireless Solution," Pervasive Computing, IEEE, vol. 5, pp. 78-85, 2006. [2] X. D. Huang, et al., "Compact ultra-wideband filter using coupled-line and short-ended stub," Electronics Letters, vol. 46, pp. 1033-1035, 2010. [3] C. Peng, et al., "Wideband bandpass filter using parallel-coupled line and step-impedance open stubs," in Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, 2009 3rd IEEE International Symposium on, 2009, pp. 1047-1049. [4] C. Nguyen, "New compact wideband bandpass filter using three parallel-coupled lines," Electronics Letters, vol. 30, pp. 2149-2150, 1994. [5] L. Ting-Shan and T. Kam-Weng, "Wideband microstrip bandpass filter using tapped wiggly-coupled lines," in Microwave and Millimeter Wave Technology, 2008. ICMMT 2008. International Conference on, 2008, pp. 178-181. [6] S. Sheng, et al., "A Compact Wideband Bandpass Filter Using Transversal Resonator and Asymmetrical Interdigital Coupled Lines," Microwave and Wireless Components Letters, IEEE, vol. 18, pp. 173175, 2008. [7] N. Thomson and H. Jia-Sheng, "Compact Ultra-Wideband Microstrip/Coplanar Waveguide Bandpass Filter," Microwave and Wireless Components Letters, IEEE, vol. 17, pp. 184-186, 2007. [8] M. K. Mohd Salleh, et al., "Wide-band microwave bandpass ring filter," in Electronic Devices, Systems and Applications (ICEDSA), 2010 Intl Conf on, 2010, pp. 317-320. [9] Y. A. Kolmakov and I. B. Vendik, "Compact ultra-wideband bandpass filter with defected ground plane," in Microwave Conference, 2005 European, 2005, p. 4 pp. [10] M. Zhewang, et al., "A novel compact ultra-wideband bandpass filter using microstrip stub-loaded dual-mode resonator doublets," in Microwave Symposium Digest, 2008 IEEE MTT-S International, 2008, pp. 435-438. [11] L. Y. Cai, et al., "A novel compact dual-wideband bandpass filter," in Microwave and Millimeter Wave Technology (ICMMT), 2010 International Conference on, 2010, pp. 56-58.
Figure 6. Photo of the proposed wideband bandpass filter prototype.
shown in Fig. 4 with the dimensions of its element. The overall surface area of the filter is 64.0 x 58.2 mm2. The measurement results of the wideband filter are compared to its simulation results and depicted in Fig. 5. It was measured on vector network analyzer. It can be seen that both results are in good agreement with each other. Measurement result shows the passband response is centre at 3.4 GHz with a fractional bandwidth of 52.4% was slightly shifted from its center frequency 3 GHz. Within the passband, the measured insertion loss is about 1.93 dB which is less than 3dB whereas the return loss of the filter is larger than 12 dB. Furthermore, the second rejection level is found to be less than 20 dB in area 5 GHz. The photo of the fabricated filter is shown in Fig. 6. IV.
CONCLUSION
A wideband filter using quarter-wavelength dual-path coupled lines is proposed and designed for wideband bandwidth. The new structure is based on the two identical paths of coupled lines that combined in parallel thus results the 3rd order bandpass filter. It can be seen that a fractional bandwidth of 52.4% at the center frequency 3 GHz was shifted to 3.4 GHz. The filter was realized on FR-4 substrate using microstrip technology and good agreement between simulated and measured results was obtained.
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