I. INTRODUCTION
Worldwide Interoperability for Microwave Access (WiMAX) technology has great potential in the development of modern communication systems, for instance, mobile broad band connectivity, wireless connectivity, providing telecommunication services and also to facilitate machine-to-machine communications. Bandpass filter (BPF) is a key component in receiver front end for frequency selection purpose. In the present scenario of multiband communication systems, single front-end devices catering to different bands simultaneously are desirable. The conventional topology of filter design fails to meet this requirement due to its large-area requirements and also suffers from uncontrollable spurious passband response [Reference Pozar1, Reference Hong and Lancaster2] which restricts designing a multiband bandpass filter. Stepped Impedance Resonator (SIR) is widely used for designing dual-band filter, where the first spurious mode is shifted to the second passband [Reference Sagawa, Makimoto and Yamashita3–Reference Huang, Chen, Chang, Chen, Wang and Houng5]. SIRs with a new coupling scheme is used to realize a compact dual-band BPF is reported in [Reference Chu and Chen6]. A new topology has been proposed to improve the response to some extent by using defected SIR [Reference Wu, Liang, Li and Qin7, Reference Wu, Liang, Qin and Li8]. A compact size dual-band characteristic is realized by implementing Meander Loop resonator with CSRR DGS [Reference Wu, Mu, Dai and Jiao9]. In [Reference Chiou, Yang, Kuo and Wu10] by inserting transmission zeros in either side of the passband a compact dual-band BPF is realized using periodic stepped impedance ring resonator.
However, a dual-bandpass filter with highly selective response in conjunction with lower insertion loss with wide stopband is still a challenging task in microwave circuit design paradigm. Here a dual-bandpass filter is realized by using two SIRs of resonating frequency 2.5 and 3.5 GHz. Impedance ratio is chosen to shift the spurious mode above 9 GHz. A new coupling scheme is also presented to combine both the resonators. Finally, interdigital capacitor (IDC) is used to increase the bandwidth of the proposed dual-band BPF.
II. DESIGN OF PROPOSED DUAL-BAND BANDPASS FILTER
The filter is designed using half-wavelength (λ/2) open end SIR. The basic structure of the λ/2 SIR is shown in Fig. 1. The resonator composed of high impedance (Z 1) section of electrical length (θ 1) followed by two low-impedance (Z 2) sections of electrical length (θ 2). The resonance condition of the resonator can be derived by
where R z is the impedance ratio of the SIR. The fundamental frequency (f 0) and first spurious frequency (fSB1) of the resonator are related by
Fig. 1. General geometry of λ/2 SIR.
Figure 2 shows the plot of impedance ratio against the normalized first spurious frequency of the SIR. It can be observed that by proper selecting the impedance ratio the first spurious mode can be shifted at high-frequency range to obtain a wide stop band.
Fig. 2. Relationship between impedance ratios with normalized first spurious resonance frequency of SIR.
The layout of the proposed dual-band BPF with two λ/2 SIRs is shown in Fig. 3. The filter is realized using a substrate of dielectric constant 2.2 and height 0.787 mm. Resonator 1 is designed to operate at the first passband of 2.5 GHz for which the impedance ratio is chosen 0.32 for which the low-impedance section impedance denoted by Z 1 = 43.12 Ω with electrical length θ 1 = 24.84° and the corresponding high-impedance section has Z 2 = 125 Ω with electrical length of θ 2 = 39.05°. Resonator 2 is realized to operate at the second passband of 3.5 GHz having an impedance ratio of 0.24 with a low-impedance section, impedance Z 3 = 29.49 Ω and electrical length of θ 3 = 17.65°. The high-impedance section has impedance Z 4 = 15.08 Ω with electrical length θ 4 = 37.63°. It is seen that for these impedance ratios the spurious modes occur beyond 7 GHz which is useful to obtain a wide stop band. An additional benefit to choose low impedance ratio is that for a specific fundamental frequency of resonance, the overall electrical length gets reduced which is helpful to realize a compact bandpass filter. In order to achieve the dual-band operation, both resonators include low-impedance sections which are connected to input and output. The spacing between the resonators and the low-impedance section is computed using a full-wave electromagnetic simulator CST Microwave Studio from which the spacing for highest coupling is determined. Parametric study yields a spacing of 0.15 mm. The frequency response of the above dual-band BPF is plotted in Fig. 4 where the bandwidth is narrow due to weak coupling.
Fig. 3. The layout of the proposed bandpass filter with l1 = 2, l2 = 1.5, l3 = 3.32, l4 = 4, l5 = 2.16, l6 = 3.75, l7 = 3, l8 = 6, w1 = 5, w2 = 3, w3 = 0.4, and w4 = 0.4 (all dimensions are in mm).
Fig. 4. EM simulated response of the proposed BPF without IDC.
To enhance the coupling further IDC is introduced between the resonator and the low-impedance section which is shown in Fig. 5. The length, spacing, and width of the fingers of IDC are extracted to achieve desired bandwidth to make it suitable for WiMAX application. To minimize the effective area of the filter the length of the high-impedance section of both the resonators is modified by bending it along the width. The equivalent circuit of the above filter is illustrated in Fig. 6. Each SIR is equal to an LC resonant tank circuit and they have capacitive coupling due to IDC with the input and output feed lines. The calculated parameters for resonator 1 are L 1 = 0.308 nH, C 1 = 11.76 pF and for resonator 2 are L 2 = 0.2 nH, C 2 = 7 pF. The series and shunt capacitances associated with interdigital sections are C 11 = 1.5 pF, C 21 = 1.3 pF, C 12 = 0.011 pF, and C 21 = 0.01 pF. These are computed using extraction methods outlined in [Reference Hong and Lancaster2].
Fig. 5. The layout of the proposed bandpass filter with IDC: g = 0.1, w5 = 0.1, w6 = 0.7, l9 = 0.7, l10 = 3.1, l11 = 0.9 (all dimensions are in mm).
Fig. 6. Equivalent circuit model of proposed dual-band BPF with IDC.
III. RESULTS AND DISCUSSION
A prototype of the proposed filter is realized on Taconic substrate of permittivity 2.2 and height 0.787 mm is displayed in Fig. 7. The frequency response obtained from EM simulation and equivalent circuit model and measurement using HP 8722C VNA of the proposed BPF with IDC is shown in Fig. 8. Good agreement is observed in the S 11 (dB) plots but with slight mismatch between the simulated and measured S 21 (dB) response. However, the S 21 (dB) obtained from equivalent circuit model closely follows the measured result. Measured S-parameter of the proposed filter depicts that for first passband of 2.46 GHz has insertion loss about 1.45 dB and for second passband at 3.51 GHz the measured insertion loss is about 1.7 dB. For the lower passband measured impedance bandwidth is 45 MHz and the higher band impedance bandwidth is 115 MHz. A transmission zero is found at 2.84 GHz, which demonstrates that the filter poses a good selectivity. This occurs due to the cross coupling between the two SIRs. The proposed filter also shows a wide upper stopband with attenuation more than 15 dB extended up to 7 GHz. A comparison with other dual-band filters as given in the reference is tabulated in Table 1.
Fig. 7. Photograph of the fabricated prototype of the proposed dual-band filter.
Fig. 8. Comparison of return loss and insertion loss of the proposed filter for electromagnetic simulation, circuit simulation, and experiment.
Table 1. Comparison of the proposed work with other related work as mentioned in the reference.
IV. CONCLUSION
This paper presents a novel layout of a compact dual-band bandpass filter using SIRs with good agreement between electromagnetic simulation, an equivalent circuit model, and measurement. IDCs are introduced between SIRs and feed lines to obtain 45 MHz measured bandwidth at first passband and about 115 MHz at second passband. By proper selection of the impedance ratio a wide stop band till 7 GHz is achieved. Folding the high-impedance section of SIRs a compact dual-band BPF is implemented with a total area of 30 × 10 mm2 only.
ACKNOWLEDGEMENTS
The authors would like to thank all the reviewers for their helpful suggestions and comments to improve the manuscript.
Pankaj Sarkar received M.Tech (Microwave Engineering) from The University of Burdwan in 2009. He started his academic pursuit as Lecturer at ITER, Siksha ‘O’ Anusandhan University, Odisha where he is currently Assistant professor in Electronics and Communication Engineering Department. His areas of interest lie in the field of microwave passive circuit design and optimization. He has over 15 publications in various national/ international journal and conferences. He is currently pursuing his PhD from Jadavpur University.
Rowdra Ghatak received his M.Tech (Microwave Engineering) from The University of Burdwan and Ph.D. (Engg) from Jadavpur University in 2002 and 2008, respectively. He initiated his career in microwave engineering as a trainee at CEERI Pilani in fabrication and testing of S-band magnetrons. He is currently an associate professor in Electronics and Communication Engineering Department of National Institute of Technology Durgapur. He is a recipient of the URSI Young Scientist Award in 2005. He has worked as co-investigator in Government of India funded project to carry out work in chaotic microwave oscillator being a member of Radionics group at the University of Burdwan. He recently received support under DST Young Scientist scheme for development of antennas for medical imaging and ground penetration RADAR. He has more than 60 publications in various National/International journals and conferences. His research interest lies in the areas of fractal antenna, metamaterials, application of evolutionary algorithms to electromagnetic optimization problems, RFID, computational electromagnetic and microwave passive circuit design.
Manimala Pal received M.Tech (Microwave Engineering) from The University of Burdwan in 2010. She started her academic endeavor as a faculty at Bengal College of Engineering and Technology for Women. Presently, she is an assistant professor in the Electronics and Communication Engineering Department of NSHM Knowledge campus, Durgapur Group of Institutions, Durgapur, West Bengal. Her areas of interest lie in design and optimization of planar bandpass filters and couplers.
Dipak Ranjan Poddar is a Professor and Emeritus Fellow in the Department of Electronics and Tele Communication Engineering in Jadavpur University. He has supervised 18 Ph.D. theses. He has 150 publications in various National/International conference and journals. His areas of research include EMI/EMC, fractal antennas, and metamaterials. He has been the principal investigator of a number of funded projects. He is a reviewer of IEEE periodicals. He is a senior member of IEEE.
