A compact dual-band bandpass filter using triple-mode stub-loaded resonators and outer-folding open-loop resonators

Transcription

1 Indian Journal of Engineering & Materials Sienes Vol. 24, February 2017, pp A ompat dual-band bandpass filter using triple-mode stub-loaded resonators and outer-folding open-loop resonators Ming-Qing Liu, Cong Wang & Nam-Young Kim* Department of Eletroni Engineering, Kwangwoon University, Seoul , Republi of Korea Reeived 4 September 2015; aepted 19 January 2017 A ompat dual-band bandpass filter with high-seletivity is proposed using triple-mode stub-loaded resonators (TMSLRs) and outer-folding open-loop resonators (OFOLRs). The proposed TMSLRs, with two idential folded shape open-stubs loated at the symmetrial sides of a mirostrip line, are used to generate the upper band while the OFOLRs are utilized to form the lower passband. Two resonators are independent ontrol of passbands, frequenies, and bandwidths. Besides, high seletivity and stopband suppression an be easily obtained due to the fat that four transmission zeros are realized near the passband edges. Measured results show that two entral frequenies ourred at 2.4 GHz and 5.4 GHz for WiMAX and WLAN appliations with a wide frational bandwidth of 13.9% and 16.7% and a low insertion loss of db and db, respetively. Keywords: Dual-band, Bandpass filter, Triple-mode, Stub-loaded resonator, Open-loop resonator, Even-odd-mode analysis The dual-band bandpass filters (BPFs) with highseletivity and ompat size have gained wide attention in reent years and beome more and more important in modern wireless systems. In partiular, there are three typial methods to realize the dual-band property. The first method uses a single resonator with ontrollable resonant responses to design the dual-band BPFs 1-3. In ref. 1, an E-shaped resonator is proposed to realize a dual-wideband filter. In ref. 2, a ring resonator is proposed to realize the dual-band harateristi with the advantages of good isolation and upper-stopband performane. In ref. 3, one square ring resonator with two open-ended stubs is proposed to realize a dualband BPF. These filters exhibit a small iruit size, but the seletivity is not good, and the seond passband is always affeted by the first passband. The seond method ombines two sets of resonators with ommon input and output ports 4,5. These two passbands an be easily and independently ontrolled, and good insertion loss an be ahieved. However, they suffer from the overall iruit size or omplex struture. The third method uses the resonator s fundamental frequeny and the seond harmoni to realize two passbands. Open-loop resonators 6-8 and stepped-impedane resonators 9,10 have been used to design dual-band BPFs in this manner. The desired operating frequenies an be obtained by ontrolling the impedane and loop *Corresponding author ( nykim@kw.a.kr) dimensions individually. However, high insertion loss and good skirt seletivity still need to be ahieved. This paper presents a ompat dual-band BPF with low insertion, whih employs two sets of resonators. One is outer-folding open-loop resonator (OFOLR) proposed on the basis of previous work 11 with resonane at the lower passband, and the other one is triple-mode stub-loaded resonator (TMSLR), whih generates the upper passband. Two different passbands an be independently ontrolled, whih allow flexible ontrol of eah bandwidth. The oupling harateristis and frequeny responses of the proposed resonators are disussed and onfirmed by simulations. In addition, four transmission zeros loated at 1.95 GHz, 3.08 GHz, 6.04 GHz and 8.33 GHz are realized to improve the skirt seletivity and stopband suppression. Analysis of the Proposed Triple-Mode Resonator Figure 1(a) shows the shemati view of the proposed TMSLR, whih inludes two folded shape open-stubs, and the distane between two parallel open-stubs is 2s. Due to the symmetrial struture, the resonators an be analysed using the even-odd-mode analysis method. Under even-mode exitation, there is no urrent flow through the symmetrial plane 12 A A, and its equivalent iruit is shown in Fig. 1(b). The even-mode resonant frequenies, f even1 and f even2, are approximately determined by the following expressions 13 :

2 14 INDIAN J. ENG. MATER. SCI., FEBRUARY 2017 f f even1 even2 4( L L s) 1 2 4( L s) (1) (2) where is the speed of light in free spae and ε denotes the etive dieletri onstant of the substrate. Under odd-mode exitation, there is a voltage null along the symmetrial plane 12 A A, and its equivalent iruit is shown in Fig. 1(). The odd-mode resonant frequeny, f odd, is approximately expressed as 13 : f odd 4L (3) Fig. 1 (a) Shemati view of the proposed triple-mode resonator, (b) even-mode equivalent iruit and () odd-mode equivalent iruit Thus, as shown in Eqs (1)-(3), the length of L 1 hanges only f even1 without affeting f even2 and f odd, whih indiates that the length of folded stubs affets only the even-mode f even1. Aording to Eqs (1) and (2), the even-modes, f even1 and f even2, an be ontrolled by tuning s, whih reveals that the oupling between two parallel open-stubs an affet the two even-modes and has little et on the odd-mode. The simulated frequeny response of the triplemode resonator against the lengths L 1 and s is shown in Fig. 2. The relationship between L 1 and f even1 is shown in Fig. 2(a). When L 1 inreases, f even1 dereases, whereas f even2 and f odd remain unhanged. As indiated in Fig. 2(b), f even1 dereases and f even2 inreases when s inreases; nevertheless, f odd is hardly hanged. Therefore, f even1 and f even2 an be individually ontrolled, but f odd remains onstant. The simulation Fig. 2 (a) Simulated frequeny response under different length L1 and (b) simulated frequeny response under different length s

3 LIU et al.: DUAL-BAND BANDPASS FILLER 15 result demonstrates the use of the aforementioned Eqs (1)-(3) aurately. In onlusion, the even-mode an be approximately determined by the oupling between two parallel open-stubs and the length of the folded stubs, while the odd-mode is onstant. Filter Design and Experimental Results The struture of the proposed filter is shown in Fig. 3. In this design, two oupled OFOLRs and two oupled TMSLRs are ombined to format the dualband BPF. Two 50 feed lines are oupled with the outer mirostrip lines of both open loop resonators. An equivalent iruit model of the proposed BPF is built for analysis, as shown in Fig. 4 14,15. Resonators 1 and 2 represent the OFOLRs, and resonators 3 and 4 represent the TMSLRs. The proposed BPF has the apability to ahieve different passband frequenies. The OFOLRs are utilized to form the lower passband with a enterfrequeny, f 1, via eletri oupling between resonators 1 and The upper passband with a enter frequeny, f 2, is realized by the TMSLRs via another eletri oupling between resonators 3 and 4. This is onfirmed by the simulated urrent density distribution, whih ourred at two different resonane frequenies in Fig. 5. It is evident that the first resonane response is dominated by the two OFOLRs, whereas the seond passband is dominated by the TMSLRs. Two resonators are independent ontrol of passbands, frequenies, and bandwidths. Two oupled triple-mode resonators should have generated 6-pole in upper band, but only 4-pole appeared in the inset of Fig. 6, noted as P 1, P 2, P 3 and P 4. This an be explained by the frequeny response of two oupled OFOLRs and two oupled TMSLRs whih are shown in Fig. 7. For oupled TMSLRs, there Fig. 5 Simulated urrent density distributions (a) at the lower resonane frequeny and (b) at the higher resonane frequeny Fig. 3 Struture of the proposed dual-band BPF Fig. 4 Equivalent iruit model of the proposed dual-band BPF Fig. 6 The simulation and measurement results of the proposed dual-band BPF (inset Fig. (6-1) shows the enlarged piture of the triple-mode passband and (6-2) shows the photograph of the fabriated dual-band BPF)

4 16 INDIAN J. ENG. MATER. SCI., FEBRUARY 2017 are two modes, M 1 and M 2, around 4 GHz and four modes, M 3, M 4, M 5 and M 6, around 5 GHz. As to the oupled OFOLRs, two transmission zeros, TZ 1 and TZ 2, an be generated near the lower passband to improve the seletivity. Meanwhile the frequeny response an be suppressed under 50 db from 2.5 GHz to 4.5 GHz by the TZ 2 whih is shown in the enlarged Fig. 7. Therefore, the two modes around 4 GHz of oupled TMSLRs are suppressed and only four modes around 5 GHz left in upper band orresponding to the 4-pole in the inset of Fig. 6. The other two transmission zeros, TZ 3 and TZ 4, near the upper passband are separately reated by the open-stub strutures and are used to widen and deepen the upper stopband, resulting in high seletivity and stopband suppression. The proposed filter is fabriated on a Teflon substrate with a low relative dieletri onstant of 2.54, a thikness of 0.54 mm and a loss tangent of Following the preeding design proess, these dimensions are obtained: L 3 = 9.82 mm, L 4 = 11.0 mm, L 5 = 12.0 mm, L 6 = 2.05 mm, L 7 = 6.5 mm, L 8 = 6.0 mm, W 1 = 1.5 mm, W 2 = 1.0 mm, W 3 = 1.0 mm, W 4 = 0.6 mm, d 1 = 0.17 mm, d 2 = 0.18 mm, d 3 = 0.18 mm and d 4 = 0.17 mm with an overall size of mm 2. The photograph of fabriated BPF is provided in the inset of Fig. 6. The simulation and measurement results of the proposed BPF are ahieved using Sonnet eletromagneti software and an Agilent 8510C vetor network analyzer, respetively. A omparison between the simulated and measured responses is desribed in Fig. 6. These results show that the lower passband is entered at 2.40 GHz, with a low insertion loss of db, a return loss of -17 db, and a frational bandwidth (FBW) of 13.9%. The upper passband of the measured results is entered at 5.36 GHz, with a low insertion loss of db, a return loss of db and an FBW of 16.7%. In addition, four transmission zeros an be ahieved for high skirt seletivity and stopband suppression. Compared with the previous work 11, triple-mode resonators realize a wider upper band with the improved frational bandwidth (FBW) by 23%. Moreover, one more transmission zero (TZ 4 ) is produed in upper band to further improve the stopband suppression. Finally, a omparison of the present work with several published dual-band BPFs is provided in Table 1. These findings show that the proposed filter exhibits a ompat size, a low insertion loss, a high seletivity and a wide FBW. Ref. Center frequeny (GHz) Table 1 Comparison between the proposed filter and the referened filters Insertion loss (db) Return loss (db) Transmission zeros Ciruit area (mm 2 ) [6] 2.4/ /1.4 18/ /5.8 [7] 2.4/ /5.2 10/ Not given [9] 2.4/ /1.1 >18/> /2.0 [11] 2.5/ / / /13.6 Present work 2.4/ / / /16.7 FBW (%) Fig. 7 Simulated frequeny response of the two oupled TMSLRs

5 LIU et al.: DUAL-BAND BANDPASS FILLER 17 Conlusions A ompat dual-band BPF for WiMAX and WLAN appliations based on the ombination of two TMSLRs and two OFOLRs is designed and fabriated in this paper. The proposed triple-mode resonators an generate one odd-mode and two even-modes in the desired band. Even-odd-mode analysis is adopted to state the working priniple. Results show that two passbands are independently ahieved and entered at 2.4 GHz and 5.4 GHz with a low insertion loss of db and db and a wide FBW of 13.9% and 16.7%, respetively. In addition, four transmission zeros loated at 1.95 GHz, 3.08 GHz, 6.04 GHz and 8.33 GHz are realized by soure-load ouplings and open-stub strutures, and good stopband performane an be easily obtained. The ompat struture, ontrollability of passbands, high seletivity, and wide bandwidth make the presented BPF an attrative option for modern wireless systems. Aknowledgments This work was supported by a Natioal Researh Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) No R1A1A and No R1D1 A1A and Business for Cooperative R & D between Industry, Aademy, and Researh Institute funded by the Korea Small and Medium Business Administration (Grant No. C ) in Referenes 1 Wu Y L, Liao C & Xiong X Z, Prog Eletromagn Res, 108 (2010) Sun S, IEEE Mirowave Wireless Compon Lett, 21(6) (2011) Liu H, Ren B, Guan X, Lei J & Li S, IEEE Mirowave Wireless Compon Lett, 23(4) (2013) Zhang X Y & Xue Q, IEEE Trans Mirowave Theory Teh, 55(10) (2007) Deng H W, Zhao Y J, Zhang X S & Zhao W, Eletron Lett, 46(17) (2010) Dai X W, Liang C H, Wu B & Fan J W, J Eletromagn Waves Appl, 22(2) (2008) Hayati M, Noori L & Adinehvand A, Eletron Lett, 48(10) (2012) Yao Z, Wang C & Kim N Y, Mirowave Opt Tehnol Lett, 55(12) (2012) Guan X H, Fu W, Li G H, Jiang S & Liu H W, Mirowave Opt Tehnol Lett, 54(4) (2010) Hayati M & Noori L, IEICE Eletron Express, 11(5) (2014) Li Y, Wang C & Kim N Y, Mirowave Opt Tehnol Lett, 56(3) (2013) Luo X, Ma J G & Li E P, IEEE Mirowave Wireless Compon Lett, 21(7) (2011) Gao L & Zhang X Y, IEEE Mirowave Wireless Compon Lett, 23(9) (2013) Ohira M & Zhewang M A, IEICE Trans Eletron, 97(10) (2014) Hong J S & Lanaster M J, IEEE Trans Mirowave Theory Teh, 44(12) (1996)