A New Compact Tri-Band Bandpass Filter Using Step Impedance Resonators with Open Stubs

This article was downloaded by: [National Kaohsiung University of Applied Sciences]

On: 31 May 2013, At: 00:29 Publisher: Taylor & Francis

Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Electromagnetic Waves

and Applications

Publication details, including instructions for authors and subscription information:

http://www.tandfonline.com/loi/tewa20

A New Compact Tri-Band Bandpass

Filter Using Step Impedance

Resonators with Open Stubs

a

Institute of Photonics and Communications, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan;, Email: [email protected]

b

Institute of Photonics and Communications, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan

Published online: 13 Apr 2012.

To cite this article: S.-K. Liu & F.-Z. Zheng (2012): A New Compact Tri-Band Bandpass Filter Using Step Impedance Resonators with Open Stubs, Journal of Electromagnetic Waves and Applications, 26:1, 130-139

To link to this article: http://dx.doi.org/10.1163/156939312798954856

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or

howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

J. of Electromagn. Waves and Appl., Vol. 26, 130–139, 2012

A NEW COMPACT TRI-BAND BANDPASS FILTER USING STEP IMPEDANCE RESONATORS WITH OPEN STUBS

S.-K. Liu* and F.-Z. Zheng

Institute of Photonics and Communications, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan

Abstract—A new tri-band bandpass filter (BPF) designed based on

two direct-coupled stepped impedance resonators (SIRs) with open stubs is proposed. Firstly, the resonant phenomenon of the stepped impedance resonator with open stubs is discussed. By properly controlling the length ratio of the SIRs and the length ratio of the stub, the center frequencies of the three passbands are respectively designed at 2.49 GHz, 3.60 GHz and 5.62 GHz, corresponding to the WLAN and WiMAX applications. Furthermore, the external quality factor and coupling coefficients are also obtained for the optimum design. Experimental results show good agreement with the simulated ones.

1. INTRODUCTION

Recently, there has been a fast-growing demand for multi-service technology in wireless communication systems. The mainstream applications are focused on the wireless local area network (WLAN) and the worldwide interoperability for microwave access (WiMAX) which correspond to IEEE 802.11b/g (2.4 GHz) and IEEE 802.11a (5.2–5.8 GHz), and IEEE 802.16-2004 (2.5/3.5/5.8 GHz) standards, respectively. In the systems, a multi-band bandpass filter (BPF) served in the radio frequency (RF) front end is required to remove unwanted frequencies and to obtain the desired signals. Several design methods such as ring resonator, uniform impedance resonator (UIR), defected ground structure (DGS), crossed resonator, stepped-impedance resonator (SIR) and stub-loaded resonator (SLR) are popular for the design of multi-band BPF [1–11]. In [1], a single periodic stepped-impedance ring structure in circular graph was used

Received 9 October 2011, Accepted 4 December 2011, Scheduled 12 December 2011 * Corresponding author: Shih-Kun Liu ([email protected]).

Compact tri-band BPF using SIR with open stubs 131

to design a dual-band BFP. Two open stubs in one of the tested cases were used to tune the frequencies for the desired passband properties, which complicates the design procedure. In [2], composite right/left-handed (CRLH) transmission lines were cooperated into ring resonators to build the dual-band BFP. The application of the filter could be hammered by its relatively large size. In [3], two ring resonators were used to construct a balanced single- and dual-band BFPs. One of the ring resonators has two open stubs. For the dual-band BFP, the two resonators have to be designed as stepped-impedance which complicates the design procedures. In [4], the SIRs were implemented in multilayered structure to construct the dual-band BPFs with narrow and wide passdual-bands simultaneously However, the two-layer structure complicates the fabrication process. For this reason, single layer design is employed to simplify the fabrication of the multi-band BPFs [5–11]. In [5], the dual-band BPF was implemented by four modified UIRs where the UIRs control the first passband and their special coupling structures tune the second passband. On the other hand, the SIR by its nature can directly control the two passbands of the dual-band BPF and thus reduces the number of resonators required for the BPFs [6–8]. In tri-band BPFs, more resonators with different types are required for tri-band operation [9– 11]. In [9], crossed resonators were used to design a tri-band BPF. The first and the third passbands can be tuned by controlling the lengths of the stubs. All of the passbands have low fractional bandwidth of about 3%. In [10], the SIRs were used along with the DGS design to construct a tri-band BPF. Since the DGS was designed on the ground plane of the substrate, the fabrication is complex. In [11], a tri-band BPF used four resonators including a pair of UIRs and SLRs, and UIRs with an interdigital capacitor, causing design complexity.

In this paper, we propose a simple method to achieve a new compact tri-band filter designed based on two direct-coupled stepped impedance resonators with open stubs for the first time as shown in Fig. 1. The resonant behavior of the SIRs in the tri-band BPF is analyzed and discussed. The three passbands of the filter can be assigned with more design freedom by appropriately selecting the length ratio of the SIR (R1) and the length ratio of the stub (R2). The external quality factors (Qe) and the fractional bandwidth (FBW ) for each resonant frequency of the filter is also discussed.

2. DESIGN PROCEDURE

The characteristics of the proposed filter are determined by the SIR resonators and the open stubs. Fig. 2 depicts the unit cell of the

132 Liu and Zheng L1 L2 L3 W1 L4 L5 W2 W3 G1 L6 T1 Por t1 Por t2 L1 L2 L3 W1 L4 L5 W2 W3 G1 L6 T1 Por t1 Por t2

Figure 1. Configuration of the

proposed direct-coupled tri-band BPF. 2 Z₂ Z₂ Z_S 2 1 1 θS Z₁ Z₁ 2 Z₂ Z₂ Z_S 2 1 1 S Z₁ Z₁ θ θ θ θ

Figure 2. Geometry of the unit

cell that composes a λ/2 SIR with an open stub.

λ/2 SIR with an open stub. The cell is composed of a SIR with two

impedance discontinuities and an open stub located on the symmetric plane of the SIR. The resonant condition of the cell can be studied by analyzing the characteristic impedance of the cell. The electrical length and the impedance of the open stub are denoted as θs and Zs, respectively. Similarly, the electrical length and the impedance of the SIR are denoted respectively as θiand Zi, where the subscripted index

i = 1, 2 for the two distinct sections of the SIR illustrated in the figure.

The input characteristic admittance of the cell can be obtained as

Yin= _Z1 2

Z1(K1− tan θ1tan θ2) + jZL(K1tan θ1+ tan θ2)

ZL(1− K1tan θ1tan θ2) + jZ1(tan θ1+ K1tan θ2)

, (1)

where the load impedance ZL is expressed as

ZL= _K jZ1K2(tan θ1tan θ2− K1)

1(K2tan θ1− tan θs) + tan θ2(tan θ1tan θs+ K2)

, (2)

and K1 = Z2/Z1 and K2 = Zs/Z1. The resonant modes of the cell can be calculated by setting Yin = 0. As a result, the odd- and even-mode resonances of the cell are obtained by respectively solving the equations:

tan θ1tan θ2− K1 = 0, (3)

and

K1(2K2tan θ1+ tan θs) + tan θ2(2K2− tan θ1tan θs) = 0. (4) To determine the three passbands, the length ratio R1 of the SIR and the length ratio R2 of the stub are adjusted so that the higher order resonant modes can be tuned over a wide frequency range. The two length ratios R1 and R2 are respectively defined as

R1 =

θ2

θ1+ θ2 = 2θ2

θT , (5)

Compact tri-band BPF using SIR with open stubs 139

2. Garcia-Perez, O., L. E. Garcia-Munoz, D. Segovia-Vargas, and V. Gonzalez-Posadas, “Multiple order dual-band active ring filters with composite right/left handed cells,” Progress In

Electromagnetics Research, Vol. 104, 201–219, 2010.

3. Lee, C.-H., C.-I. G. Hsu, H.-H. Chen, and Y.-S. Lin, “Balanced singled- and dual-band BPFs using ring resonators,” Progress In

Electromagnetics Research, Vol. 116, 333–346, 2010.

4. Weng, M.-H., S.-K. Liu, H.-W. Wu, and C.-H. Hung, “A dual-band bandpass filter having wide and narrow bands simultaneously using multilayered stepped impedance resonators,”

Progress In Electromagnetics Research Letters, Vol. 13, 139–147,

2010.

5. Yang, R.-Y., H. Kuan, C.-Y. Hung, and C.-S. Ye, “Design of dual-band dual-bandpass filters using a dual feeding structure and embedded uniform impedance resonators,” Progress In Electromagnetics

Research, Vol. 105, 93–102, 2010.

6. Huang, C.-Y., M.-H. Weng, C.-S. Ye, and Y.-X. Xu, “A high band isolation and wide stopband diplexer using dual-mode stepped-impedance resonators,” Progress In Electromagnetics Research, Vol. 100, 299–308, 2010.

7. Wu, Y.-L. C. Liao, and X.-Z. Xiong, “A dual-wideband bandpass filter based on E-shaped microstrip SIR with improved upper-stopband performance,” Progress In Electromagnetics Research, Vol. 108, 141–153, 2010.

8. Ma, D.-C., Z.-Y. Xiao, L.-L. Xiang, X.-H. Wu, C.-Y Huang, and X. Kou, “Compact dual-band bandpass filter using folded SIR with two stubs for WLAN,” Progress In Electromagnetics

Research, Vol. 117, 357–364, 2011.

9. Chu, Q. X., F. C. Chen, Z. H. Tu, and H. Wang “A novel crossed resonator and its applications to bandpass filters,” IEEE Trans.

Microw. Theory Tech., Vol. 57, 1753–1759, 2009.

10. Lai, X., C.-H. Liang, H. Di, and B. Wu, “Design of tri-band filter based on stub loaded resonator and DGS resonator,” IEEE

Microw. Wireless Compon. Lett., Vol. 20, No. 5, 265–267, 2010.

11. Zhang, X. Y., Q. Xue, and B. J. Hu, “Planar tri-band bandpass filter with compact size,” IEEE Microw. Wireless Compon. Lett., Vol. 20, No. 5, 262–264, 2010.

12. Pozar, D. M., Microwave Engineering, 2nd edition, Wiley, New York, 1998.