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Aug 10, 2011 - is presented in this letter. The MMR ... The letter shows modal resonant frequencies against the ... By introducing two side stubs to the tri-mode.
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 8, AUGUST 2011

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Novel UWB Bandpass Filter Using Stub-Loaded Multiple-Mode Resonator Qing-Xin Chu, Member, IEEE, Xiao-Hu Wu, and Xu-Kun Tian

Abstract—Novel compact ultra-wideband (UWB) bandpass filter (BPF) using stub-loaded multiple-mode resonator (MMR) is presented in this letter. The MMR is constructed by loading three open stubs in a uniform-impedance resonator, i.e., one stepped-impedance stub at the center and two uniform-impedance stubs at the symmetrical side locations. Five modes, including two odd modes and three even modes, could be designed within UWB band, and two transmission zeros generated by the stepped-impedance stub improve the passband selectivity greatly. The letter shows modal resonant frequencies against the step-impedance-stub-loaded parameters, which can control the even modes flexibly, while the odd modes remain the same. A compact planar UWB BPF is simulated, fabricated and measured. The simulated and measured results are in good agreement and show good in-band filtering performance and sharp selectivity. Index Terms—Bandpass filter (BPF), compact, multiple-mode resonator (MMR), ultra-wideband (UWB).

I. INTRODUCTION

S

INCE the release of ultra-wideband (UWB) frequency spectrum covering 3.1–10.6 GHz for commercial communication application in 2002 [1], the UWB bandpass filters (BPFs), with good in-band transmission characteristics and sharp selectivity, are highly demanded. Great interests in investigating the broadband filter led to the development of multi-mode resonator (MMR) type UWB BPF [2]–[7]. In [2], a tri-mode UWB BPF using MMR of stepped-impedance configuration was originally reported, and the passband covered a frequency range from 2.96 to 10.67 GHz, but this stepped-impedance MMR-based filter suffered from a high insertion loss, narrow upper stopband as well as worse selectivity. Filter based on tri-mode electromagnetic bandgap (EBG) embedded MMR is reported to widen the upper stopband [3]. The stub-loaded MMR with four modes within UWB band was presented to improve the upper stop-band [4], [5]. Recently, UWB filters using quintuple-mode stub-loaded MMRs were presented. By introducing two side stubs to the tri-mode stepped-impedance MMR, two additional modes could be located within UWB band, and by using the open stub and Manuscript received May 25, 2011; accepted June 14, 2011. Date of publication July 08, 2011; date of currentversion August 10, 2011. This work was supported by the Science Fund of China (60801033), the State Key Laboratory of Millimeter Waves (K201112), and the Fundamental Research Funds for the Central Universities (2011IM0025). The authors are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou, China (e-mail: [email protected]; [email protected]) Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2011.2160526

Fig. 1. Proposed UWB BPF.

short stub at the center, the even modes could be tuned while the odd mode is fixed, and the selectivity is also improved [6]. In [7], the stepped-impedance stub loaded resonator was used, and the designed five-mode UWB filter showed good filtering performance and sharp selectivity, but suffered from large size. In this letter, a novel compact planar quintuple-mode MMR is presented. The MMR is constructed by loading three open stubs in a uniform-impedance resonator, i.e., one stepped-impedance stub at the center, and two uniform-impedance stubs at the symmetrical side locations. There are five modes, including two odd modes and three even modes within the desired band, and two transmission zeros generated by the stepped-impedance stub are at the lower and upper cutoff frequencies. The two odd modes could be located within the UWB band by properly designing the horizontal uniform-impedance resonator and the two side stubs. Otherwise, the even modes could be flexibly tuned by the stepped-impedance stub while the odd modes are fixed. A compact planar UWB BPF using the presented stub-loaded MMR, as shown in Fig. 1, is simulated, fabricated, and measured. Measured results are in good agreement with simulated predictions. II. STUB-LOADED MULTIPLE-MODE RESONATOR Fig. 2(a) is the basic structure of the proposed stub-loaded MMR. It is a conventional uniform-impedance resonator with three open stubs. The uniform-impedance resonator with width and length is in the horizontal position, while three open stubs are vertically loaded, i.e., one open stubs with , length at the stepped-impedance of width center and two open stubs with uniform-impedance of width , length at the symmetrical sides. Since the presented MMR is symmetrical in structure, the odd- and even-mode method could be applied to analyze it. when odd-mode The voltage is null along the plane excitation is applied, and it results in the equivalent circuit of Fig. 2(b). As it can be seen, the middle point of MMR is short circuited and the stepped-impedance open stub at the center

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 8, AUGUST 2011

Fig. 3. Resonator characteristics of the proposed MMR with varied stub length L and stub location L . Associated parameters: L L : ,W :, : ,L : ,W : ,L : ,W : (all in millimeters). W

=10

= 9 45

=08

= 3 68

+ = 11 11 =44

=05

Fig. 2. (a) Basic structure of proposed stub-loaded MMR. (b) Odd-mode equivalent circuit. (c) Even-mode equivalent circuit.

could be ignored. From the resonance condition the resonate modes of odd mode could be extracted

, (1)

For the even-mode excitation, the equivalent circuit is shown in Fig. 2(c). The resonate condition of even mode is summarized

(2) Seen in (1), the solutions which correspond to the odd mode resonances of the proposed MMR are dependent on the parameters of the horizontal uniform-impedance resonator and the two side-loaded stubs. The resonator characteristics of the proposed MMR with varied stub parameters are carried out simultaneously under weak coupling. Fig. 3 is the resonator characterisand . There are tics of the proposed MMR with varied five modes, i.e., two odd modes and three even modes, besides and are also observed. As two transmission zeros changes from 0 to 4 mm, and decrease significantly, while it has a little effect on the other three modes , and . For a much wider tunable range, the side-stub location and width could be used. Besides, the two transmission zeros which are located at the lower and upper cutoff frequencies respecor changes. tively are fixed while The solutions of (2) correspond to the even modes of the MMR. Comparing (2) with (1), it indicates that the even modes could be controlled by the parameters of the stepped-impedance varied from 1.0 to stub at the center. In Fig. 4, as stub width 7.0 mm, the first even mode frequency and the third even decrease dramatically while the two odd mode frequency and are fixed, with little change in being obmodes is unchanged served. Finally, when the total length and increases from 1.0 to 7.0 mm, tends to be lower, and becomes larger. Moreover, when the parameters of stepped-

Fig. 4. Resonator characteristics of the proposed MMR with varied stepped: ,W : ,L : , impedance stub. Associated parameters: L W : ,L L : ,W : ,L : (all in millimeters).

=10

+

= 13 13

=08

Fig. 5. Comparison of the simulated j those shown in [6] and [7].

S

j

= 8 98 = 0 5 = 2 13

= 3 35

of the proposed UWB BPF against

impedance stub is adjusted, the two transmission zeros and have the same tendency with and , respectively. It should be noted that all the stub parameters have a little impact on the mode . Since it is of uniform-impedance at the can be set to be half-wavelength horizontal position,

CHU et al.: NOVEL UWB BPF USING STUB-LOADED MULTIPLE-MODE RESONATOR

represent for 3 dB bandwidth and 30 dB where bandwidth of passband, respectively. As it can be seen, the proposed UWB BPF has sharper selectivity than those previously reported, and it has almost the same filtering performance with the one in [7], but the proposed stub-type UWB filter has a size reduction of 33.6%, compared with the one in [7]. Fig. 6 is the photograph of the fabricated filter. The substrate used is with a relative dielectric constant of 2.55 and a thickness of 0.8 mm. The dimensions optimized by IE3D are , , , , , , , , , , , , , , , , (all in millimeters). Fig. 7 is the simulated and measured results of proposed UWB BPF, and good agreement between simulated and measured results are observed. The 3 dB passband covers the range of 3.1–11.1 GHz, and it has a fractional bandwidth of 117%. The measured return loss is better than 10 dB within the UWB passband. In addition, the group delay within the UWB passband is between 0.25–0.70 ns. Owing to the two transmission zeros in the lower and upper cutoff frequencies, sharp selectivity is achieved, and the upper stopband of 20 dB level is extended to 17.1 GHz.

Table I COMPARISON WITH THE REPORTED UWB BPFS



405

is the free space wavelength at 6.85 GHz.

Fig. 6. Photograph of the fabricated UWB filter.

IV. CONCLUSION

Fig. 7. Simulated and measured frequency responses of fabricated UWB BPF.

at (4.61 GHz). Consequently, by finely designing the two side open stubs, the two modes and could be located within the desired band, and the even modes could be flexibly tuned by the stepped-impedance stub while the odd modes are fixed. The resonant frequencies of proposed MMR under weak coupling are plotted in Fig. 5. Five resonant peaks could be obviously observed, and two transmission zeros at the lower and upper cutoff frequencies are introduced by the center stub. III. SIMULATION AND MEASUREMENT RESULTS Based on the above analysis, by applying a strong feed coupling to the presented stub-loaded MMR as shown in Fig. 1, i.e., the parallel-coupled feed lines with two aperture-backed at the two sides [8], an ultra-wideband BPF is realized. Fig. 5 depicts of the proposed UWB the comparison of the simulated BPF against those shown in [6] and [7]. Table I is the comparison of proposed UWB filter with the reported ones. The “S.F.” stands for the skirt factor of UWB passband, defined as (3)

Compact planar UWB BPF using the stub-loaded MMR is presented in this letter. The MMR is constructed by loading three open stubs in a uniform impedance resonator, and five modes, including two odd modes and three even modes within the desired band are combined to realize UWB passband. The two odd modes could be located within the UWB band by properly designing the horizontal impedance resonator and the side stubs and the even modes could be flexibly tuned while the odd modes are fixed. Otherwise, two transmission zeros generated by the stepped-impedance stub are at the lower and upper cutoff frequencies, resulting in a sharp passband performance. The simulated and measured results show low passband insertion loss, good return loss and sharp selectivity. REFERENCES [1] “Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission System,” FCC, Washington, DC, Tech. Rep. ET-Docket, Apr. 2002, pp. 98–153. [2] L. Zhu, S. Sun, and W. Menzel, “Ultra-wideband (UWB) bandpass filters using multiple-mode resonator,” IEEE Microw. Wireless Compon. Lett., vol. 15, no. 11, pp. 796–798, Nov. 2005. [3] S. W. Wong and L. Zhu, “EBG-embedded multiple-mode resonator for UWB bandpass filter with improved upper-stopband performance,” IEEE Microw. Wireless Compon. Lett., vol. 17, no. 6, pp. 421–423, Jun. 2007. [4] R. Li and L. Zhu, “Compact UWB bandpass filter using stub-loaded multiple-mode resonator,” IEEE Microw. Wireless Compon. Lett., vol. 17, no. 1, pp. 40–42, Jan. 2007. [5] S. W. Wong and L. Zhu, “Quadruple-mode UWB bandpass filter with improved out-of-band rejection,” IEEE Microw. Wireless Compon. Lett., vol. 19, no. 3, pp. 152–154, Mar. 2009. [6] H. W. Deng, Y. J. Zhao, L. Zhang, X. S. Zhang, and S. P. Gao, “Compact quintuple-mode stub-loaded resonator and UWB filter,” IEEE Microw. Wireless Compon. Lett., vol. 20, no. 8, pp. 438–440, Aug. 2010. [7] Q. X. Chu and X. K. Tian, “Design of UWB bandpass filter using stepped-impedance stub-loaded resonator,” IEEE Microw. Wireless Compon. Lett., vol. 20, no. 9, pp. 501–503, Sep. 2010. [8] L. Zhu, H. Bu, and K. Wu, “Broadband and compact multi-pole microstrip bandpass filters using ground plane aperture technique,” Proc. Inst. Elect. Eng., vol. 147, no. 1, pp. 71–77, 2002.