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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 8, AUGUST 2006

Capacitive-Ended Interdigital Coupled Lines for UWB Bandpass Filters With Improved Out-of-Band Performances Sheng Sun, Student Member, IEEE, and Lei Zhu, Senior Member, IEEE

Abstract—This letter presents a novel ultra-wideband (UWB) microstrip bandpass filter on a microstrip line with improved out-of-band performances. A multiple-mode resonator (MMR) is first constituted to equally allocate its first three resonant frequencies in the 3.1–10.6-GHz UWB band. Two capacitive-ended interdigital coupled lines are then formed to assign their transmission zero towards the fourth-order resonant frequency of this MMR, thereby suppressing the first spurious passband. Moreover, two outer arms in the interdigital lines are properly tapered to compensate the phase imbalance or group delay near the UWB upper-end relying on extra capacitive-ended stubs. And finally, two UWB filters with one- and two-MMRs are designed and implemented to experimentally demonstrate the improved out-of-band performances, i.e., widened/deepened upper-stopband and sharpened rejection skirts outside the UWB passband.

Fig. 1. Schematics of the three MMR-based UWB BPFs. (a) Filter-A. (b) Filter-B. (c) Filter-C.

Index Terms—Bandpass filter (BPF), interdigital coupled line, multiple-mode resonator (MMR), out-of-band, ultra-wideband (UWB).

I. INTRODUCTION

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ITH the recent tremendous progress in ultra-wideband (UWB) technology, it has become very imperative to explore various planar bandpass filters (BPFs) with specified ultrawide passbands. Since the UWB mask was defined in the U.S. as a 3.1–10.6-GHz band in 2002 [1], extensive work has been reportedly made so far to make up a variety of planar UWB BPFs using several unique approaches, e.g., nonperiodical shunt-stub loading [2], [3], composite lowpass–highpass topology [4], [5], cascaded broadside-coupling [6], and multiple-mode resonator (MMR) [7]–[9]. Of them, the MMR-based UWB BPF is initiated in [7] and it is constructed by allocating the three resonant frequencies of the MMR into the concerned UWB passband while setting up the coupling peak of quarter-wave parallelcoupled lines at the center frequency, i.e., 6.85 GHz. Following this simple idea, the three types of planar UWB BPFs have been implemented on the conventional microstrip-line [7], aperturebacked microstrip-line [8], and hybrid microstrip/CPW structures [9]. These UWB filters show excellent bandpass behaviors of insertion loss less than 0.5 dB and group delay variation less than 0.25 ns in the whole UWB passband as confirmed in the experiment. As with any other transmission-line based BPFs, these UWB filters unfortunately suffer from spurious harmonic passbands

Manuscript received January 25, 2006; revised April 27, 2006. The authors are with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (e-mail: [email protected]. sg). Digital Object Identifier 10.1109/LMWC.2006.879492

Fig. 2. Frequency-dependent transmission responses of the constituted MMR 10.8 and h 1.27 mm under weak coupled on the RT/Duroid 6010 with " excitation.

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above the dominant UWB passband, thus producing an unexpected narrow upper-stopband, e.g., 11.2 to 13.8 GHz reported in [7]. In addition, a single-stage MMR-based UWB filter exhibits unsatisfactory out-of-band rejection skirts below and above the UWB passband, whereas the coupling degree of parallel-coupled lines [7] is usually not sufficiently strong as requested in exploring a UWB filter on a microstrip line with a high return loss in the UWB passband. In this letter, an improved class of UWB BPFs is presented by driving the MMR resonator with interdigital coupled lines of enhanced coupling degree. Fig. 1(a)–(c) depicts the schematics of the three UWB filters to be considered, namely, filter-A, filter-B, and filter-C. First of all, a MMR resonator needs to be constructed to quasi-equally allocate the first three resonant freand , in the interested UWB passband quencies, as denoted in Fig. 2. In this way, the fourth resonant frequency, , near 13.7 GHz unfortunately contributes to the lowest spurious passband in the resultant UWB filter. Second, interdigital

1531-1309/$20.00 © 2006 IEEE

SUN AND ZHU: CAPACITIVE-ENDED INTERDIGITAL COUPLED LINES

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Fig. 3. Layouts of the three interdigital microstrip coupled lines: (a) Type-A, (b) Type-B, and (c) Type-C.

Fig. 5. Comparison among the predicted S -parameter magnitudes and group delays of the three proposed UWB BPFs in Fig. 1(a)–(c).

Fig. 4. Predicted frequency responses of the three interdigital coupled-lines in Fig. 3.

coupled lines of enhanced degree are used to make up Filter-A with increased return loss in the UWB passband and their modified counterparts with capacitive-ended loading and/or tapered strip shape are then formed to reallocate their first transmission as mentioned in [10] so as to make up Filter-B and zero to Filter-C, respectively. After comparative study is executed on electrical performances of these three single-stage UWB filters, two UWB filters with one- and two-stage MMR resonators are designed, implemented, and measured. II. CHARACTERIZATION OF THREE PROPOSED UWB FILTERS Fig. 3(a)–(c) shows the three types of microstrip interdigital or double-parallel coupled lines, namely Type-A, Type-B, and Type-C. The Type-A structure is expected to achieve much tighter coupling degree than the conventional parallel coupled line used in the initial UWB filter [7]. The latter two structures with capacitive-ended loading are formed to allocate their . coupling zero to the fourth-order resonant frequency, Fig. 4 plots the frequency-dependent scattering parameters of these coupling structures via Agilent ADS software. As compared with the coupled line of Type-A, the coupling zero in the type-B moves down from 14.4 to 13.5 GHz as the bottom and top strip arms are capacitively terminated by open-ended stubs. To achieve the balanced UWB passband behavior, the

two strip arms with tapered configuration are properly formed in the Type-C structure. In this respect, the two transmission poles within the passband can be compensated in a balanced way while the coupling zero still keeps at 13.5 GHz, as can be seen in Fig. 4. Fig. 5(a) and (b) represents the simulated -parameter magnitudes and group delays of the three proposed UWB filters with a single MMR resonator, as shown in Fig. 1, in association with the three coupling structures in Fig. 3. With the help of the tightened coupling degree of the double-parallel coupled lines, in the UWB passband, Filter-A achieves an almost flat frequency response of insertion loss close to 0 dB and its return loss is entirely raised above 20 dB as compared to 10 dB in the initial UWB filter [7]. In Filter-B, the first spurious harmonic near 13.5 GHz is totally removed due to transmission zero reallocation [10] of the installed capacitive-ended stubs. However, its return loss in the high end of the UWB passband unexpectedly drops off from 22 dB (Filter-A) to 12 dB (Filter-B). Also, the upper maximum in group delay rises up from 0.30 to 0.37 ns around 10.6 GHz, implying that the phase linearity of this filter becomes worsened. By forming the tapered strip arms in Filter-C, the return loss improves, especially in the upper side of UWB passband, and its value in the whole UWB band is now above 22 dB again as shown in Fig. 5(a). More importantly, the relevant upper maximum in group delay moves down to 0.30 ns as can be observed from Fig. 5(b). Looking at the three curves in Fig. 5(b) together, the group delay in Filter-A and Filter-C varies in between 0.20 and 0.30 ns, thus achieving a maximum variation of 0.10 ns that is smaller than 0.25–0.30 ns reported in [7]–[9].

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 8, AUGUST 2006

Fig. 6. Predicted and measured results of single-stage UWB BPF. Fig. 7. Predicted and measured results of two-stage UWB BPF.

III. IMPLEMENTATION AND EXPERIMENTAL VERIFICATION In this section, two UWB filters are implemented on the 1.27 mm RT/Duroid 6010 with a substrate thickness of and dielectric constant of 10.8. Fig. 6(a) depicts the photograph of the fabricated one-stage UWB filter, i.e., Filter-C in Fig. 1(c). Fig. 6(b) shows the predicted and measured frequency responses of -parameter magnitudes and group delays. They are found in very reasonable agreement, confirming that the proposed UWB filter is capable of reducing the insertion loss within the UWB passband and also widening the upper-stop bandwidth. The measured return and insertion losses are found to be higher than 14 dB and lower than 1.3 dB over the UWB passband, respectively. Meanwhile, the measured group delay slightly varies between 0.20 to 0.30 ns as predicted. Fig. 7(a) is the photograph of a fabricated UWB BPF with two MMR resonators, which are coupled together via a parallel-coupled line. This filter is constituted herein in order to further improve the out-of-band performances in both lower and upper stop-bands. Fig. 7(b) shows the predicted and measured results. In the UWB passband, the two return losses keep higher than 14 dB and the group delay varies between 0.35 and 0.65 ns. Outside the UWB passband, the lower- and upper-band skirts get sharpened to a great extent while the upper-stopband with the insertion loss above 20 dB occupies an enlarged range of 11.8 to 15.9 GHz. Discrepancy between measured and simulated results may be caused by approximate wide-band calibration and parasitic effects of the SMA connector. IV. CONCLUSION A novel MMR-based UWB BPF with improved out-of-band performance is presented and implemented in this letter. By properly forming two capacitive-ended interdigital coupled lines and linking them with a single MMR resonator in two sides, a single-stage UWB filter is initially constructed and its

performance is extensively investigated in theory to demonstrate its capability in suppression of first-order spurious passband as confirmed in the experiment. Moreover, a two-stage UWB filter is designed and fabricated to prove its improved in-band and out-of-band performances with reduced insertion loss in the passband, sharpened rejection skirts outside the passband, and widened/deepened upper-stopband. REFERENCES [1] FCC, “Revision of part 15 of the commission’s rules regarding ultra-wideband transmission systems,” Tech. Rep. ET-Docket 98–153, FCC02–48, Federal Communications Commission, Apr. 2002. [2] H. Ishida and K. Araki, “Design and analysis of UWB bandpass filter,” in Proc. IEEE Topical Conf. Wireless Comm. Tech., Oct. 2003, pp. 457–458. [3] J.-S. Hong and H. Shaman, “An optimum ultra-wideband microstrip filter,” Microw. Opt. Technol. Lett., vol. 47, no. 3, pp. 230–233, Nov. 2005. [4] C.-L. Hsu, F.-C. Hsu, and J.-T. Kuo, “Microstrip bandpass filters for ultra-wideband (UWB) wireless communications,” in IEEE MTT-S Int. Dig., , Jun. 2005, pp. 679–682. [5] W. Menzel, M. S. R. Tito, and L. Zhu, “Low-loss ultra-wideband (UWB) filters using suspended stripline,” in Proc. 2005 Asia-Pacific Microw. Conf., Dec. 2005, vol. 4, pp. 2148–2151. [6] K. Li, D. Kurita, and T. Matsui, “An ultra-wideband bandpass filter using broadside-coupled microstrip-coplanar waveguide structure,” in IEEE MTT-S Int. Dig., Jun. 2005, pp. 675–678. [7] 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. [8] L. Zhu and H. Wang, “Ultra-wideband bandpass filter on aperturebacked microstrip line,” Electron. Lett., vol. 41, no. 18, pp. 1015–1016, Sep. 2005. [9] H. Wang, L. Zhu, and W. Menzel, “Ultra-wideband (UWB) bandpass filters with hybrid microstrip/CPW structure,” IEEE Microw. Wireless Compon. Lett., vol. 15, no. 12, pp. 844–846, Dec. 2005. [10] S. Sun and L. Zhu, “Periodically nonuniform coupled microstrip line filters with harmonic suppression using transmission zero reallocation,” IEEE Trans Microw. Theory Tech., vol. 53, no. 5, pp. 1817–1822, May 2005.