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© 2009 Wiley Periodicals, Inc.

A QUASI-ELLIPTIC WIDEBANDBANDPASS FILTER USING A NOVELMULTIPLE-MODE RESONATORCONSTRUCTED BY AN ASYMMETRICCOMPACT MICROSTRIP RESONANTCELL

Lin Li, Zheng-Fan Li, and Qi-Fu WeiDepartment of Electrical and Electronic Engineering, ShanghaiJiaotong University, No. 800 Dongchuan Road, Shanghai 200240,China; Corresponding author: lilin_door@hotmail.com

Received 14 July 2008

ABSTRACT: This article proposes a novel wideband bandpass filterusing a novel microstrip multiple-mode resonator (MMR). This MMR iscomposed of a section of open-ended microstrip-line and an asymmetriccompact microstrip resonant cell (ACMRC) connected to the open-endedmicrostrip-line section at its central point. Its three resonant modes areemployed in this article to construct the wider passband of the proposedfilter. In addition, multiple transmission zeros brought by the ACMRC willhelp to create a quasi-elliptic response and extend the stopband. Finally, ademonstration filter is designed, fabricated and tested to show the validityof the proposed structure. Both simulated and measured results show thatthe proposed filter has a good performance. © 2009 Wiley Periodicals, Inc.Microwave Opt Technol Lett 51: 713–714, 2009; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24168

Key words: wideband bandpass filter; multiple-mode resonator (MMR);asymmetric compact microstrip resonant cell (ACMRC); quasi-elliptic

1. INTRODUCTION

As a key block of wideband wireless communication systems,microwave wideband bandpass filters (BPF) have found manyapplications in modern communication systems. Consequently,much attention has been paid to microwave wideband filter design,and several schemes have been developed recently [1–3]. How-ever, the size reduction and performance improvement of wide-band microwave filters are still a great challenge today [1–3].

In this article, we present a wideband BPF using a novelmicrostrip multiple-mode resonator (MMR) [4]. The new MMR iscomposed of a section of open-ended microstrip-line with anasymmetric compact microstrip resonant cell (ACMRC) [5, 6]tapped at the open-ended microstrip-line’s mid. With the help of

the ACMRC, three resonant modes are provided to cover a widerpassband; and significantly, multiple transmission zeros can becreated at both sides of the passband to create equasi-ellipticresponse. Finally, a wideband filter is designed, fabricated, andmeasured to validate the design principle advanced in this article.

1.1. TheoryFigure 1 illustrates the wideband BPF using the proposed MMR.The MMR is composed of a section of open-ended microstrip-lineand an ACMRC connected to the open-ended microstrip-line sec-tion at its central point. And two symmetric parallel coupled linesare employed to couple the MMR to the input/output transmissionlines.

The three resonant modes of the proposed MMR in Figure 2can be used to construct the passband of the BPF. One is thedominant mode of the half-wavelength resonator composed of theopen-ended microstrip-line. Two other modes are the dominantmode of the resonator composed of a half of the open-endedmicrostrip-line and the ACMRC, and its second-order mode, re-spectively. The resonant frequency of the half-wavelength resona-tor is mainly determined by the parameter l1, but it will vary withthe parameter W1. Actually the parameter l1 may be taken firstly asabout �g/4 at the central frequency, where �g is the waveguidewavelength of the electromagnetic wave on the microstrip-line.And when the dimensions of the open-ended microstrip-line are

Figure 1 The topology of the proposed wideband filter

Figure 2 The resonant modes of the proposed NMR

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 3, March 2009 713

given, the two other resonant frequencies of the quarter-wave-length resonator are mainly determined by the dimension of theACMRC. Therefore, the two resonant frequencies and the band-width of the BPF can be controlled easily by tuning the dimensionof the ACMRC. On the basis of the process described earlier, thewideband BPFs with different bandwidths may be designed. Be-sides, to support the desired wider stopband, a proper couplingbetween the MMR and the input/output transmission lines and agood impedance matching at both the input and the output ports ofthe filter must be provided, which can be achieved by adjusting thedimensions of the parallel coupled lines. With this design process,a wideband BPF can be realized easily.

And because of the slow-wave effect of the ACMRC, theoverall length of filter will be shortened remarkable, which con-tributes a lot to filters’ size reduction.

Significantly, just as shown in Figure 2, two zeros are observedjust beside the passband, which is brought by the ACMRC. Ob-viously, the two zeroes just beside the passband will create aquasi-elliptic response, which upgrade the proposed filters’ perfor-mance drastically without sacrificing the proposed filters’ pass-band performance.

2. RESULTS AND DISCUSSION

On the basis of the abovementioned analysis, a microstrip wide-band BPF is designed and fabricated. The filter is fabricated on a1.5-mm-thick dielectric substrate with a relative dielectric constantof 2.65. The correlative parameters are shown as below: W1 � 0.8mm, l1 � 19 mm, W2 � 0.8 mm, l2 � 19 mm, W31 � 0.25 mm,W32 � 4.25 mm, l31 � 1.5 mm, l32 � 2 mm, l33 � 2 mm, W41 �0.25 mm, W42 � 10.25 mm, l41 � 2.8 mm, l42 � 5 mm, l43 � 5mm, gap � 0.25 mm. And the photograph of the fabricated filteris displayed in Figure 3.

Figure 4 shows the measured results for the proposed filter,together with the EM simulation results. The simulated and mea-sured results display a good agreement. The filter has a centerfrequency of 2.86 GHz. Measured data show that its passband isfrom 1.8 to 3.95 GHz, which indicates its relevant fractionalbandwidth of about 75%. The minimum insertion loss in thepassband is 0.6 dB, whereas the simulated one is lower than 0.2dB. This insertion loss would be mainly due to the effect of theinput/output subminiature A connectors, and to the dielectric lossof the substrate as well as the resistance loss of the metal strip.

Significantly, four transmission zeros are observed at around1.59 GHz, 4.735 GHz, 5.455 GHz, and 6.715 GHz, respectively.At all the four zeros over 40 dB attenuation can be provided.Further, the transmission zeros can contribute to provide a sharptransition from the passband to the stopband, thus a quasi-ellipticresponse is realized. Meanwhile, with the help of the multiplezeros, the BPF has a high out-of-band rejection level which isbelow 20.0 dB over the frequency range of 0–1.7 GHz, and below30 dB over 4.3–6.8 GHz.

3. CONCLUSION

This article presents a novel microstrip MMR composed of asection of open-ended microstrip-line with an ACMRC tapped atthe mid. Its three resonant modes are used to construct the pass-band of a wideband BPF. The design principle of the widebandBPFs using this new proposed MMR is discussed and a simpledesign sample is provided to demonstrate the filter’s performance.Both measured and simulated results show that the BPF has a goodperformance including a low insertion loss, a sharp transition fromthe passband to the stopbands, and a high out-of-band rejectionlevel. The BPFs may be applied in the design of microwavecircuits with a wide operation frequency band and UWB commu-nication systems.

REFERENCES

1. J.T. Kuo and E. Shih, Wideband bandpass filter design with three-linemicrostrip structures, Proc Inst Electron Eng 149 (2002), 243–247.

2. L.H. Hsieh and K. Chang, Compact, low insertion-loss, sharp-rejection,and wide-band microstrip bandpass filters, IEEE Trans MicrowaveTheory Tech 51 (2003), 1241–1246.

3. L. Zhu, H. Bu, and K. Wu, Broadband and compact multi-pole micro-strip bandpass filters using ground plane aperture technique, Proc InstElectron Eng 149 (2002), 71–77.

4. L. Zhu, S. Sun, and W. Menzel, Ultra-wideband (UWB) bandpass filtersusing multiple-mode resonator, IEEE Microwave Wireless ComponLett 15 (2005), 796–798.

5. K.M. Shum, T.T. Mo, Q. Xue, and C.H. Chan, A compact bandpassfilter with two tuning transmission zeros using a CMRC resonator, IEEETrans Microwave Theory Tech 53 (2005), 895–900.

6. Q. Xue, K.M. Shun, and C.H. Chan, Novel 1-D microstrip PBG cells,IEEE Microwave Wireless Compon Lett 10 (2000), 403–405.

© 2009 Wiley Periodicals, Inc.Figure 3 The photograph of the fabricated filter

Figure 4 The simulated and measured results for the fabricated filter

714 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 3, March 2009 DOI 10.1002/mop