736 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 11, NOVEMBER 2014

A High-Selectivity Tunable Dual-BandBandpass Filter Using Stub-LoadedStepped-Impedance Resonators

Bin You, Long Chen, Yaping Liang, and Xuan Wen

Abstract—A novel varactor tunable dual-band bandpass filter(BPF) using stub-loaded stepped-impedance resonators is pro-posed in this letter. Compared with the traditional tunablefilters, the source-load coupling and T-shape stub-loaded linesare employed in this design. The proposed BPF architecture hasthe advantages of high selectivity and less control voltages. Inthe overall tuning range, the proposed filter is designed with5–6 transmission zeros and more than 30 dB rejection betweenthe two passbands. Meanwhile, only one control voltage is neededfor each passband. A prototype of this filter is fabricated andmeasured. The measurement results show great agreement withsimulated results, which show that the first passband can betuned in a frequency range from 0.8 to 1.02 GHz, and the secondpassband varies from 2.02 to 2.48 GHz.

Index Terms—Dual-band, high selectivity, stub-loaded res-onators, tunable bandpass filter (BPF).

I. INTRODUCTION

E LECTRONICALLY tunable/reconfigurable filters areone of the most essential microwave components for

multi-band communication systems due to their potential forsize and complexity reduction. Recently, in exploring theadvanced multi-band wireless systems, tunable bandpass filters(BPF) for RF devices have become quite popular [1]–[3].However, there still exist many unsolved problems for thesereported tunable BPFs. The tunable BPFs in the past publica-tions usually do not have enough signal selectivity for moderncommunication systems. Meanwhile, dual-band tunable filtersrequire multiple dc bias voltages [4]–[6], which increase thecomplexity of the circuits and the difficulty of tuning. Based onthe above issues, a high-selectivity tunable dual-band bandpassBPF using stub-loaded stepped-impedance resonators (SL-SIR)is firstly proposed, as shown in Fig. 1. The proposed filter struc-ture offers independent dual-passband characteristics withimproved stopband characteristics and less control voltages.

Manuscript received June 04, 2014; accepted July 31, 2014. Date of publica-tion August 29, 2014; date of current version November 04, 2014. This workwas supported by Zhejiang Province Natural Science Foundation of China undergrant LY14F010020 and the Oversea Returnee Research Project Fund.B. You, L. Chen, and Y. Liang are with the Circuits and Systems Key Lab-

oratory of the Ministry of Education, Hangzhou Dianzi University, Hangzhou,Zhejiang, China.X. Wen is with the Nokia Siemens Networks Technol. (Beijing) Co. Ltd.,

Hangzhou Zhejiang, China.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.2014.2348322

Fig. 1. Structure of the proposed tunable dual-band BPF.

II. DESIGN OF A MULTI-TRANSMISSION ZEROS SL-SIR FILTER

The passive microstrip structure of the proposed tunable filtercan be regard as a fixed multi-transmission zeros (TZ) SL-SIRfilter. SL-SIRs have been used to design the multi-band BPFs orthe tunable BPFs for their advantages of small size and multi-mode characteristics [6], [7]. In this letter, we proposed a novelimproved SL-SIR and its application in high-selectivity tun-able dual-band BPF design. As shown in Fig. 2 compared withthe traditional tri-mode SL-SIR, the source-load coupling andT-shape stub-loaded lines are employed in this design. The tri-mode SL-SIRs with one open stub and one short stub have an in-herent TZ in the upper stopband, while the source-load couplingare adopted to generate two more TZs, therefore, total six TZsare realized in this dual-band BPF design to improve the selec-tivity. The strength of the source-load coupling can be used totuned the frequency of TZs. The T-shape stub-loaded lines areutilized to reduce the number of control voltages which will beintroduced in the next section. Similar to the stub-loaded res-onators in [8], odd- and even-mode analysis can be used forcharacterizing the resonator. The proposed SL-SIR has threeresonant modes: , , and . can be totallycontrolled by , , and . When is fixed,and are mainly controlled by the short stub and theopen stub, respectively. In practice, the short stub realized bya through hole in this filter. The three resonant modes have tosatisfy the size condition as . The ad-mittance ratios, which can be expressed as , are used in allthree resonant modes to reduce the resonator’s size and widenthe upper stopband bandwidth. The parasitic passband would bemoved more than three times away from the original center fre-quency by designing [9].To prove the above characteristics, a 1.05 GHz/2.7 GHz

high-selectivity dual-band BPFs with absolute-bandwidths of

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YOU et al.: HIGH-SELECTIVITY TUNABLE DUAL-BAND BPF USING SL SIRS 737

Fig. 2. Microstrip circuit models (without lumped elements).

TABLE ITHE DIMENSIONS OF THE PROPOSED FILTER (UNIT: mm)

Fig. 3. Simulated and measured results of the multi-TZs SL-SIR filter.

170 MHz/240 MHz is designed and fabricated. The substratewe used in this work is the Rogers RT/Duriod 6010 with

, and . Thedimensions are listed in Table I. The S-parameter simulationand measurement results are shown in Fig. 3. The first passbandwith the center frequency of 1.05 GHz has less than 0.9 dBinsertion loss and greater than 20 dB return loss. The secondpassband with the center frequency of 2.7 GHz has less than1.1 dB insertion loss and greater than 18 dB return loss. In ad-dition, the six TZs provide a better cutoff rate in the stopband.

III. DESIGN OF A TUNABLE DUAL-BAND FILTER

For this tunable filter design, total eight identical varactorsare loaded onto the open-ends, as shown in Fig. 1. The tradi-tional stub-loaded tunable filters usually need two types of con-trol voltages for each passband [4]–[6]. In this tunable filter de-sign, the T-shape open stubs are utilized, which makes the pro-posed filter only need one control voltage for each passband. Tosimplify the analysis, is assumed and filter B, as shownin Fig. 1, is chosen as an example. , are the total capac-itance for series connection of dc block and , , respec-tively. For the odd- and even-mode resonant condition, and

can be expressed as

(1)

Fig. 4. Calculation results of against B and R.

Fig. 5. Simulated results of SL-SIR against C. (a) With T-shape open stub.(b) Traditional design.

(2)

where is the phase velocity, and. If we remove all varactors, the proposed

tunable filter become a fixed filter which mentioned above. Weassume the fixed center frequency is and the fractional band-width (FBW) is B. When we add the varators, we define

, which is associated with the tunable elements. Then, , , can be expressed as , ,, , respectively. When the center frequency

is tuned, the ratio of to can be derived as

(3)

According to (3), the ratio of to is only influ-enced by B and R. In Fig. 4, based on (3), the results shows

in narrow-band filters. It proves that the pro-posed tunable filter only need one control voltage for eachpassband if the FBW is narrow enough. Simulated results inFig. 5 show that the proposed SL-SIR is tuned well with onlyone control voltage, while the traditional one is not.A high-selectivity tunable dual-band BPF using the SL-SIRs

is fabricated and measured. The passive microstrip structureis as same as the one mentioned above. The variable capaci-tances are implemented by the JDV2S71E varactors. As shown

738 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 11, NOVEMBER 2014

Fig. 6. Simulated andmeasured results of the proposed tunable dual-band filter.Lower passband frequency is unchanged. , , 7 V, 20 V.

Fig. 7. Simulated, measured results and the photograph of the proposed tunabledual-band filter. Higher passband frequency is unchanged. , 4.4 V, 20 V,

.

in Fig. 1, in filter A, the RF chokes are 22 nH. In filter B, theRF chokes are 100 nH. All dc blocks are 1 pF. Fig. 6 and Fig. 7plot the simulated and measured results of the tunable dual-bandBPF. In Fig. 6, it can be seen the second band frequency varyingfrom 2.02 to 2.48 GHz while the first band frequency is un-changed and fixed at 1.02 GHz. In Fig. 7, it can be seen thefirst band frequency varying from 0.80 to 1.02 GHz while thesecond band frequency is unchanged and fixed at 2.48 GHz. In

TABLE IICOMPARISON WITH PRIOR DUAL-BAND TUNABLE BPFS(FT DENOTES

FREQUENCY TUNING RANGE, CV DENOTES TYPES OF CONTROL VOLTAGES,A DENOTES INDEPENDENTLY TUNABLE PASSBAND

the overall tuning range, the measured passband return lossesare all better than 15 dB and the passband insertion losses are1.12–2.93 dB and 1.45–4.89 dB, respectively. Moreover, themeasured results show that the maximum passband center fre-quency ratio is about 3.1. We have not seen a larger center fre-quency ratio in the previous publications. It shows a large fre-quency shift of the harmonic bands of the first passband is ob-tained. The performance comparisons of the proposed tunableBPF with other tunable BPFs are summarized in Table II. Com-pared to previously filter designs, this filter design can provideindependently tunable dual-band passbands characteristics withhigh selectivity and less control voltages.

IV. CONCLUSION

This letter presented a novel varactor tunable dual-band BPFusing SL-SIRs. The source-load coupling are employed to im-prove the selectivity. In the overall tuning range, the proposedfilter is designed with 5–6 transmission zeros and more than30 dB rejection between two passbands. This letter presents thebrief analysis of a novel T-shape open stub, which can reducethe types of control voltages in a narrow band tunable filter de-sign. Both the simulated and measured results indicate that theproposed tunable dual-band bandpass filter has the advantagesof high selectivity and less control voltages.

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