Compact Filtering Power Divider With Enhanced ... - IEEE Xplore

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 9, SEPTEMBER 2013

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Compact Filtering Power Divider With Enhanced Second-Harmonic Suppression Xiu Yin Zhang, Senior Member, IEEE, Kai-Xu Wang, and Bin-Jie Hu, Senior Member, IEEE

Abstract—This letter presents a compact power divider integrated with high-selectivity bandpass resoponses. A novel topology is proposed to integrate five resonators and a resistor to realize the dual functions of the power division and filtering. Two transmission zeros are generated near the passband edges and another one is created at the second harmonic frequency which enhances the rejection levels. For validation, a filtering power divider operating at 920 MHz is implemented with more than 20 dB isolation. The circuit size is , featuring compact size. Index Terms—Bandpass filter (BPF), compact size, integration, power divider.

I. INTRODUCTION

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OWER dividers and bandpass filters (BPFs) are indispensable building blocks in many RF front-ends. These two components occupy a considerable area, especially at low gigahertz frequency region. Thus, it is beneficial to integrate them for size reduction. Recently, some integrated designs were proposed. There are three typical design methods. One method is to cascade the filtering circuit with power dividers. The filtering structure is cascaded with the T-junction [1] or embedded in the power divider [2], resulting in dual functions of power dividing and filtering. The second one is to replace the quarter-wavelength transformers in Wilkinson power dividers with BPFs [3]–[6]. Besides the two methods, the filtering and power splitting circuits can be merged together to obtain dual functions [7], [8]. In this letter, a novel filtering power divider with enhanced second-harmonic suppression is proposed. The circuit consists of a half-wavelength resonator and four quarter-wavelength resonators as well as an isolation resistor. The resistor is added between the two resonators to get good isolation and matching performance. Mixed electric and magnetic coupling as well as cross coupling is realized among the resonators that can generate two transmission zeros near the passband edges and one at the second-harmonic frequency. Thus, the out-of-band rejection is better than our previous work in [6], whereas the isolation performance is enhanced and the size is much smaller. Based on the proposed idea, an experimental circuit is designed. The circuit occupies the compact size of . Good power division and bandpass responses are observed in the experiment. Manuscript received May 05, 2013; revised June 19, 2013; accepted July 02, 2013. Date of publication August 02, 2013; date of current version August 30, 2013. This work was supported by the NSFC under Grants 61271060, 61001055, U1035002, and by NCET-10-0402. The authors are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510641, 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.2013.2274993

Fig. 1. Configuration of the proposed filtering power divider.

Fig. 2. Schematic of the proposed filtering power divider.

II. CIRCUIT DESIGN A. Circuit Configuration Fig. 1 shows the configuration of the proposed filtering power divider. The circuit consists of one half-wavelength resonator and four quarter-wavelength resonators. The schematic diagram is shown in Fig. 2. Resonators 2 and 3 as well as Resonators and are symmetrically arranged. Port 1 is connected to the midpoint of resonator 1. Ports 2 and 3 are tapered at Resonators 2 and , respectively. The signals from port 1 are fed to the resonator 1, then equally split and coupled to the other four quarter-wavelength resonators. A resistor is added between the resonators 3 and to enhance the isolation and impedance matching. The proposed circuit includes the equivalent power splitting circuit and filtering circuit with the center frequency of 920 MHz and fractional bandwidth of 6%. The detailed analysis is as follows. B. Analysis of Power Splitting Circuit The proposed circuit is symmetrical and odd-even-mode analysis can be adopted. If odd-mode excitation is applied to

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Fig. 3. (a) Odd-mode equivalent circuit. (b) lation resistance. Dimensions of the circuit are , , , , , , , ,

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 9, SEPTEMBER 2013

for various values of the iso, , , , , .

ports 2 and 3, the resonator 1 becomes two quarter-wavelength resonators denoted as resonator 1 . The equivalent circuit is shown in Fig. 3(a). According to the design principle for power dividers, port 2 need be matched to 50 . Since the coupled resonators function as impedance converter network to convert to 50 , the matching can be realized by tuning the resistance or changing the inter-resonator coupling strengths. However, if the coupling strength is altered, the filtering responses will be changed. Here the matching of port 2 is mainly fulfilled by changing the resistance . As shown in Fig. 3(b), various resistance results in different matching status at port 2. Because of the impedance-transformer network, the resistance is much larger than that in conventional dividers. If even-mode excitation is applied to ports 2 and 3, the equivalent circuit is shown in Fig. 4(a). From the point view of power dividers, this part functions as an impedance- transformer network which converts the 50 impedance at port 2 to the 100 impedance at port 1. Apart from this function, the even-mode equivalent circuit can also act as a filtering circuit as detailed below. C. Analysis of Filtering Circuit The topology of the equivalent filtering circuit is shown in Fig. 4(b). It consists of three quarter-wavelength resonators. The real and dash lines denote the electrical and magnetic coupling, respectively. Mixed electric and magnetic coupling as well as cross coupling are realized and thus transmission zeros can be generated.

Fig. 4. (a) Even-mode equivalent circuit. (b) Schematic of the even-mode equivalent circuit. (c) Simulated results of the filtering circuit. Dimensions of the circuit are the same as those given in the caption of Fig. 3.

In this design, the filter is designed with the same passband frequency and bandwidth as the filtering power divider. The coupling strengths between the three resonators are as follows: , , . This filter can be designed following the design procedure in [9]. The simulated responses are shown in Fig. 4(c) and good filtering performance is observed. Due to the mixed electric and magnetic coupling as well as cross coupling, three transmission zeros are generated. One transmission zero is located at the second harmonic frequency or 1.85 GHz, which can be used to reject the second harmonic responses generated by active components in transceivers. III. CIRCUIT IMPLEMENTATION The proposed circuit can be designed as follows. Firstly, the desired passband frequency can be obtained by tuning the the length of resonator 1 to be equal to half guided wavelength and others to be quarter guided wavelength. Secondly, the three resonators are combined using the configuration in Fig. 4(a) and the coupling strength is tuned to obtain a bandpass response with given specifications. The third step is to adjust the resistance to obtain the desired matching status at port 2 as shown in Fig. 3(a). Finally, the two filtering structures as well as the

ZHANG et al.: COMPACT FILTERING POWER DIVIDER WITH ENHANCED SECOND-HARMONIC SUPPRESSION

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passband. The photograph of the fabricated filter is shown in the insert plot of Fig. 5. The simulation is carried out using IE3D and the results are measured on the network analyzer Agilent E5071C. As shown in Fig. 5, the measured center frequency is 920 MHz, with the fractional bandwidth of 6.5%. The measured insertion loss of and are 3.96 dB and 3.99 dB. The passband return loss of , and are greater than 20 dB. The isolation is higher than 20 dB from DC to , which is better than that in [2], [8] with around 10 dB isolation and that in [4], [6] with more than 20 dB isolation only within a narrow frequency band. Two transmission zeros are generated at 0.75 GHz and 1.15 GHz, resulting in quasi-elliptic bandpass responses. Due to the transmission zero at 1.85 GHz or , the rejection level is more than 45 dB, resulting in enhanced second harmonic suppression. This design is different from those in [10] adopting the slow-wave structure and [11] using the stub and extended lines to obtain harmonic suppression without filtering responses. The comparison is tabulated in Table I. It is seen that the circuit size is smaller than the single-band counterparts in [2], [4], [6], and [8]. IV. CONCLUSION This letter has presented a compact power divider with quasielliptic bandpass responses. A new topology has been proposed and different resonators are combined to realize the power splitting and filtering functions. The design methodology and experimental results have been presented. Compact size as well as low insertion loss has been obtained. One transmission zero is generated at second harmonic frequency which is useful to suppress the spurious responses generated by active components. With the feature of compact size, high selectivity and multifunction, the proposed circuit is attractive for wireless communications. Fig. 5. Simulated and measured results of the filtering power divider. (a) & . (b) , & .

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TABLE I COMPARISON WITH PREVIOUS WORK

resistor are symmetrically arranged to form the circuit shown in Fig. 1 and then fine tuning is performed. A demonstration filtering power divider is implemented on the Rogers RO4003 substrate, with the relative dielectric constant of 3.38, the thickness of 0.81 mm and the dielectric loss tangent of 0.0027. The layout parameters in Fig. 1 are as follows: , , , , , , , , , , , , , , . The overall size of this circuit is , where is the guided wavelength at the center frequency of

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