A Compact Microtrip Filtering Power Divider Made Of Novel Coupled Resonators Yifeng Chen, Xianling Liang, Weiren Zhu, Liang Liu, Junping Geng, Ronghong Jin, Maohua Zhu and Guanshen Chenhu Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, China. E-mail: email@example.com Abstract—We present the design of a compact microstrip filtering power dividers by utilizing the coupling between adjacent resonators. For the purpose of effectively reducing the circuit area and achieving a good isolation, four novel resonators are precisely designed and arranged. Numerical simulation shows that a filtering power with size as compact as 0.16 λg by 0.16 λg can achieve a high isolation over 20 dB. 1 2 Input Port Output Port λ/4 3 Output Port 4 Isolation Port Keywords—filtering power divider (FPD), microstrip, novel resonator, compact structure. 3λ/4 I. INTRODUCTION Fig. 1. Geometry structure of the typical 3 dB hybrid ring. Port 1 With the development of modern wireless communication systems, the miniaturization of RF front-end components is highly demanded. Integrating multiple functionalities into one component is an effective way to reduce the circuit area. A power divider can split an input power into several parts, which is frequently used in feeding networks in antenna array systems. Meanwhile, an RF filter is also an essential component in modern RF front-ends, which is used to filter out undesired signals while pay little deterioration to useful signals. In many systems, both the two components are indispensable. It is, therefore, necessary to integrate them into one component for size reduction and low loss. In recent years, there were two design methods proposed to meet the requirement. One method is to cascade the filtering circuit with power dividers . Another method is to replace the quarter-wavelength transformers in Wilkinson power dividers with bandpass filters . Other than these two methods, the couplings between adjacent resonators can also be used for the integration of power dividers and filters . By replacing the quarter/three-quarters-wavelength-lines of the 3dB hybrid ring with the coupling of resonators, the filtering power divider (FPD) can be realized. Then, the circuit area can be reduced significantly as the FPD is transformed into the coupled-resonator network. In this paper, to further reduce the circuit area, a coupledresonator network is used to design the FPD, which composed of four novel resonators. The resonators are well placed for achieving good isolation and filtering response. The FPD has a size as compact as 0.16 λg× 0.16 λg and simulated isolation higher than 20 dB within the whole passband. II. FILTERING POWER DIVIDER DESIGN Resonator 1 t1 Resonator 2 d21 l2 t2 w2 d31 d42 Port 2 t3 Resonator 3 Resonator 4 t4 d43 Port 3 50Ω Fig. 2. Schematic layout of the proposed FPD. 2&3 and none for port 4. At the same time, port 2 and port 3 are isolated each other. If port 4 is loaded by a 50 Ω resistor, the 3 dB hybrid ring can be regarded as a 3 dB power divider. The schematic layout of the proposed FPD is shown in Fig. 2, which is composed of four resonators coupled with each other. The magnetic resonance can be managed to be out-of-phase with the electric resonance . Therefore, the quarter-wavelengthlines and three-quarters-wavelength-lines in a 3 dB hybrid ring can be mimicked by the electromagnetic coupling between resonators. Particularly, the quarter-wavelength-line is achieved by the electric couplings while the three-quarters-wavelengthline is realized by the magnetic coupling. The typical structure of the 3 dB hybrid ring is shown in Fig. 1. The input power from port 1 divides evenly between ports 978-1-5386-3284-0/17/$31.00 ©2017 IEEE 2225 AP-S 2017 Figures 3(a) and 3(b) show the even and odd modes of the proposed novel resonators, respectively. Since the resonant frequency of the odd mode is much higher than that of the even mode, the coupling between these two modes is negligibly weak. It is seen that the even mode of the resonator is transformed from the quarter-wavelength resonator shown in Fig. 3(c). The geometric parameters of the proposed FPD are listed in III. SIMULATION RESULTS The simulated S-parameters are shown in Fig. 4. It is seen that the simulated insertion losses |S21| and |S31| are approximately 4 dB, which is caused by the filtering response of the bandpass filter and the insertion loss of the 3 dB power divider. Besides, the isolation between ports 2 and 3, |S32|, is higher than 20 dB within the whole passband. However, as a tradeoff of the excellent isolation performance, the bandwidth is relatively narrow. Table II compares the proposed FPD with other works. It is seen that the proposed FPD owns not only the smallest size and lowest insertion loss, but also a good in-band isolation as well. (a) (b) TABLE II. (c) Fig. 3. (a) Even mode of the resonator, (b) Odd mode of the resonator (c) Quarter-wavelength resonator. COMPARISON BETWEEN THE PROPOSED FPD AND THE OTHER WORKS Insertion loss (dB) Isolation (dB)  6.4 >15 0.49×0.38  6 >24 0.31×0.22  4.4 >30 0.19×0.19 This work 4 >20 0.16×0.16 Size (λg2) IV. CONCLUSION 0 -10 Magnitude (dB) In summary, we have presented a compact microstrip FPD based on a coupled-resonator network consisting of four novel identical resonators. Each resonator performs as a quarterwavelength-line in the 3 dB hybrid ring. These resonators are precisely placed for achieving good isolation and filtering responses. The proposed FPD has a compact size of 0.16 λg by 0.16 λg with simulated isolation over 20 dB at the frequencies of interest. The insertion losses are approximately 4 dB including the 3 dB loss from power divider within the whole passband. S11 S21 S31 S32 -20 -30 -40 -50 ACKNOWLEDGMENT -60 -70 0.50 0.75 1.00 1.25 1.50 1.75 This work was supported by the National Natural Science Foundation (61671416, 61471240, 61571298 and 61571289) 2.00 Frequence (GHz) REFERENCES Fig. 4. S-parameters of the proposed FPD.  Table I. The substrate is Neltec NH9338 with a relative dielectric constant of 3.38 and a thickness of 0.508 mm. The widths of w2 and w1 are set to be 1.137 mm and 3.071 mm, respectively, which are corresponding to the microstrip line characteristic impedances of 50 Ω and 25 Ω. Due to the folded line design, the proposed FPD only has a size of 27 mm × 27 mm, which equals to 0.16 λg × 0.16 λg (λg is the guided wavelength at the center frequency of 1.2 GHz). TABLE I.    DIMENSION OF THE PROPOSED FPD (ITEM: MM) d21 d31 d42 d43 t1 t2 t3 t4 l1 l2 l3 0.49 0.6 0.6 0.35 6.8 6.8 6.5 6.8 15.5 11.5 5.5  2226 S. W. Wong and L. Zhu, “Ultra-wideband power divider with good inband splitting and isolation performances,” IEEE Microwave and Wireless Components Letters, vol. 18, no. 8, pp. 518-520, Aug. 2008. J. Y. Shao, S. C. Huang, and Y. H. Pang, “Wilkinson power divider incorporating quasi-elliptic filters for improved out-of-band rejection,” Electron. Lett., vol. 47, no. 23, pp. 1288–1289, Nov. 2011. P. Cheong, K.-I. Lai, and K.-W. Tam, “Compact Wilkinson power divider with simultaneous bandpass response and harmonic suppression,” in IEEE MTT-S Int. Dig., 2010, pp. 1588–1591. C. F. Chen, T. Y. Huang, C. C. Chen, W. R. Liu, T. M. Shen, and R. B. Wu, “A compact filtering rat-race coupler using dual-mode stubloaded resonators,” in IEEE MTT-S Int. Dig., Montréal, QC, Canada, Jun. 2012, vol. 1, pp. 1–3. C. F. Chen and Y. L. Cheng, “Compact microstrip filtering power dividers with good in-band isolation performance," IEEE Microwave and Wireless Components Letters, vol. 24, no. 1, pp. 17-19, Jan. 2014.