1549-7747 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TCSII.2016.2634555, IEEE Transactions on Circuits and Systems II: Express Briefs Tunable Dual-Band Filter and Diplexer Based on Folded Open Loop Ring Resonators Weijie Feng, Member, IEEE, Ying Zhang, Wenquan Che, Senior Member, IEEE Abstract- Tunable dual-band filter and diplexer based on folded open loop ring resonators (OLRRs) are proposed in this paper. Two pairs of OLRRs of different center frequencies are utilized to couple with microstrip transmission lines in these two devices, and there is no extra matching network in the diplexer structure. Maximal magnetic coupling and high isolation can be obtained by placing the resonators at proper positions along the feeding lines. The passband/channel frequencies of the dual-band filter/diplexer can be tuned independently by varying DC bias voltages applied to the varactors loaded at one end of the resonators. In addition, a broad harmonic suppression can be obtained by choosing appropriate lumped elements adding to the OLRRs. The performances of the proposed devices can be verified by the simulated and measured results. Index Terms—Tunable, dual-band filter, diplexer, OLRRs, harmonic suppression I. INTRODUCTION LECTRONICALLY tunable/reconfigurable filters and diplexers have been becoming a popular trend with the more and more crowded spectrum environment, for they can cover different center frequency signals within one component [1]-[13]. Some researches about tunable dual-band filters have been done in recent years [5]-[6]. In [5]-[6], tunable dual-band filters with two absolutely independent tunable passbands are proposed, respectively, but the filter in [5] has a low return loss and poor out-of-band performance, and the filter in [6] uses so many varactors which increases the designing complexity and measuring difficulty. Moreover, for the diplexers with fixed channels, they can be designed by combing two filters of different channel frequencies with a matching network. However, this method cannot be applied to tunable diplexer directly. Because the input impedance of each channel to the common input port will vary with the changing channel frequency, which must lead to mismatching at the input port. So it is rather hard to design a matching network working over the entire tuning range for both channels. That is why researches on tunable diplexers are still challenging [7]-[8]. The half-wavelength folded open loop ring resonators (OLRRs) are first proposed in [9], and they have been widely used to design dual-band filters, balun filters and multiplexers [10]-[12]. In this This work was supported by the National Natural Science Foundation of China (6140010914, 61571231), Natural Science Foundation of Jiangsu Province (BK20140791) and the 2014 Zijin Intelligent Program of Nanjing University of Science and Technology. Wenjie Feng, Ying Zhang, and Wenquan Che are with the Department of Communication Engineering, Nanjing University of Science & Technology, 210094 Nanjing, China (e-mail: zhangying__0726@163.com, fengwenjie1985@163.com, yeeren_che@163.com) paper, simple tunable dual-band filter and diplexer based on OLRRs are proposed. For these two prototypes, they both use two pairs of OLRRs with different frequencies with microstrip coupled lines. Maximal magnetic coupling and high isolation can be received by placing the resonators at proper positions along the transmission lines. The passband/channel frequencies can be tuned independently, by changing varactor capacitances without influencing another passband/channel characteristic. And a broadband harmonic suppressed characteristic can be achieved. The tunable dual-band filter and diplexer using OLRRs are designed and fabricated. II. ANALYSIS OF PROPOSED TUNABLE DUAL-BAND FILTER AND DIPLEXER In this section, tunable dual-band filter and diplexer based on folded open loop ring resonators are analyzed in detail. The analysis of the resonator is shown in Part A, the analysis of the tunable dual-band filter/diplexer are illustrated in Part B and Part C, respectively, finally the structure parameters are given in Part D. A. Analysis of the Resonator The configuration of the proposed resonator is shown in Figs. 1(a)-(b). There is a varactor loaded in the end of the half-wavelength OLRR, a capacitor and a resistor are attached at the center of the OLRR. For the folded OLRR in Fig. 1(a), it can be modified as Fig. 1(b), a reference port is added to derive the input admittance. The input admittance is tan tan L in V L Y jY Y j C Y Y jY        (1) where tan ( tan ) 1 L Y YRC jY C j RC           (2) Where C V is the capacitance of the loading varactor, and θ is the half electric length of the transmission line. The resonate frequency can be calculated from (3) Im[ ] 0 in Y  (3) It should be noted that there are several resonate frequencies existing in this resonator, but the fundamental resonate frequency is chosen as the center frequency. It can be seen from (1) and (3), the frequency depends on the parameter C V , C and R. But the capacitance of the varactor is the decisive factor of the frequency variation, C and R have little effect on it. So by changing the DC bias voltages applied to the varactors adding at the resonator can E