IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS , VOL. 15, NO. 12, DECEMBER 2005 835 Serially-Connected Series-Stub Resonators for Narrowband Coplanar-Stripline Bandpass Filters Ning Yang and Zhi Ning Chen Abstract—In this letter, a coplanar stripline (CPS) series res- onator composed of serially-connected series-stubs is proposed. This kind of resonator provides two transmission zeros close to the resonant frequency. With the proposed structure, the reactance slope parameters of the resonators are significantly increased and adjustable by changing the length ratio of the two series stubs. This is useful for designing narrowband filters which is difficult to realize with conventional series CPS resonators. The structures of the resonators are optimized for a lower radiation loss and used to design a narrowband Chebyshev bandpass filter. The simulation and experimental results show that the proposed filter provides multiple transmission zeros and high selectivity. Index Terms—Coplanar stripline (CPS), series resonator, series stub, transmission zero. I. INTRODUCTION I N MODERN communication systems, the components have to be small and allow low power consumption for integra- tion. Researchers are taking efforts to miniaturize the module by burying the peripheral components, including antennas, filters, and amplifiers into multilayer substrate. Therefore, it is neces- sary for the radio frequency (RF) system to be co-designed. In fact, many RF components can be designed with balanced in- puts/outputs (I/O), such as amplifiers, mixers, and antennas with symmetric structures. The design of RF components with bal- anced structures can remove the requirement of unbalance–bal- ance transition which may cause loss to the system. The coplanar-stripline (CPS) was introduced as a uniplanar structure with balanced configuration [1] and has found wide application for uniplanar microwave circuits, such as filters, dividers, mixers, and antenna feeding networks [2], [3]. It is efficient in use of wafer area and has all the advantages of coplanar waveguide (CPW) in design of uniplanar circuits while via holes or air-bridges are not needed. The propaga- tion characteristics and modeling of the discontinuity of CPS have already been studied and a few CPS filters have been constructed in the past years [4]–[6]. A kind of short-end series-stub resonator by etching slots in the strips were used to design a CPS bandpass filters [6]. These filters have some advantages over microstrip filters, such as ease of fabrication and compactness. However, this structure is the candidate only for wide-band filters because the series stub resonator is obliged to a low reactance slope parameter and a low external Manuscript received May 18, 2005; revised August 9, 2005. The review of this letter was arranged by Associate Editor M. Mrozowski. The authors are with the Institute for Infocomm Research, Singapore 117674; (e-mail: ningyang@motorola.com). Digital Object Identifier 10.1109/LMWC.2005.860022 Fig. 1. Transmission models of (a) the conventional CPS series resonators and (b) the proposed serially connected series-stub resonator. value, which is limited by the slot width etched in the strips. Meanwhile, the desired high skirt selectivity performance in the filter design could be achieved by increasing the order or by introducing transmission zeros close to the passband. This letter proposes a CPS stub resonator formed by serially connected short-circuit series stubs which are directly etched within the strip conductors. The series connection of the stubs is capable of providing high impedance level. Therefore, the pro- posed resonators can be used to design narrowband filter in that the reactance slope parameters are increased. The external value of the resonator is adjustable by the length ratio of the two series stubs. Another advantage of this kind of resonator is that two transmission zeroes are obtained near the resonant frequency, which can improve the skirt selectivity in the filter design. To minimize the radiation loss from the series-stub res- onator, this work presents an improved resonator structure. A narrowband CPS bandpass filter using the proposed resonator is designed and fabricated, and its performance is characterized numerically and experimentally. II. RESONATOR DESCRIPTION Fig. 1(a)–(b) compares the transmission models of a conven- tional CPS series stub resonator [6] and the proposed serially connected series-stub resonator. In the conventional design, a shorted-end 2 series stub is used as the series resonator. The 2 length stub is folded to make the filter more compact. The input impedance of the stub resonator can be expressed by , where is the resonant frequency and is the characteristic impedance of the stub line. The reactance-slope parameter of the resonator is 2 . To obtain a higher external is required to have a higher value. For the CPS, high impedance can be obtained by increasing the gap between the two strip conductors. However, the in- crease of the gap is constrained by width of the strips, and furthermore, the radiation loss and coupling will be increased. Usually the characteristic impedance of a CPS is within the range of 30–80 . Using the 1/4 wavelength transmission line 1531-1309/$20.00 © 2005 IEEE