IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 8, AUGUST 2008 503 Wideband Parallel-Strip Bandpass Filter Using Phase Inverter Ka Wai Wong, Leung Chiu, Student Member, IEEE, and Quan Xue, Senior Member, IEEE Abstract—A new wideband parallel-strip bandpass filter using phase inverter with stubs is proposed. With the wideband phase in- verter in the parallel-strip line structure, the measured fractional passband bandwidth is 123% with a flat group delay response. Also, over 90% impedance bandwidth is obtained for the return loss higher than 20 dB. The theoretical simulation, electromagnetic simulation, and measured results show good agreement with this proposed design. Index Terms—Bandpass filter (BPF), parallel-strip line, phase inverters, wideband filters. I. INTRODUCTION F OR modern microwave systems, the bandpass filter (BPF) is an important component in the front-end circuit design. Particularly, the wideband BPF is always desired in a wide- band system. In theory, it is realized that the conventional par- allel coupled line can be designed for larger fractional band- width (FBW). However, for wideband BPFs, they usually need a tight coupling gap distance, which is very difficult to be real- ized during the fabrication process [1]. In [2], it was proposed using three-line microstrip structures, which is an alteration of coupled line by adding one more coupled section to implement a wideband BPF, and its FBW ranges from 40% to 70% with the critical value of the small gap spacings among the coupled lines. Hence, it is not convenient to use such kind of conven- tional filters for designing the wideband BPFs. Recently, various designs have been proposed with different kind of techniques. In [3], a wideband BPF using dual-mode ring resonators with feed lines and tuning stubs was proposed, in which the stubs are used for controlling the rejections and its corresponding bandwidth of this design is about 54%. In addition, some multilayer structures were proposed to use in the wideband BPFs [4]–[6]. In [4] and [5], microstrip/coplanar waveguide resonators structure were used to control the res- onant modes to implement a wideband response. For [6], the slotline incorporated with microstrip structure was employed to control the broadside-coupling and resulted in a wideband characteristic. The use of parallel-strip line has many advantages such as easy realization of the balanced microwave components, simple implementations for the low characteristic impedance line, se- ries stubs, and also the phase inverter. The phase inverter is a Manuscript received February 11, 2008; revised May 8, 2008. Published Au- gust 6, 2008 (projected). This work was supported by the Hong Kong Research Grant Council under Grant CityU122407 and City University of Hong Kong Strategic Research Grant 7002026. The authors are with the State Key Laboratory of Millimeter Waves, Depart- ment of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China (e-mail: oliver_wkw@hotmail.com). Digital Object Identifier 10.1109/LMWC.2008.2001001 Fig. 1. Schematic of the proposed wideband BPF. passive element, which is convenient for implementing a 180 phase shift with a small area. In the literature, only a few BPF designs were implemented on parallel-strip line using the char- acteristics of parallel-strip lines, as shown in [7]. Therefore, in this letter, a new implementation of parallel-strip wideband BPF using phase inverter with stubs is proposed. The computed, sim- ulated and measured results are well matched with the matrix manipulation analysis which exhibits wide FBW and obtains a flat group delay response as well. II. DESIGN OF THE PROPOSED WIDEBAND BANDPASS FILTER A. Basic Operation of the Wideband BPF The proposed wideband BPF is implemented by a parallel- strip structure. The schematic diagram of the proposed wide- band BPF is shown in Fig. 1. It consists of three sets of par- allel-strip lines, namely , and , with character- istic impedances of , and , respectively, and a 180 phase inverter. The physical length of and are 4 long at center frequency while is 2 long, where is the guided wavelength of the parallel-strip line. In order to analyze the filter easily, the filter can be divided into the upper part and the lower part. At , i.e., 1 GHz for passband, the input impedance at point A appears to be an open circuit since it is half-wavelength 2 long from the end of the open circuit stub . Consequently, the signals from input port (Port1, 50 ) pass mainly to the output port through the lower path, and its response contributes a passband at center frequency. For stopband frequency response at 2 , i.e., 2 GHz, the input impedance appears to be an open circuit (from point B) because is quarter-wavelength long (at ) from an open end. The two open circuit stubs are seemingly neglected in this situation. Thus, the signals from input port divide into two components. 1531-1309/$25.00 © 2008 IEEE