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Mathematical Problems in Engineering
Volume 2014, Article ID 985064, 6 pages
Research Article

Fabrication of Compact Microstrip Line-Based Balun-Bandpass Filter with High Common-Mode Suppression

1Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan
2Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan
3Department of Avionic Engineering, Air Force Academy, Kaohsiung 820, Taiwan
4Department of Electrical Engineering, Cheng Shiu University, Kaohsiung 833, Taiwan
5Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan

Received 15 August 2014; Accepted 18 September 2014; Published 30 September 2014

Academic Editor: Teen-Hang Meen

Copyright © 2014 Chia-Mao Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


A new type of balun-bandpass filter was proposed based on the traditional coupled-line theory and folded open-loop ring resonators (OLRRs) configuration. For that, a new device with both filter-type and balun-type characteristics was investigated and fabricated. Both magnetic and electric coupling structures were implemented to provide high performance balun-bandpass responses. The fabricated balun-bandpass filters had a wide bandwidth more than 200 MHz and a low insertion loss less than 2.51 dB at a center frequency of 2.6 GHz. The differences between the two outputs were below 0.4 dB in magnitude and within 180 ± 7° in phase. Also, the balun-bandpass filter presented an excellent common-mode rejection ratio over 25 dB in the passband. An advanced design methodology had been adopted based on EM simulation for making these designed parameters of OLRRs, microstrip lines, and open stubs. The measured frequency responses agreed well with simulated ones.