《Balanced-to-Unbalanced Bandpass Filters and the Antenna Application》.pdf

2474 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 11, NOVEMBER 2008 Balanced-to-Unbalanced Bandpass Filters and the Antenna Application Chung-Hwa Wu, Chi-Hsueh Wang, Shih-Yuan Chen, and Chun Hsiung Chen, Fellow, IEEE Abstract—Novel balanced-to-unbalanced (balun) bandpass fil- maintain good balun performance within the desired passband. ters are presented and carefully examined. By properly converting Over the past few years, several balun bandpass filters have a symmetric four-port balanced-to-balanced bandpass filter to a been developed. Some of them were based on the conventional three-port device, with two ports forming a balanced output and one port remaining unbalanced, a novel three-port balun band- Marchand baluns [1]–[4] and the others were based on lumped pass filter with both functions of balun and bandpass filter may be center-tapped transformers [5]–[7]. Although those balun band- realized. Specifically, a balun bandpass filter is implemented with pass filters have shown both characteristics of balun and band- its center frequency at 0.99 GHz and a 3-dB fractional bandwidth pass filter at the same time, their passband responses may not  of 10.1%. The implemented balun bandpass filter presents be designed and the in-band balun performance may not be an excellent in-band balance performance with common-mode re- jection ratio   better than 41 dB (the corresponding ampli- optimized. tude and phase imbalances are within 0.003 and 1 ) over the pass- Recently, four-port balanced-to-balanced bandpass filters band. In this study, a wideband balun bandpass filter is also imple- with desired differential-mode ban