A Dual-Band Balun BPF Using Double-Sided Parallel-Strip Line

In this paper, a novel dual-band balun bandpass filter (BPF) is designed by using double-sided parallel-strip line (DSPSL) with an intermediate conductor plane. Half wavelength resonators and open stub loaded resonators are utilized to provide third-order filtering responses in each band. Due to the frequency-independent out-of-phase feature of DSPSL, two balanced outputs are achieved with 180° phase difference. In addition, the proposed coupling topology introduces multiple transmission zeros providing high frequency selectivity and good out-of-band rejection. A well-designed BPF is fabricated and measured with two passbands centering at 2.89 GHz and 4.81GHz, respectively. Good filtering and balun performance are realized from the well-agreed simulated and measured results.