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 degrees 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.