A Wide Stopband Dual-Band Bandpass Filter Based on Asymmetrical Parallel-Coupled Transmission Line Resonator

A self-coupled resonator consisting of asymmetrical parallel-coupled microstrip transmission lines is proposed to design wide stopband dual-band bandpass filters. Due to the even-to-odd-mode analysis limitation to asymmetrical structure, a multiport analysis method is developed and applied to analyze the proposed self-coupled resonator and dual-band filter. According to the analysis, the proposed self-coupled resonator features a small frequency ratio of the second to the first resonances and controllable spurious responses, and its transmission zeros (TZs) are placed close to the position out-of-band spurs in order to achieve wide stopband. Moreover, a feed line consisting of tapped line loaded coupled line is investigated, resulting in flexible adjustment of the second passband external quality factor (
${ \boldsymbol Q}_{ \boldsymbol e}$
) with a minor influence on the first passband
${ \boldsymbol Q}_{ \boldsymbol e}$
. A dual-band BPF prototype with a frequency ratio of 1.59 is proposed and validated. It is operated at 2.34 and 3.72 GHz with the insertion loss of 1.05 and 1.86 dB, respectively. A return loss better than 20 dB is achieved at both passbands. The out-of-band rejection is better than 30 dB within the frequency range from 4.19 to 11.24 GHz, and the harmonic suppression is 39.97 dB at 14.88 GHz and 61.41 dB at 18.6 GHz. The measured results verify the analytical method presented in this article well.