Compact Dual-Mode Balun Bandpass Filter with Improved Out-of-Band Performance

Abstract

A compact dual-mode balun bandpass filter with improved out-of-band performance is proposed based on a ring resonator. In order to excite the two degenerate modes and obtain filtering balun performance, three parallel-coupled lines are placed at three excitation ports along the ring resonator. By properly arranging the angles between the excitation arms, four transmission zeros can be realized to improve stopband performance. Finally, a prototype of the designed balun bandpass filter is implemented and fabricated. The measured results show a good agreement with those obtained from the simulation, thus verifying our proposal with good in-band filtering and out-of-band performance.