Sharp Roll-Off Bandpass Filter Using Coupled Lines and Stepped-Impedance Stubs

A fifth-order bandpass filter (BPF) using three pairs of coupled lines and two shorted stepped-impedance stubs is proposed in this paper. Five transmission poles (TPs) within the passband and six deep transmission zeros (TZs) from 0 to 2f0 (f0:passband center frequency) can be realized through input admittance calculation. For demonstration, a sharp roll-off BPF prototype centered at 2 GHz with good in-band characteristics (insertion losses and return losses) and high stopband rejections is designed, whose 3-dB fractional bandwidth is of 34%.