Quasi multi-mode resonator for wideband filter applications

This paper presents a compact planar bandpass filter that comprises of multiple coupled-line sections to exhibit a wideband elliptic function response. The equivalent circuit representing the proposed resonator structure was synthesized using a conventional coupled-line J-inverter model. The electrical length of the transmission-line connecting the two inverters is λ/6, thus effectively reducing its physical size compared to existing designs. The proposed filter structure using triple coupled quasi multi-mode resonators provides a frequency response with a wide 3-dB fractional bandwidth of 50%, and the two transmission zeros at the upper and lower stop-bands provide a highly sharp cut-off response with a low passband insertion-loss and good return-loss characteristics.