Compact quad-mode bandpass filter with sharp cut-off using folded parallel-coupled microstrip lines and semi-annular resonator

A quad-mode microstrip bandpass filter with compact size, wide stopband and sharp lower and upper cut-off bands is designed. The proposed filter uses low impedance feeding network, folded parallel coupled microstrip lines (FPCML) and semi-annular middle resonator. The low impedance feeding network is utilized to enhance the coupling of the FPCML structure and to achieve a sharp lower and upper cut-off frequencies with extended upper stopband. Meanwhile, the low impedance semi-annular resonator is used to produce multiple resonant modes within the operating band and two transmission zeros to isolate that band from the stopbands. The proposed filter with a centre frequency of 2.45 GHz is designed and optimized using ADS Momentum. The filter shows excellent performance with 0.5 dB insertion loss, more than 26 dB attenuation at the wide stopband, more than 77 dB/GHz rate of cut-off at the edges of 35% fractional bandwidth passband.