Wideband microstrip bandpass filter based on modified parallel-coupled line topology

A wideband technique based on the modification of parallel-coupled line microstrip bandpass filter (BPF) is presented in this paper. The wideband response can be obtained by using a half-wavelength transmission line (TL) connected between the two cascaded of quarter-wavelength parallel-coupled lines. Since a conventional half-wavelength TL can be formed by a quarter-wavelength parallel-coupled line resonator with a three-line conductor, a BPF with compact circuit size and symmetrical structure can be achieved. By applying a T-shaped TL in the cascaded configuration, the wideband BPF not only give more overall compact filter´s size but also further suppress the first spurious response. The prototype of cascaded BPF is designed with the center frequency of 2 GHz and fabricated on an FR-4 substrate in the microstrip structure. Good agreement between the simulated and measured results is obtained.