Miniaturized bandpass filter with high selectivity and wide stopband using dual-layer structure

A miniaturized bandpass filter with high selectivity and wide stopband is proposed using the dual-layer structure. The broadside-coupled microstrip/interdigital defected ground structure (DGS) cell is introduced to allocate center frequency with two close transmission zeros. Based on the proposed cells, an interdigital-coupled scheme is employed to achieve strong passband enhancement with high selectivity. To further improve the passband perfomance and extend the stopband bandwidth, the defected microstrip structure (DMS) is proposed. The measured filter operated at f₀ of 2.6 GHz has the merits of an excellent selectivity (i.e., strong attenuation slope with 420.9 dB/GHz in the lower and 484.9 dB/GHz in the upper transition bands) and wide stopband up to 3.75f₀.