Wide stopband bandpass filter based on dual-plane microstrip/interdigital DGS slot structure

A novel bandpass filter with a wide stopband is presented. The proposed filter is based on a dual-plane structure consisting of a microstrip and a defected ground structure (DGS) slot with an interdigital shape. Using this structure, the filter not only creates two transmission zeros near the passband, but also exhibits a wide upper stopband. The equivalent-circuit model is derived for the filter. A filter centred at 2.5 GHz with 14.8 fractional bandwidth is designed and implemented, which shows a minimum passband insertion loss of 1.78 dB and a wide stopband with rejection better than 22 dB up to 12f₀, where f₀ stands for the passband centre frequency.