Parallel feed bandpass filter with high selectivity and wide stopband

A quasi-elliptic function bandpass filter (BPF) based on parallel feed topology is presented. The parallel feed topology is realised by splitting the input/output port of the filter into two paths through a T-junction to excite a pair of resonators simultaneously. This configuration contributes to further improving the selectivity of the filter by introducing more transmission zeros (TZs) near the passband. In addition, a defected microstrip structure is utilised to enhance the out-band attenuation and spur suppression. In order to verify the proposed topology, a fourth order BPF with cross-coupling is designed, fabricated and measured. The measured frequency responses agree well with simulated ones. Four TZs can be obtained with two TZs on each side of the passband.