Stub-loaded resonator and asymmetric parallel-coupled lines for dual notched-band implementation in wideband bandpass filter

A High-selective wideband bandpass filter (BPF) with dual notched-band implemented by using stub-loaded resonator and asymmetric parallel-coupled lines is presented in the paper. Firstly, a pair of λ/2 open-ended stubs is tap-connected to a λ/2 uniform impedance resonator at two symmetrical locations with respect to its center, and they are further electrically connected by a microstrip line section. Such an arrangement leads to bring out a notched-band owing to the out-of-phase cancellation of two dissimilar signal paths in a wide passband. The electrical path connecting the two open-ended stubs is tuned so as to adjust the notched-band to the desired frequency location. Secondly, Asymmetric parallel-coupled lines are applied to the input and output (I/O) to produce another notched-band in the passband, and furthermore the location of the notched-band can be adjusted by changing the length of the stubs loaded on the coupled lines. Finally, a prototype filter is designed and fabricated, and its measured results are provided to verify the predicted frequency response.