Simultaneous 2nd and 3rd spurious passband suppression design for coupled-line microstrip bandpass filter

In this paper, a novel coupled-line microstrip bandpass filter with significant suppression in both 2^nd and 3^rd spurious passbands is presented. In contrast to the conventional ones, two modifications are made in this novel topology. One of these is the extension of the coupling gap of the resonators, which is mainly concerned for 2^nd harmonic suppression. In addition, 3^rd harmonic suppression is achieved by the perturbation of the resonators´ strip widths. In order to demonstrate the proposed filter performance, three prototype filters are designed at 900 MHz with 10% fractional BW. From the simulation and experimental results, over 15 dB suppression in the second harmonic and about 10 dB third harmonic suppression have been obtained in the proposed filter topology.