An Alternate Circuit for Narrow-Bandpass Elliptic Microstrip Filter Design

This letter demonstrates the dual transmission zeros (TZs) of bandpass elliptic prototype filters that can be directly implemented with two resonators in microstrip. An experimental filter based on the proposed alternate circuits is designed and fabricated. In order to improve the rejection of stopband, two additional TZs are introduced to the proposed filter. By means of directly implementing an elliptic bandpass filter in microstrip, the insertion loss of the presented filter can be less than 0.8 dB, and the Δf3dB/Δf20 dBresult is better than 0.583, which means the proposed filter has a good selectivity.