Designing dual-band pass filter by coupled resonators

Dual-band pass filters pass signals at two distinct frequency regions with a specified bandwidth. In this work a dual-band pass filter based on coupled open-loop rectangular resonators is designed. The microstrip filter is designed to pass signals at uplink and downlink frequencies of the 3G cellular networks, 1920 MHz and 2110 MHz respectively. The original design of the dual-band filter is the sixth order filter by shifting the feed positions so that the signals get two different paths due to the parallel circuit of the filter. The designed filter works at the first frequency at 1.91 GHz with a bandwidth of 90 MHz and reflection and insertion loss of 23.9 dB and 1.09 dB, respectively. The second frequency of the filter is located at 2.16 GHz with a bandwidth of 110 MHz and reflection and insertion loss of 17 dB and 1.24 dB, respectively. The measurement results verify the designed filter very well.