Design of tunable microstrip diplexer with reconfigurable filtering characteristics based on dual-mode square loop resonators

A novel compact microstrip diplexer with tunable centre frequencies and reconfigurable filtering characteristics at both channels is presented. The designed diplexer is constructed by coupling two dual-mode square loop resonators (DMSLRs) having different electrical lengths. The DMSLRs are investigated by virtue of the coupled line analysis to evaluate the resonance frequencies, transmission zeros and out-band insertion loss level more conveniently. Centre frequencies of both channels can be independently tuned by the varactor diodes utilised instead of perturbation and reference capacitances. Depending on the specialised values of perturbation and reference capacitances, filtering characteristics of both channels can be reconfigured in terms of transmission zeros. The designed diplexer was fabricated and tested for the experimental studies and the measurements were obtained in a good agreement with the predicted results. Tuning ranges of the measured centre frequency in the first channel are between 1.23 and 1.43 and 1.35 and 1.64 GHz for the filtering characteristics with right- and left-side transmission zeros, respectively. In the second channel, they are 2.07-2.31 and 2.14-2.51 GHz for the filtering characteristics with real- and imaginary-axis transmission zeros, respectively. Isolation between two channels has been obtained better than 22 dB.