Research On Novel Dual-Mode Dual-Band Filters

Recently, with the rapid development of communication industry, microstrip dual-mode filters, microstrip dual-band filters and even multi-band filters have on one hand satisfied the request of multi-frequency and multi-mode communication systems and the request of the trend of compactness on the other hand. They have a wide bright application foreground.The paper laid emphasis on the combination of microstrip dual-mode filters and microstrip dual-band filters. It was analyzed seriously that how dual-mode resonators can form a dual-band filter and how tunable dual-band filters works with varactors. An extremely large perturbation in a dual-mode resonator and the cascading method based on mirror theory are first proposed in this paper. A novel capacitance loaded square loop resonator is designed and the loaded metal branches not only reduce the size of the resonator but also change the average current distribution which eliminates the coupling interference of the two passband between two resonators. The coupling strength of the two passbands can be controlled independently. A novel dual-mode dual-band bandpass filter is realized by cascading two capacitance loaded square loop resonators. Besides, the application of a novel coplanar waveguide (CPW) feeding structure with two branches is also proposed in this paper. The external quality factors of the two passbands can be controlled independently by the areas of each branch. Three types of filters are designed based on the analysis. They are direct coupling dual-band filter, source-load coupling dual-band filter and CPW feed dual-band filter and comparisons are made about the three types of dual-band filters. Measured results agree well with the simulations.Moreover, a novel varactor loaded whole wavelength closed loop resonator is proposed in the paper. The resonance mechanism and tuning ways are analyzed, approximate calculations are made and simulations are achieved. The comparison of the results and error analysis are presented detailedly. Finally, a second-order tunable dual-band bandpass filter is realized and it has the advantage of wide tuning range and constant absolute band width. Measured results agree well with the simulations.