Design Of Dual-Band/Tri-Band Planar Filters Using Stepped Impedance Resonators

With the phenomenal development in the communication industry, wirelesscommunications have become one of the most popular technologies in this century.Many new wireless communication systems, such as GSM, WIMAX, WCDMA,WLAN and UWB systems are proposed, verified, realized and adopted, which havespurred a tremendous development in microwave filters. The demand on enhancing theutilization of frequency spectrum and compatibility of different communicationsystems has spurred the design technology for dual-band and tri-band filters. In thisthesis, some resonators with easily controlled resonance frequencies and couplingstructures are presented, and different multiband filters with good characteristics aredesigned using these resonators.Firstly, the structures of stepped impedance resonators （SIRs） are introduced.Emphasis is placed on the analysis of two-section and three-section SIRs, and theprinciple of dual-band-characteristic generated by the SIRs.Secondly, a miniaturized dual-band planar filter based on two-section SIR isdesigned. The SIR is folded, and the public-coupling-feeding is used in the filter forminiaturization. The return losses are better than13dB and15dB in these twopassbands, the insertion losses at the center frequencyf1=2.4GHz andf2=5.2GHzare0.36dB and0.4dB, and the relative bandwidths reach7.5%and7.1%respectively.Then, a planar filter is designed, and its resonant frequency could be controlled bythe length of stub loaded in the center of the SIR. The return losses are better than16dB and15.1dB in these two passbands, and the insertion losses at the centerfrequencyf1=2.4GHz andf2=5.8GHz are0.17dB and0.25dB, and the relativebandwidths reach16.3%and17.9%respectively. A transmission zeros generated at7GHz improves the out-of-band performance.Finally, a tri-band planar filter based on three-section SIR is designed. Its centrefrequencies are1.8GHz for GSM,3.5GHz for Wimax and5.2GHz for WLAN. Thesecond and third resonant frequencies can be controlled by loading a stub in the centerof the three-section SIR. The return losses are better than13dB,21.2dB and18.1dB inthese three passbands. The insertion losses at the center frequencyf₁=1.8GHz,f₂=3.5GHz andf3=5.2GHz are0.19dB,0.29dB and0.18dB, and the relativebandwidths reach11.1%,10.3%and3.8%respectively. Three transmission zeros at 2.2GHz,4GHz and5.6GHz are evident, and out-of-bands suppression are39dB,34dBand46dB respectively.