Design Of Balanced Multi-band Bandpass Filters

With the continuous progress of wireless communication technologies,such as GPS,WLAN,Wi MAX and mobile cellular systems,especially the gradually maturation of 5G technology in recent two years,multi-band technology has become a research hotspot.On the other hand,compared with the single-end counterparts,balanced circuits have attracted substantial attention and been more and more utilized because of their high immunity to external interference signals.To achieve miniaturization,multi-band and high performance,two balanced dual-band BPFs and a balanced quad-band BPF are designed and fabricated in this thesis.The main contents of this thesis mainly include the following aspects:1.A wide stopband dual-band balanced filter centered at 2.57 GHz and 3.41 GHz is designed based on the theory of the stub-loaded resonators(SLRs).By employing a U-shaped half wavelength microstrip resonator and a folded stub-loaded resonator(FSLR),two DM passbands with controllable center frequencies and fractional bandwidth(FBW)are realized.Due to the symmetry of the resonators,the BPF can be analyzed by odd-even mode theory.Two DM passbands with controllable center frequency can be obtained.In addition,the harmonics are appropriately suppressed and a wide stopband is realized by adjusting the physical lengths and coupling distance between the resonator and the feed lines.Moreover,the BPF achieves high CM rejection,low insertion losses and better out-of-band selectivity.2.A dual-band balanced BPF based on stepped impedance resonator(SIR)is designed in this thesis.By employing two symmetrical folded stepped impedance split resonant rings(FSISRR),two DM passbands with controllable center frequency are realized.The center frequencies can be adjusted by the characteristic impedances or physical lengths of the high and low impedance lines in SIRs.Five transmission zeros are generated on both sides of the DM passbands by 0-degree feeding and multi-path transmissions,which significantly enhances the steepness of the passbands and out of band selectivity.3.A novel capacitive interdigital coupling line is proposed in this thesis.Based on this structure,a quad-band balanced BPF with high selectivity and low insertion losses is designed.The proposed novel capacitive interdigital coupling line is composed of two symmetrical capacitive interdigital coupling lines,which can enhance the coupling strength of the source-load and increase the transmission path in the feeding line.Thereupon then enhance the out-of-band suppression and selectivity.In order to achieve four independently controllable DM passbands,two pairs of different folded asymmetric stub-loaded resonators(FASLR)are loaded on both sides of the capacitive interdigital coupling line.In addition,eight TZs are generated around the four DM passbands to improve the selectivity of the BPF significantly.These TZs are introduced by the novel interdigital coupling line and the cross coupling of each structure.The proposed BPF can achieve controllable DM center frequencies,low insertion losses,high CM suppression and sharp passband roll-off skirts,which indicates that the proposed balanced quad-band BPF has a good potential to be utilized in the balanced multi-band communication systems.