Research On High Temperature Superconducting Filters With Spurious Passband Suppression

With the rapid development of wireless communication technologies,problems of the spectrum resource scarcities and frequency interferences are increasing seriously.To address these problems,high-performance receive front-ends with high sensitivities and superior anti-interference abilities are in great demand.Because of characteristics of low insertion loss,high out-of-band rejection,and high selectivity,high temperature superconducting,?HTS?filters have been widely used to fabricate high-performance front-ends.However,as planar devices,HTS filters also suffer from unwanted spurious bands.Interference signals can leak into the receiver front-ends through spurious passbands,which will seriously deteriorate performances of whole communication systems.Therefore,research on HTS filters with spurious passband suppression has important significance.In this dissertation,three kinds of novel structures are presented to realize spurious suppression performance of filters.?1?A ground surrounded quasi-interdigital structure?GSQIS?is proposed.Compared to conventional interdigital structure,the GSQIS can be conveniently loaded on a quarter-wavelength microstrip resonator to obtain a large ratio of the first spurious frequency f_s to the fundamental resonant frequency f₀,while without changing the original coupling strength of a filter.A compact four-pole high temperature superconducting?HTS?filter is designed and fabricated using the proposed resonators.The measured 80 dB rejection stop-band of the filter is up to 1040 MHz,which is 7.3 times of the fundamental frequency.The size of the filter layout is 0.033?_g×0.017?_g,where?_gis the guided wavelength at143 MHz.?2?An asymmetric stepped impedance resonator?ASIR?,which consists of two stepped impedance transmission-lines and an interdigital capacitor?IDC?structure,is proposed.Compared to conventional ASIR,the proposed resonator can adjust the ratio?f_s/f₀?of the first two resonant harmonics by changing the capacitor value of IDC.And the proposed resonator has a smaller size and a higher f_s/f₀ratio.An equivalent model is used to analysis the fundamental frequency f₀of the resonator.Moreover,a novel IDC structure is introduced in the SIR,which can obtain a better suppression performance than conventional IDCs.For demonstration,a small high-temperature superconducting?HTS?bandpass filter is designed and fabricated using the proposed ASIRs.The stopband of the filter is extended up to 4.0 f₀and 3.5 f₀with 30 dB and 60 dB out-of-band rejection levels,respectively.The size of the filter layout is 0.16?_g×0.13?_g,where?_gis the guided wavelength at 922 MHz.?3?A self-embedding asymmetric stepped asymmetric impedance resonator?SE-ASIR?topology is proposed.By embedding stepped impedance resonators in themselves,circuit sizes of SIRs can be reduced effectively,while the ability to control spurious modes of SIRs still remains.Therefore,SE-ASIRs are suitable for designing filters with wide stopbands and miniaturized sizes.Furthermore,the construction process of the SE-ASIR is described in detail,and an equivalent model of the SE-ASIR is proposed.For demonstration,a high-temperature superconducting bandpass filter centered at 1112 MHz is designed and fabricated for demonstration.The filter circuit size is 0.28?_g×0.12?_g,where?_gis the guided wavelength at 1112 MHz.The measured result agrees well with the simulation and shows that the out-of-band rejection is better than 60 dB up to 4088MHz,which is about 3.7 times of the fundamental frequency.