Research On Electromagnetically Tunable And Multilayer SIW Filters

Due to the rapid development of wireless communication and a variety of communication modes and services existing in different bands, there is an increasing demand on multi-band or multi-mode reconfigurable RF front ends. As one of the core components in the reconfigurable RF front ends, tunable RF filters play an important role to select or suppress the desired frequency bands. Hence, it is quite meaningful to focus on the issues of high selectivity, large tuning rang and compact RF filters.In this thesis, first, the plane filter loaded with varactor diodes is studied. Second,magnetically tunable SIW filters are proposed. Since the bandwidth of the transceiver is always constant, we adopt a constant bandwidth scheme to make sure that the absolute bandwidth or fractional bandwidth during the tuning range can keep constant in our proposed tunable filter designs, then can improve the practicality, simplify the bias circuit and preserve compact sizes. Then, the realization of the electromagnetic coupling in vertically coupled SIW resonators is studied. Finally, compact SIW filters with harmonic suppression and high selectivity are proposed. The main contents of this research include:(1)Electrically tuned bandpass filters: First of all, single-band and dual-band constant bandwidth tunable bandpass filters are proposed. In addition, the two passbands of the dual-band bandpass filter can be independently tuned. Among the two filters, the single-band tunable bandpass filter is realized by a none-shorted ring resonator loaded with varactors.Meanwhile, a dual-band bandpass filter is designed by using two dual-mode resonators. The bandwidths and selectivities of the two passbands are chosen according to the odd and even modes of the resonators. The proposed filters are both with high tuning rate and simple controlling circuits.(2)Electronically tuned bandstop filters: First of all, sing-band and dual-band bandstop units with independently controlled center frequency and bandwidth are established. Then,two-, three-order sing-band bandstop filter and dual-band bandstop filter are designed while combining with compact series structure. All the proposed filters show the advantages of independently controlled center frequencies and bandwidths and compact sizes. Thanks to the analysis on the bandwidth and coupling coefficient of the bandstop units during the tuning,tunable two- and three-order bandstop filters with constant bandwidth are designed with loaded varactors. The proposed tunable bandstop filters have the advantages of high suppression levels and compact physical sizes.(3)Electrically tunable bandpass filter with bandstop units: First of all, a three-order tunable bandpass filter with constant bandwidth is proposed. Then, combining the bandstop units, extra transmission zeros are introduced to suppress the strong interfere signal that emerges around the passband. Thanks to the easily controlled center frequency of the bandstop units, the introduced transmission zeros can be freely controlled by the loaded capacitors of the bandstop units, while the passband are preserved at the same time. The size and controlling circuits of the proposed filter are compact and simple, respectively.(4) Magnetically tuned SIW filter: First of all, the specifics of the effect permeability of the ferrite under different magnetic bias are analyzed. Then, the proper location is chosen to load the ferrite slabs according to the magnetic field distribution of the SIW higher-order mode TE202. Finally, the magnetically tuned SIW higher-mode filter with constant bandwidth is designed by choosing appropriate coupling windows. Under the same fabrication platform,the proposed high-order mode tunable filter can be operated at a higher frequency band.(5) Multi-layer SIW bandpass filters: First of all, the electromagnetic field distribution of SIW resonators under vertically coupling is analyzed. The electric, magnetic and mixed couplings of fundamental and high-order modes are proposed according to coupling theory.Furthermore, two harmonic suppressed bandpass filters and one high-order cross-coupling bandpass filter with harmonic suppression are designed thanks to the independence of the coupling of fundamental modes and high-order modes. The proposed filters have compact sizes, high out-of-band rejection and high selectivity.This thesis proposes different kinds of electromagnetically tuned filters, which are helpful to the development of the tunable filters with large tuning range and high selectivity. All the proposed tunable filters are with constant bandwidths and easy controlling strategy.Meanwhile, the proposed multilayer SIW filters have compact sizes, high selectivity and out-of-band rejection, leading to good practicality.