Research On Miniaturized Bandpass Filters Based On Multilayer Folded Substrate Integrated Waveguide Technology

Miniaturized,low-cost,and high-performance microwave filters are the core of many design problems in wireless communications.The frequency spectrum of electromagnetic waves is limited and should be allocated according to the application.Whether it is to limit the radiation of high-power transmitters within the specified frequency band or to prevent the receiver from receiving interference outside the working frequency band,filters need to be used.To obtain certain phase or delay characteristics,such as pulse compression or broadening,or to compensate for phase distortion caused by other filters or dispersion structures,it is also necessary to apply high-performance microwave filters.Miniaturized,high-performance,low-cost,and easy-to-integrate planar filters have also been widely researched.Substrate Integrated Waveguide（SIW）technology,as a new low-cost on-chip technology,provides the possibility to design high-performance microwave and millimeter-wave devices.However,for the microwave low-frequency band,the lateral size of SIW is still too large compared to planar transmission lines such as microstrip lines and strip lines.The application of multi-layer SIW technology can effectively solve the problem of excessively large lateral dimensions of microwave devices and achieve size miniaturization.Based on the multilayer FSIW technology,this paper designs a series of miniaturized high-performance microwave bandpass filters with self-encapsulation characteristics.The main research content of this article is mainly divided into the following points:First,two miniaturized waveguide bandpass filters with different frequency responses are proposed based on the folded substrate integrated waveguide（FSIW）and complementary split-ring resonators（CSRRs）.The application of folding technology can reduce the lateral size of the filter by half.The fractional bandwidth（FBW）of the first band-pass filter is 35.5%,and the insertion loss（IL）in the passband is about 1.2 d B.The CSRRs are resonate above the cutoff frequency of the FSIW.The narrowband bandspass filter is realized by three CSRRs which resonate below the cutoff frequency of the FSIW.Therefore,the miniaturization of the filter can be further achieved.In both designs,all CSRRs are etched on the middle metal layer of the FSIW.Compared with filters based on SIW technology,it can reduce the radiation loss caused by the incomplete ground plane.At the same time,due to the self-encapsulation characteristics of FSIW,the designed band-pass filter can be easily integrated with other planar circuits while achieving miniaturization.Second,the characteristics of the quarter-wavelength folded substrate integrated waveguide cavity（QFSIW）are studied,and the direct-coupled bandpass filter is designed.To improve the selectivity of the bandpass filter,two cross-coupled bandpass filters are designed.The cross-coupled signal paths are realized by stripline and I-shaped slots respectively.A three-pole cross-coupled bandpass filter is designed and fabricated using the stripline coupling and the QFSIW cavities.A transmission zero（TZ）in the upper stopband is realized.A fourth-order cross-coupled filter based on the novel I-shaped negative coupling structure and the QFSIW cavities is designed,fabricated,and measured.Two TZs are realized in the upper stopband and lower stopband respectively.High selectivity and compact size can be achieved by using the QFSIW cavities and the cross-coupled technology.All the slots are etched in the middle metal layer of the cross-coupled bandpass filter.Owing to the self-packaged characteristic,the proposed filters can be integrated with other planar circuits.Third,to further realize the miniaturization of the band-pass filter,two single-cavity dual-mode filters based on a QFSIW cavity were designed.The high-order degenerate modes TE₁₀₃ and TE₃₀₁in the QFSIW cavity are used to form the passband.The feed lines are perpendicular to each other and the degenerate mode is divided into odd mode and even mode relative to the feed lines.The signals passing through the odd mode and the even mode are canceled at a specific frequency,thereby introducing a transmission zero in the upper or lower passband of the filter.The TE₂₀₂mode in the resonant cavity acts as a non-resonant node（Non-Resonanting Node:NRN）,which can introduce additional cross-coupling paths between the source and the load to generate transmission zero points to improve the performance of the filter.Based on the single cavity dual-mode FSIW bandpass filter,to improve the selectivity and the out-of-band suppression,a series of high-performance and miniaturized FSIW bandpass filters based on the box topology are proposed.Two FSIW resonators that have been folded once along the middle virtual magnetic wall and a QFSIW cavity are cascaded to form a box-like filter.The FSIW cavity works in the fundamental mode TE₁₀₁ mode and the QFSIW resonant cavity is a dual-mode resonator,in which TE₁₀₃ and TE₃₀₁ are degenerate modes.Different coupling modes between single-mode resonators and dual-mode resonators are studied.The same coupling topology,namely box-type coupling topology,is used to design and implement band-pass filters with different transmission responses,including three fourth-order single-mode filters and a dual-band bandpass filter.