| Recently,with the rapid development of modern wireless communication,the performance requirements of microwave and millimeter wave communication system become increasingly higher.Devices in the system are developing towards high performance,high integration,low cost,and low loss.As microwave devices with frequency selective functions,filters and diplexers,which play a key role in filtering unwanted frequency signals and reducing interference,have received widely attention in theoretical research and practical applications.Their performance directly determines the performance of RF front-end and even the entire communication system.Substrate Integrated Waveguide(SIW)technology has attracted widespread attention in the research of filters and diplexers due to its advantages of high Q value,low insertion loss,high power,low cost,and ease of integration.Compared with rectangular and circular SIW cavities,triangular substrate integrated waveguide(TSIW)cavities have the advantage of being more flexible in shape while maintaining similar performance.They can be arranged appropriately to form more compact devices.In this article,TSIW cavities are discussed and employed to design filters and diplexers.The main research contents are as follow :(1)The resonant frequencies and transverse field components of two classical TSIW resonators,isosceles right angle TSIW and equilateral TSIW,are discussed in detail.Firstly,the electric field distributions and resonant frequencies of each mode in these two resonators are rigorously derived.Then,based on this and coupling matrix,compact fourth order and sixth order bandpass filters are designed.The fourth order filter consists of isosceles right TSIW cavities,and the sixth order filter consists of equilateral TSIW cavities.Both TSIW cavities are in their fundamental mode.A pair of transmission zeros in the filters is observed due to introducing a negative cross coupling structure.After that,an approximate formula for the fundamental mode resonant frequency of an isosceles TSIW cavity with a vertex angle of 72°is given.Based on this cavity,a fifth order bandpass filter with a transmission zero at upper sideband is designed.All three filters are fabricated and measured,The measured results agree well with the simulation ones,which verifies the validity of theoretical derivation.In addition,the proposed filter has the advantages of low cost,compact structure,good selectivity,and easy integration with other planar circuits.(2)A coupling matrix transformation is presented to change the sign of the coupling coefficient.Transforming the negative coupling coefficient to the diagonal of the coupling matrix,the response can be controlled by resonant frequency shifting instead of the negative coupling structure.Then,combined with the transformed coupling matrix,a mixed mode band-pass filter based on dual mode equilateral TSIW cavity and single mode isosceles right TSIW cavity is realized.In order to shift the resonant frequency of the equilateral TSIW cavity,a perturbation metal via is introduced at the top corner of the triangle.Then,for eliminating the perturbation metal via in the equilateral TSIW resonator,a mixed mode band-pass filter based on dual mode isosceles TSIW resonator and single mode isosceles right TSIW resonator is designed.Two transmission zeros are implemented in both filters.Finally,the two devices are fabricated and measured.The measured results are in line with the simulation ones.The presented filters eliminates the complex negative coupling structures,reduces the complexity of circuit processing,and makes it easier to package and integrate.At the same time,it has also verified the feasibility of TSIW cavities in multimode filter design.(3)A diplexer based on mixed mode isosceles right TSIW resonator is proposed.A third order diplexer is formed using a dual mode isosceles right TSIW cavity instead of the T-junction structure in traditional diplexers,combing with two single mode isosceles right TSIW cavity at each channel.The dual mode cavity provide a transmission pole for each channel,so the diplexer is more compact compared to traditional T-junction structures.In addition,based on the field distributions of two modes in the dual mode isosceles right TSIW cavity,the positions of the coupling windows and the feeding microstrip line is adjusted properly.The isolation between two channels is 45 d B.The diplexer has good insertion loss and return loss,and the simulation and measured results agree well.In addition,the diplexer has the advantages of easy processing and integration.(4)The equilateral TSIW cavity is folded along the median to form a folded TSIW cavity.Firstly,the folded TSIW cavity is analyzed.Then,based on a folded TSIW cavity and two equilateral TSIW cavities,a three order bandpass filter with a two-layer structure is designed.A coupling circular hole is etched in the middle mental layer between feeding microstrip lines,leading to introducing of non resonating node(NRN)and resulting in three transmission zeros in the designed filter.One transmission zero is at lower side and the other two are at upper side.Finally,the filter is processed and measured.The measured results are consistent with the simulation ones. |
