Study On Cross-coupling Technique For Substrate Integrated Waveguide Bandpass Filter

Combining the inherent advantages of waveguide resonator filter namely high quality, highpower capacity and the advantages of micro-strip line namely light weight, small size, cheap priceand easy to be integrated, Substrate Integrated Waveguide (SIW) filters become popular for filterdesign in the past decade. In general, there are two ways to maintain transmission zeros (TZs), thatis cross-coupling technique and muti-mode technique. As another key point, crossing couplingtechnique, which adds extra coupling between non-adjacent resonators, is theoretical and practical.Firstly, based on the canonical topology of cross coupling resonator, stacked four-cavity SIWbandpass filters, with input and output laying on the same layer, are achieved. Detailed discussionsof the impacts of the coplanar microstrip input/output structure, non-coplanar probe input/outputstructure and the coupling slot, coupling circle are made to modify the design parameters.Furthermore, taking price and difficulty into account, fabrication of the2.5GHz SIW bandpass filterusing cross coupling technique with micro-strip input/output ports has been done. Measured resultsbasically agree with the simulation, showing the2%narrow bandwidth and2.47GHz centrefrequency. Moreover, the influence of the relative layout of resonator cavities for filter design and aimproved layout is demonstrated according to the analysis and verification of stacked three-cavitySIW filter.Secondly, in order to study the SIW bandpass filter based on another cross coupling topology,coplane four-cavity and stacked four-cavity SIW bandpass filters are proposed. The simulationresults show that these two cross coupling topologies four-cavity SIW filters have the sameperformance. Especially for the new coplane four-cavity SIW cross coupling filter, it shows a goodcharacter when it works at different modes such as the the dominant mode, high mode or bothmodes. In addition, the stacked four-cavity SIW cross coupling filter can futher reduce the sizecompared with the coplane structure.Finally, a SIW bandpass filter using dual-mode technique to achieve two transmission zeros isdescribed step by step. The comparison between the dual-mode SIW filter and the cross couplingSIW filter shows that the cross coupling technique suits for narrow bandwidth SIW filters while thedual-mode technique can reduce the size of SIW filter effectively. Therefore, it ends with an idea ofthe combination of these two technologies to design multi-cavity multilayer stack SIW filter.