Structural Optimization And Capacitive Cross-coupling Design Of Dielectric Cavity Filters

With the rapid development of modern wireless communication,the performance of the base station filter needs to meet the more stringent requirements.Metal coaxial cavity filter and dielectric cavity filter with high performance dielectric resonators are the two common base station filters,and the latter has the advantages of low loss,high temperature stability and miniaturization.It is necessary to provide higher quality signals to meet the increasing demand of data.Therefore,how to make full use of the high performance dielectric resonators to design the whole filter is very important for production applications,especially how to select the cavity layout structure,how to optimize the overall parameters to shorten the production time and reduce the size to reduce costs.Direct-coupling is crucial to the filter curve,first of all,in this paper,a bandpass dielectric resonators filter with six cylindrical cavities was designed by using the circuit simulation software ADS?Automation Device Specification?and the electromagnetic simulation software HFSS?High Frequency Structure Simulator?.In order to introduce the cross-coupling in the later stage,three different structures were compared to obtain the best topological configuration of the cavity and it proved that U-type cavity structure was the best.In particular,according to the same technical requirement,six-pole bandpass cavity filters were designed with cubic cavities by using ring or rod feeder ports in U-type topology.Finally,the experimental results are as follows:the center frequency?f₀?is 2.62 GHz,the band width?BW?is 50 MHz,the insertion loss?IL?in the passband|S₂₁|<0.5 dB,the return loss?RL?in the passband|S₁₁|>20dB,the out-of-band rejection|S₂₁|>40 dB@f₀±60 MHz.It is proved that the cube cavity has wider range of applications.Transmission zeros can be generated by introducing cross-coupling structure in order to further enhance the out-of-band suppression.According to the theory of capacitive cross-coupling,based on the four-pole filter of direct-coupling,a U-type rod was introduced to generate transmission zeros.Two transmission zeros were generated in the high and low band of curve by using cascaded quadruplet?CQ?topology and one transmission zero was generated in the low band of curve by using cascaded triplet?CT?topology through adjusting the position and size of the coupling structure.Comparing the S parameter curves of direct-coupling and cross-coupling,it can be seen that the transmission zeros generated by cross-coupling enhanced the out-of-band rejection|S₂₁|?f₀±60 MHz?in the case of meeting the requirements of the indicators?f₀,BW,IL in the passband,RL in the passband?.To further verify the experimental results,according to the characteristics of the cylinder and cube cavity,vertical U-type bars in CQ capacitive cross-coupling were added to the previous direct-coupling six-pole filters,and it can be found that the out-of-band suppression was also enhanced.So the proposed capacitive cross-coupling structures are reasonable and feasible.After the completion of the direct-coupling and cross-coupling,the higher requirements of the technical indicators were proposed.The higher requirements of eight-pole filter with cubic cavities are as follows:f₀ is 2.62 GHz,BW is 50 MHz?2.56 GHz².61 GHz?,IL in the passband|S₂₁|<1 dB,RL in the passband|S₁₁|>20 dB,the out-of-band rejection is|S₂₁|>60dB@f₀±60 MHz,the input/output impedance is 50?.Two U-type topology structures with four cavities respectively were connected to simulate the direct-coupling.On the basis of it,the horizontal U-type bar as cross-coupling structure was added to introduce CQ capacitive coupling.And the transmission zeros were generated at the high band 2.6870 GHz and low band 2.5218 GHz.By comparing the results of adding the cross-coupling structure,it can be concluded that both the left and the right out-of-band rejection have been improved,and the overall out-of-band performance was better than the technical requirements.In addition,the design example of a eight-pole dielectric cavity filter by using CT capacitive cross-coupling with cylindrical cavities was also presented with technique targets such as the center frequency?f₀?=2.605 GHz,BW=50 MHz?2.58 GHz².63 GHz?,IL in the passband|S₂₁|<1 dB,RL in the passband|S₁₁|>20 dB,the out-of-band rejection|S₂₁|>60 dB@f₀±60 MHz,the input/output impedance 50?.Finally,the simulation results showed that a transmission zero was successfully generated in the low band?2.5168 GHz?,and the out-of-band suppression at 2.545 GHz was 89.8295 dB.Compared with 87.9135 dB in the direct-coupling at the same frequency,the left out-of-band rejection was enhanced.After the simulation debugging,according to the specific parameter values,AutoCAD software was used to draw the cavity specific size.Through the assembly and debugging,the measured results basically reached technical expectations and there were some differences from the simulation results due to the influence of machining error and ambient temperature.Generally,the feasibility and practicability of the direct-coupling and cross-coupling design were verified.