The Research And Design Of A Kind Of Directional Coupler And A Kind Of Band-pass Filter Used In TV Broadcast Communication

Directional couplers and filters,as two important parts of many microwave circuits,are widely used in modern microwave communication system.Because each port is impedance matched,directional couplers can be easily embedded into other circuits or sub-systems.Coaxial cavity filter is widely applied in modern wireless communication,digital TV broadcasting,satellite navigation,radar and other remote sensing and telemetry system,for its high power capacity,low insertion loss,and far spurious pass-band.The research of this thesis is based on the project of a TV transmission equipment company,the requirement is to design one directional coupler and one narrow-band band-pass filter,which is used in television broadcast system.Due to the limited spectrum resources which television broadcast system can use,the bandwidth of channel is less than 10 MHz.So here are the requirements: Firstly,the center frequency of the directional coupler and filter is 600MHz;Secondly,high power capacity,high isolation and temperate pass-band of the directional coupler is needed;Finally,the filter should have the characteristics of narrow pass-band,high band rejection,low insertion loss and high power capacity.According to the requirements of the project,this thesis mainly deals with the directional coupler and filter.Through analyzing the structure of strip-line and coaxial cavity,in this thesis,one directional coupler based on air strip-line and one kind of narrow band-pass filter based on coaxial cavity are designed.Details are as follows:1)At First,together with the history and basic theoretical knowledge of directional couplers and filters,the resource and the significance of this study is introduced,which is the foundation of this thesis.2)The calculation method of strip-line,coupled line and couple strip-line is analyzed in the thesis.According to the theoretical knowledge,a 3d B directional coupler which is based on air-strip-line is designed.Then,it should meet the requirements of bandwidth of 16.7%,the VSWR being less than 1.2 and the isolation greater than 20 d B.The influence of each parameter on the performance of the directional coupler is analyzed.At last,the directionalcoupler is fabricated and measured,and the simulated and measured results are also shown in the thesis.3)The coupling mode,coupling structure,and port structure of coaxial resonator is analyzed in the thesis firstly.Then,a kind of narrow band-pass filter based coaxial cavity is designed according to the analysis.Its bandwidth is 1.25%,the insertion loss is about 0.5d B,and the band rejection is up to 60 d B.After simulation and optimization,the filter is fabricated and measured,and the simulated and measured results have been compared and they agree well with each other.