Research Of Harmonic-suppression And Multifrequency Cavity Filter

Microwave filter is an indispensable part of wireless communication system and plays a very important role in the process of transmitting and receiving signals from RF terminals.The rapid development of wireless communication technology has brought great convenience to people.In addition to the resonance at the basic frequency,the microwave resonant circuit also generates high-order harmonics at the fundamental multiple frequency.These unnecessary harmonics will affect the frequency selection and denoising performance of the filter.The higher harmonics of the cavity filter appear more cluttered than the microstrip structure filter.Therefore,it is important to suppress the higher harmonics in the cavity filter.On the other hand,how to design a multi-frequency filter that can adapt to various communication standards has gradually become a hot research topic.At present,most of the research focuses on multi-frequency microstrip filters,but there are few studies on the cavity structure.The design of low-cost,low-volume and multi-performance cavity multifrequency filters is of great significance.The research work based on cavity filter structure mainly studies the realization of cavity filter harmonic suppression and the design method of cavity multi-frequency filter.Its work content is roughly divided into the following three parts:(1)The coaxial cavity harmonic suppression filter has been studied.The coaxial resonator is a quarter-wave resonator,the resonator generates higher harmonics at three times and five times of the fundamental frequency.Three methods for suppressing cavity harmonics are proposed based on coaxial cavity filter characteristics are proposed.(a)A method of adding an auxiliary resonator to the excitation port is proposed.The excitation port and the energy of the main resonator at the triple frequency and the fifth frequency are transferred to the excitation column and the auxiliary resonator,thereby suppressing the first and the second parasitic passbands of the filter.And two auxiliary resonant units are designed to realize the harmonic suppression wide-resistance coaxial cavity filter.(b)A capacitive load filter based on step-impedance resonator(SIR)structure is proposed.Due to the suppression of harmonics by SIR structure and load capacitance,a coaxial SIR capacitor-loaded harmonic suppressionfilter is designed which the harmonic suppression effect is more than 8 times the basic frequency.(c)A coaxial cavity harmonic suppression filter based on branch loading is proposed.The first harmonic of the resonator is staggered by the branch loading technique,so that the filter does not resonate at the triple frequency,and the first harmonic of the filter is suppressed.(2)The rectangular waveguide harmonic suppression filter is studied.A harmonic suppression method based on rectangular waveguide filter is proposed.By adding ridge processing to the rectangular waveguide,not only the volume of the filter can be greatly reduced,but also the out-of-band suppression performance is better,The out-of-band suppression effect is more than 3 times the basic frequency.(3)The cavity multi-frequency filter was studied.A dual-frequency filter is designed based on the coaxial cavity.For the open cavity branching of the coaxial cavity,a new resonant mode can be introduced,and the position and length of the branch loading affect the frequency ratio of the harmonic to the fundamental frequency.A dual-band cavity wideband filter based on branch loading is proposed.The first passband center frequency is 0.9 GHz and the 3 dB bandwidth is 9%.The second passband center frequency is 2 GHz,the 3 dB bandwidth is 7.2%,and a transmission zero is introduced.