Research On Quasi Optical Cavity Test System Based On Spectrum Analyzer

At present,the research and development of high-precision and low-loss material parameter testing system in millimeter wave and terahertz band is urgent.The quasi-optical cavity has the advantages of simple structure,easy processing and high inherent quality factor.It is a common structure for developing the related test system in this frequency band.Therefore,the focus of this thesis is to build a quasi-optical cavity test system working in W-band based on spectrum analyzer through the theoretical analysis of quasi-optical cavity quality factor and the demonstration of the scheme of precise measurement of quality factor.As for the theoretical analysis,this thesis first discussed the definition and measurement methods of the quality factor of quasi-toptical cavity,and selected the structure of transmission measurement system.Through the analysis of its equivalent circuit,the relationship between the complex transmission coefficientS₂₁ and frequency f,on load quality factorQ_L and inherent quality factor₀Q is deduced.For the calculation of quality factor,the thesis introduced the test principle and calculation formula derivation of three analysis methods in detail:the 3d B method,Lorentz fitting method and resonance curve area method（RCA）,so as to improve the accuracy of the test system.As for the implementation process,Firstly,this thesis formulated an external harmonic mixing scheme suitable working in W-band based on the theoretical basis of the spectrum analyzer and the principle of harmonic mixing.Secondly,this thesis established the simulation model of conical horn antenna according to the design index in HFSS,and drew the engineering drawing.The simulation gain of the antenna is more than 30 d B,and the relative bandwidth is more than 30%.The matching performance of the antenna is favorable,and the measured results are basically consistent with the simulation curve.Finally,this thesis analyzed the the distribution of the field in the quasi optical cavity after loading the beam splitter based on the diaphragm coupling theory,and designed and processed the beam splitter model.Finally,this thesis built the double concave quasi optical cavity test system based on spectrum analyzer.The thesis uses Python to write the test program,which realized the automatic frequency scanning of the signal source and the spectral line reading function of the spectrometer.Then,the theis compiled the curve fitting and quality factor calculation programs of three analysis methods.Through the analysis of the measurement accuracy of the three methods,we can know that the measurement accuracy of Lorentz fitting method is close to the resonance curve area method,and the measurement accuracy of resonance curve area method is relatively higher.The system realized the accurate measurement of the quality factor of the quasi optical cavity,and has significant engineering application value.This system can be used to measure the dielectric property parameters of dielectric materials and the reflectivity of metals in W-band,especially for the test and development of high-power vacuum window materials.