Thermally Induced Passive Intermodulation Mechanism Of Microwave Transmission Line

With the rapid development of the wireless communication,passive intermodulation(PIM)has become one of the most critical issues affecting the performance of communication system.PIM refers to the phenomenon that high power RF signals are mixed together via the passive devices,spurious signals are generated due to the inherent nonlinearity of the device.PIM has always been a difficult problem in wireless communication systems.The low-power interference signal will increase the noise floor of the receiver,reduce the communication quality and communication capacity.PIM has become an important index for evaluating the reliability of communication systems,further research on PIM issues is necessary.This thesis is aimed at the microwave transmission line devices which widely exist in wireless communication systems.With the actual printed circuit board manufacturing process,the thesis analyzed and modeled the PIM problem of microwave transmission lines.The details are as follows:1.The power loss prediction model of transmission line based on fractal geometry was developed.Based on the Weierstrass-Mandelbrot(WM)model,the rough topography of the substrate-conductor interface can be represented by fractal dimension and contour scale parameter,and the power spectral density method was used to identify the fractal parameter from the actual measurement data.In order to study the influence of the substrate-conductor interface on the power loss simulation,a power loss prediction model based on the electromagnetic field analysis theory was developed from the continuity assumption of conductor conductivity.Furthermore,with the parameters such as equivalent conductivity and surface impedance parameter,the influence of rough interface on the power loss and self-heating phenomenon was analyzed.2.Research on PIM power level prediction model for microwave transmission lines was developed.In order to solve the problem of PIM modeling in microwave transmission lines,considering the actual thermal characteristics of the device,the effects of design parameters on the periodic temperature distribution of the device were analyzed.Based on electrothermal coupling modulation effect,the expression of thermally induced PIM per unit length of the transmission line was deduced.Based on the point-source PIM generation mechanism,the forward and reverse PIM power level prediction models of microwave transmission line were established.3.Experimental verification of the PIM prediction model were accomplished.According to different manufacture technology of copper foil,with the rough topography measurement data and power spectral density identification method,the fractal dimension and contour scale parameters of different copper foil were obtained.The forward PIM power levels were calculated under different design conditions,and the corresponding transmission lines prototype were designed.The theoretical predictive value and experimental measurement value were compared to verifies the accuracy of the prediction model,and the design factors affecting the PIM performance of transmission line were analyzed.Then,the trends of forward PIM behavior under different conditions were obtained.At the same time,the lowPIM design guidelines for microwave transmission lines were proposed from the perspective of engineering feasibility and circuit design.