Research On The Modeling Of Passive Intermodulation In RF Connectors And The Effect Of Signal Interference

Passive intermodulation(PIM) is a kind of interference phenomenon in communication system.With the development of wireless communication technology,the widespread application of high-power transmitters,transceiver antennas,dense spectrum resources and the demand of high-sensitivity receivers make the PIM problem of passive components increasingly prominent and become an extremely important factor limiting the system capacity.Among all the passive devices,radio frequency(RF)connectors are considered as the most widely used and diverse components which play a critical role in connecting different modules and transmitting signals.When the connector works in the atmospheric environment,the terrible environment will result in the degradation of the contact surface and the change of the material properties,having some negative effects on the PIM performance.Therefore,PIM performance of the RF connector has become a key factor that restricts the signal integrity of the entire system,which is extremely important to the reliability of the communication system.In this thesis,the PIM mechanism in RF connectors and the effect of the interference signals in entire system are theoretically modeled and experimentally studied.This research is composed of four parts.They are respectively the PIM mechanism caused by contact nonlinearity,the PIM mechanism caused by material nonlinearity,the effect of degradation in RF connectors caused by harsh environment on PIM performance,and the PIM fault location and interference mitigation analysis.These four researches are interrelated.The paper demonstrates from device modeling to system analysis,from micro-surface to macro-circuit,from theoretical deduction and simulation analysis to experimental verification.The PIM characteristics of RF connectors and the influence of interference signals are comprehensively expounded.The research results have significant value for the design of low-PIM components and the selection of connectors with low-PIM requirements.At the same time,the research results can also be extended to other passive devices and systems.The main contributions of this dissertation are concluded as follows:1.Based on the physical modeling and nonlinear mathematical analysis of contact surface,the mechanism model of passive intemmodulation caused by contact is established.An equivalent circuit model of connection junction is developed.The micro-physical parameters such as contact pressure,surface roughness and contact area are correlated with the macro-circuit parameters(resistance and capacitance).According to the conduction mechanisms,the distinguish of different nonlinear transfer functions on the PIM modeling is comparatively analyzed.The nonlinear conduction mechanism based on the model of power cut-off distortion at the contact point is established.The influence of micro change of contact surface on the power performance of macro intemiodulation products is discussed from the physical mechanism.The validation experiment is designed and conducted to verify the availability and accuracy of the proposed models.2.Based on the nonlinear effect of magnetic materials in the coating and base metal of connector,the mechanism model of passive intermodulation caused by material nonlinearity is established.An equivalent finite element analysis(FEA)model of the cross section of inner conductor is proposed to study the current density distribution in different metal regions at different frequencies.According to the hysteresis effect of magnetic metals,a nonlinear resistance model induced by the current in magnetic region is theoretically concluded.Consequently,a theoretical prediction model of IM product power is derived.The connector samples with different coating materials are designed,processed and produced,which are used in the PIM experiments to verify the availability and accuracy of the proposed models.3.Based on the strong oxidation environment,vibration environment and temperature change environment,the prediction model of passive intermodulation characteristics caused by connector degradation under different harsh environments is developed.Under different environmental stresses,the failure mechanism of the connector is analyzed,and the degradation characteristics caused by environmental stresses are extracted.The theoretical prediction model of the relationship between the degradation characteristics,such as the corrosion rate of the contact surface,the shape variables produced by the contact,the physical properties of the material(permeability,conductivity,hardness,etc.)and the power of the intermodulation products was established.A series of accelerated tests of different environmental stresses are designed and carried out for producing the required degradation connector samples.Relevant experimental results are given to verify the validity of the predicted models.4.Based on the superposition effect of multiple intermodulation points,the PIM characteristics of a series system are studied.By using the fault characteristics of the IM products in different types of RF connectors,a novel method of locating the abnormal PIM points is proposed based on BP(Error Back Propagation)neural network.Considering the methods of active signal compensation and passive nonlinear network insertion,four schemes of system intermodulation interference reduction are proposed.Through simulation and experimental tests,the advantages of different mitigation methods are compared and analyzed,and the effectiveness of the proposed scheme is verified.