A Study Of Passive Intermodulation In Microwave Band

In recent years, passive intermodulation (referred to as PIM) is observed as oneof the key technical problems with the development of high-power, large-capacity,ultra sensitive communication system. PIM is a form of nonlinear distortioncaused by the electromagnetic nonlinearity of passive components,such asantennas,cables and duplexers.The passive nonlinearity is negligible when the inputis very small. However,passive weakly nonlinearity can not be ignored while theinput power is very high. Generally, the nonlinear distortion generated by the activedevice can be mitigated by filters. However,PIM distortion products is so close tothe operating frequency that it can fall into the operating band instead of beingfiltered. This is one of the reasons why it can not be cancelled. In addition, PIMdistortion often appears behind the filter,so it is difficult to eliminated completely.Severe distortion will not only limit the sensitivity of the receiver, but also degradethe performance of the entire system,or even destory it. The mechanisms of PIM iscomplex,often are combined results of various physical processes,so people tend toutilize their experience and behavior models to describe the reasons behind it.In thispaper,we developed a a better understanding of the causes and behavior of PIM inmicrowave systems,particularly microstrips on PCB.Firstly, we explain the main physical causes of passive nonlinearity, introducethe mathematical models and analytical methods. Secondly,we inquire into themechanism of microstrip line on the printed circuit board. Two equations areintroduced from the point of physics and phenomenology respectly,which areintended to explain the characteristics and rules of PIM occuring in microstrips. Thesimulation and analysis results shows certain relationships between the strip length,microstrip width and substrate material properties and PIM of microstrip.Somemethods of low PIM microstrip design are put farward also.Finally,we study themeasurement technology of PIM. A obvious PIM distortion is observed when thepower of the input signals is27dBm respectively.The experimental results from aPIM testing system established by oueself,some testing datas on antennas and a setof microstrip lines,are given and discussed. Additionally,a short-circuit line testingmethod,one of methods based on the standing wave principle,is presented,which canimprove the measurement dynamic range of our system by17dB or more.