Modelling of passive intermodulation in RF systems

Sammanfattning: Modern communication systems are increasing in complexity, power and dynamicrange. As a result, weaker signals can be detected which improves rangeand quality of service. However, this also results in greater sensitivity to spurioussignals which can impact system performance. One of the mechanismsthat spurious signals can be created is by mixing that occurs by the nonlinearbehavior of components. These components are typically active, such astransistors and diodes. Recently, there has been increased focus on studyingnonlinearities of passive components. These are usually considered linear andoperate under low power, such as connectors and waveguides. Such elementsexhibit weak nonlinearity and can produce measurable levels of unwanted signalswhen enough power is injected into the system. Since the source is passive,these signals are referred to as passive intermodulation products.In this thesis, the fundamental behavior of passive intermodulation is exploredand modelled. A literature review of physical sources is conducted andkey contributors to nonlinearities are identified. A general model is developedfor both discrete and distributed structures and can be used to characterize thecurrent-voltage relation, regardless of the physical origin of nonlinearity. Additionally,different numerical methods are presented for calculating the currentamplitudes of individual spectral components, allowing for extraction ofpower of intermodulation products. The models are first verified through comparisonwith commercial circuit simulation software. The effects of losses anddispersion in transmission lines on passive intermodulation generation are alsostudied.Finally, the model is used for characterizing a transition from stripline tomicrostrip line. The presence of other nonlinear components is separatelycharacterized and accounted for. The developed model is shown to be ableto closely fit the third order intermodulation products. Additionally, it canpredict higher order intermodulation products with a good degree of accuracyfor two out of three components measured.