ying weakly nonlinear behavior. Analyzing time-varying weakly nonlinear behavior builds on per- turbation techniques and linear time-varying system analysis. It thereby proceeds along the same lines of reasoning as used for time-invariant weakly nonlinear system analysis [Wamb98a]. Developing (symbolic) methods for time-varying weakly nonlinear sys- tem analysis would contribute yet another step towards a true top-down analysis (and hence design) of electronic systems and circuits. Chapter 5 Modeling oscillator dynamic behavior If man were restricted to collecting facts the sciences would be only a sterile nomenclature and he would never have known the great laws of nature. It is in comparing phenomena with each other, in seeking to grasp their relation- ships, that he is led to discover these laws... -Pierre-Simon Laplace scillators are key building blocks in almost all of today's communication systems. Because of their importance, we need a clear understanding of the fundamentals of their behavior. This understanding must be accompanied by methods for accurate analysis and for the generation of compact behavioral models. The dynamic behavior of many oscillators is characterized by a mixture of fast- and slow-varying time con- stants. On the one hand, we observe the system's fast-varying oscillations. On the other hand, the properties of these oscillations, e.g. amplitude and phase, vary slowly over time. This kind of behavior results in long simulation times when handled by means of traditional algorithms (e.g. SPICE). In this chapter, we focus on methods that allow us to separate an oscillator's fast- and slow-varying dynamics. These methods are used for the improvement of simulation efficiency, for behavioral model extraction and for the symbolic characterization of an oscillator's dynamic behavior. As such, they are a step towards improved oscillator design methods. The methods presented in this chapter are based on perturbation analysis and averaging techniques. Oscillator perturbation analysis relies on pa