Английски език
In this chapter, first the parametric principle is illustrated by two simple examples, one mechanical and one electrical. Then the realization of time varying reactances is explained, followed by a short history of "parametric electronics". This survey demonstrates the importance of parametric circuits in the field of low-noise microwave electronics as well as explains the organization of this book. 1.1 The Parametric Principle An oscillating system comprising a single or several time-varying energy storing elements is called a parametric system; usually the variations are harmonic functions of time. Everybody knows one example of a mechanical parametric system from his childhood, namely, a swing. Therefore, we will start with this example though as it turns out, a swing is a rather compli cated parametric system. Fortunately, the electrical parametric systems, which form the object of this book, are simpler. Figure 1.1 shows such a swing. If it is removed from its equilibrium position and the child stands on it in a fixed attitude, the swing oscillates with a certain amplitude, the magnitude of which decreases with time due to the mechanical friction of the system. To increase the amplitude of oscil lation, the child changes positions during swinging: it crouches and straightens in a certain way twice during one cycle of the swing.
