each of these building blocks. This book focuses on techniques that can be used to model the behavior of mixers, oscillators and PLLs. Note that Fig. 2.5 only considers receiver architectures. Transmitters, however, have similar structures. In this case, the I- and Q-channels serve as inputs that are fed to digital-to-analog converters (DACs). The signals are then combined, upconverted to RF and transmitted through the antenna which is driven by a power amplifier (PA). Modeling DACs and PAs is, however, not a topic in this book3. 2.2.2 Properties of frontend building block behavior The construction of good frontend building block models requires us to incorporate all relevant prior knowledge. As will be illustrated in subsequent chapters, exploiting this knowledge helps us to improve model quality. This, for instance, makes simulations run faster. In what follows, we give a brief overview of some properties in common to many building blocks that occur in telecommunication frontend architectures: their almost linear nature, the presence of widely spaced time constants and the presence of stochastic (noisy) components in their behavior. Note that subsequent chapters also exploit other properties. However, these properties are often tied to a single building block. It is therefore not relevant to discuss them here. 2.2.2.1 Almost linear Most building blocks in telecommunication frontends are, by design, intended to be- have linearly. Here, linear should be interpreted in its most general, time-varying set- ting. As will be discussed in depth in chapter 3, up- and downconversions, induced by a multiplication y(t) = u(t) X cos(wot) of an input signal u(t) with some periodic carrier signal cos(wot), are linear operations: the principle of superposition still holds. This stands contrary to popular belief that considers multiplying two non-constant signals as a nonlinear operation. The key issue to observe is that one of the signals in the product -the carrier signal- is known at the time when the mixer is designed: It d