TABLE OF CONTENTS Although most of this book is concerned with analyzing noise in circuits, we frequently require a system-level, black box characterization. This leads to a two-port noise model in which the two-port is noiseless and has an equivalent thermal noise source at the input. This is, by far, the most common way to characterize noise in components and to predict their performance in systems.
Such a model is necessarily idealized, thus placing restrictive assumptions on the nature of the system in which it is used. Most important is the requirement of matched interfaces; that is, each port is terminated in its standard impedance. Later in this chapter, we examine the implications of this restriction.
In Section 2.3.1 we examined thermal noise. We noted that the noise power available from a resistor or any other lossy medium, such as an antenna pointed at an absorber, is simply KT ? f, where K is Boltzmann's constant and T is its absolute temperature. This thermal noise is a white, stationary, zero-mean Gaussian random noise process. Most remarkably, the noise spectral power density, KT, depends only on temperature, and, therefore, the temperature fully defines the available noise spectral power. Noise temperature is equivalent to available spectral power density; the quantities differ only by a proportionality constant. Since we are dealing with linear circuits, we can ignore Dr. Boltzmann and his constant momentarily, and define the noise by its temperature alone. If...
