Introduction

In Chapters 2 through 4 we have shown how a single formalism can be used to describe the gain and frequency response of both direct and external modulation links. We continue with that same approach in this chapter. However, we will see that because different noise sources dominate in each type of link, the specific form of the link noise model depends on the type of link.

Up to this point all the signal sources we have dealt with were deterministic, in the sense that we could express their output voltage at any instant of time in terms of a known function of time, say v(t). In the case of the noise sources discussed here, there are – at present – no known expressions for any of the noise sources that give the noise source output as a deterministic function of time. Consequently we are forced to use the next best description, which is to describe the noise source output in terms of its statistical properties.

There are many statistical descriptors that could be used; by far the most common one for describing noise sources in electrical and optical applications is the mean-square value. There are primarily two bases for the popularity of the mean-square value. One is that it can be derived from the statistical distribution for the noise source, without ever knowing the underlying deterministic function. The other reason is that the mean-square value corresponds to the heating effect generated by the noise source.