8.1 Introduction

In any classical system (i.e. non-quantum) the ultimate limit of sensitivity will be set either by interference or by random signals that are produced within the system. In this chapter we are not concerned with the situations where interference sets the ultimate limit and so henceforth, we will consider only situations where the ultimate sensitivity is set by random signals. The minimum possible value for these random signals is generally set by physical phenomena collectively called noise. If we are interested in determining the limit of sensitivity of a system then we need to measure the random signals which determine that limit. Alternatively, we may wish to design a system to reach a chosen level of sensitivity in which case we need to have methods to allow the calculation of the level of random signals that are to be anticipated in the system.

In this chapter, we are particularly interested in systems that are sensitive to electromagnetic signals, generally in the microwave and radiofrequency (RF) region of the spectrum. However, some of the principles apply at any frequency, or even to systems that are not concerned with electromagnetic signals.

Random signals produced in an electrical system are usually called electrical noise. The concept of noise is familiar to anyone who has tuned an AM radio to a point between stations where the loudspeaker will produce a hissing noise which is attributable to the electrical noise in the system. This example also illustrates an important general point...