AP 3302 Pt. 3
Section 1
CHAPTER 2
Basic Requirements Of A Pulse-Modulated Radar System
display of target information. The received echoes are very weak and may be only a few micro-volts. The input voltage required by the indicator however to give a reasonable display may be 10 volts or more. Thus the receiver must have a high gain.
As we shall see later there is a limit to the amount of useful gain which can be obtained in practice. The main limitation is noise. The interference we get in television in the form of random flashes of light on the screen is the result of unwanted electrical variations which have either entered the receiver circuit from an outside source (e.g. the unsuppressed ignition system of a motor car) or been generated within the receiver itself. This interference is referred to as noise. The main sources of noise within a receiver are mentioned in Part 1 of these notes and will be dealt with in more detail later.
It is sufficient for us to know at this stage that the noise level of a receiver has a marked effect on the performance of the equipment. If the indicator uses a simple type A display (Fig 9a) the noise voltages are applied to the Y-deflecting plates of the c.r.t. together with the signal echoes to give the effect which we call 'grass' on the trace. If the noise level is sufficiently high
