12-3] TYPES OF DISPLAYS 613 



collection and data processing. For example, a ground mapping radar 

 may produce 100,000 picture elements per second. As will be shown in 

 Paragraph 12-7, a human operator can interpret information at the rate 

 of about 30 bits a second. Obviously a large time-scale change is required 

 if full use is to be made of the radar information. 



12-3 TYPES OF DISPLAYS 



The basic types of displays are categorized by (1) the method of signal 

 insertion, (2) the method of deflecting the signal into an understandable 

 pattern on the indicator, and (3) the type of information that is displayed. 



The display device itself is some form of electrical-to-light transducer. 

 For most applications, the cathode ray tube — or one of its many variations 

 — is the most readily adaptable element. Thus, this discussion will be 

 confined to the types of display presentations which can be utilized with a 

 cathode ray tube. Aural presentations and other types of optical displays 

 will be discussed at the end of the chapter. 



In a cathode ray tube, the intelligence signal can be used either to move 

 or to intensity-modulate the electron beam. Amplitude-modulated displays 

 are achieved on the cathode ray tube (CRT) by insertion of the signal in 

 the deflection circuit. The signal amplitude is shown in such a display as a 

 variable along a time base sweep. Intensity-modulated displays, on the 

 other hand, are achieved by modulating the cathode ray tube electron beam 

 by inserting the signal on the grid or cathode of the tube. Amplitude of the 

 signal is therefore displayed as a variation in light intensity on the CRT 

 phosphor screen. 



Amplitude-Modulated Display. The simplest and oldest radar 

 indicator utilized a type A presentation in which only target range is 

 displayed. Fig. 12-5 shows this presentation. The sweep of the A scope 

 starts simultaneously with the transmitted pulse and moves linearly with 

 time from left to right. The video signals (target returns) are impressed on 

 the vertical deflection plates. They give a short vertical deflection (pip) 

 at distances along the horizontal trace corresponding to the two-way travel 

 time of the transmitted pulse. 



Between sweeps the cathode ray tube is biased to cutoff by applying a 

 negative pulse to the grid of the tube. The biasing of the tube improves the 

 display by blanking out the illumination that would ordinarily build up 

 (incident to receiver noise) at the start of the sweep. Other signals that 

 may be added to this display include range marks, which are displayed 

 along the range axis, and range notches which can be strobed along the axis 

 to measure the precise position of a target. This position is measured as a 

 function of the d-c voltage required to move the notch to the target position. 

 The range marks and range notches have been displayed in the type A scope 



