632 



DISPLAY SYSTEM DESIGN PROBLEMS 



modulated presentation of most fire-control and search radar indicators 

 effects a signal which is a combination of pulse correlation and integration. 

 Target information is presented on the scope not as a dot but as a line 

 made up of a series of dots where each dot represents one pulse return. 

 The scanning motion causes the dots to be displaced one from another. 

 Depending upon the radar parameters — beamwidth, scan speed, PRF, 

 and spot size — overlap of the dots may or may not occur. The ratio of 

 length of the line made up of a series of dots to the picture size is usually 

 proportional to the ratio of the antenna beamwidth to the azimuth scan 

 pattern. Integration is effected by superposition of the signal return along 

 the dash line during the antenna dwell time. 



The ideal integrator assumes no information decay as the summing 

 process is made. If the display device were a perfect integrator of a pulse, 

 a signal-to-noise power ratio improvement of as much as sjN might be 

 realized (noncoherent integration). The use of a perfect integrator, 

 however, would necessitate the erasure of screen information on the 

 successive frames. If this were not done, noise pulse variations would 

 eventually saturate the integrating device. 



Conventional cathode ray tubes are not perfect integrators. The 

 phosphorescent characteristic of all phosphors follows a temperature- 

 independent exponential decay law of Boe'"^ or a temperature-dependent 

 power law decay of 5o/~". The exponent n varies greatly with temperature, 

 duration of pulse and degree of excitation. Depending on the type, decay 

 phosphor approaches the /"" type as the depth and distance of the activator 

 from the parent atom is increased. Both heating and electrical conduction 

 accompany a decay of the /~" type. 



Typical buildup and decay of CRT phosphors are as shown in Fig. 12-13. 

 The average tube bias, the pulse width, the beam current and the PRF 



Brightness 



Time 



Fig. 12-13 Buildup and Decay of Light for Intermittent Excitation of e °" Type 



Phosphor. 



