618 DISPLAY SYSTEM DESIGN PROBLEMS 



antenna is not obscured by the minimum spot size. This type of display 

 distorts the very near information since all points along the circumference 

 of the circle are in reality a common point (the transmitting point). This 

 type of display is used sparingly in conjunction with the conventional PPI 

 as a closing stage in navigational or homing displays. 



Other general radar displays have been cataloged^ such as the E scope 

 for height finding, the F scope, G scope, and I scope for error correcting, 

 the M scope and N scope for range reading. These presentations are not 

 common in airborne radar displays. 



Several general comments which apply to all types of intensity-modulated 

 displays are in order. A sacrifice in bandwidth is made in the intensity- 

 modulated display as compared with the type A scope since the phosphor 

 excitation is more confined. When the spot size is greater than the product 

 of pulse length and sweep speed, then several spots overlap, integrating the 

 video information contained in the overlap period. Thus the display acts 

 like a low-pass filter with a buildup time of 



n„ = I (12-2) 



where T6„ = display build-up time, sec 



d = spot size, cm 



^.V = sweep speed, cm /sec 



or a bandwidth of 



B, = l/n„cps. (12-3) 



When the pulse to be reproduced has a length greater than the display filter 

 buildup time (or has a bandwidth less than that of the display), then it is 

 passed (displayed) with little distortion and no adjacent noise pulse 

 integration occurs. However, when the pulse length is shorter than T^u 

 (or the signal bandwidth is greater than that of the display), then the 

 signal is distorted and adjacent signal and /or noise pulses are integrated, 

 degrading detection capability and possibly picture quality. 



The voltage-to-brightness transfer function of the display device is also 

 important for intensity-modulated displays. A square-law relationship, 

 for example, tends to preserve a linear relationship between signal power 

 and brightness. This factor is discussed in more detail in Paragraph 12-6. 



Another important factor is the total brightness range. If this is smaller 

 than the range of signal amplitudes, then video compression will occur, 

 with aconsequent loss of some of the signal intelligence. 



•''See Chapter 1 of reference 1 given at the end of this chapter. 



