752 BELL SYSTEM TECHNICAL JOURNAL 



extremely large overloading signals are encountered. These signals may 

 represent strong radar return echoes from objects in the vicinity of the radar 

 antenna or may be due to enemy jamming signals. The ability of the radar 

 receiver to recover in a short time following such serious overloading is an 

 extremely important design consideration. In the case of jamming signals 

 of a continuous-wave or long-pulse form, their effectiveness can be minimized 

 by the inclusion of a high-pass network at the input of the video amplifier. 



In the case of radar systems which employ intensity modulated displays 

 of the B, C, and PPI forms the maximum useful brightness that can be 

 attained is limited by "blooming," a phenomenon in which the cathode-ray 

 tube spot on the fluorescent screen undergoes a sudden defocussing when the 

 brightness is increased beyond a critical value. In addition, halation effects 

 are quite pronounced in these long-persistence cascade layer screens and 

 contribute to an undesired masking effect when large areas of extreme bright- 

 ness are encountered. In these cases it is extremely important to limit the 

 maximum ampUtude of the signal that can be impressed upon the indicator. 

 The usual radar video amplifier includes an amplitude limiting stage located 

 early in the amplifier chain whose operating conditions are such as to be 

 driven to plate current cut-off by signals which exceed a preselected 

 amplitude. 



The range of useful brightness of the cascade screen radar indicator is 

 severely limited in comparison with the extreme amplitude range of the 

 received radar signals. As has been discussed, the maximum useful bright- 

 ness has been seen to be limited by halation and defocussing effects while the 

 minimum brightness threshold is controlled by halation and ambient viewing 

 conditions. These limitations of the viewing tube result in a criticalness 

 of adjustment required of the radar operator to achieve the optimum per- 

 formance of the radar system. In an effort to improve the general repro- 

 duction efficiency of the military radar system, several circuit forms have 

 been devised whereby the amplitude of the indicator signal is related to the 

 received radar signal in a nonlinear fashion. In certain instances two paral- 

 lel amplifier paths have been provided where one path operates in a normal 

 fashion until overload is reached when the second transmission path, de- 

 signed to properly transmit the higher amplitude signals, becomes effective. 

 In this manner two relatively linear amplification regions are provided with 

 a step or amplitude limiting region interposed. Such a nonlinear circuit 

 arrangement has been referred to as "duo-tone", indicative of the two 

 reproduction regions employed. Another video nonlinear characteristic 

 which was employed in a certain airborne radar bombing equipment de- 

 veloped toward the end of the war was of a logarithmic form realized by a 

 two-path amplifier design. In general this nonlinear treatment of the 

 radar signal amplitude has proven capable of reducing the critical adjust- 



