THE RADAR RECEIVER 775 



information available and upon the indicator deflection methods to be 

 employed. 



The sweep deflection problems may be separated into two quite distinct 

 categories dependent on the speeds of operation involved. The slow-speed 

 deflection systems originate with the required deflection of the beam of the 

 indicator cathode-ray tube in accordance with the instantaneous position 

 of the axis of propagation of the antenna structure. Since the antenna 

 structures with which we are concerned at radar frequencies are of compara- 

 tively large dimensions, their motional velocities are extremely limited and 

 accordingly the electrical information describing these slow mechanical 

 changes contains only low-frequency components. The deflection problem 

 associated with this information, in general, offers little difficulty to the radar 

 receiver designer. Commonly employed methods of slow-speed sweep 

 deflection include the use of potentiometers, selsyn generators or variable 

 capacitors mechanically linked to the deflection axes of the antenna struc- 

 ture and associated circuits of relatively simple form whereby the electrical 

 changes in the characteristics of these devices are more or less directly 

 impressed upon the proper deflection axis of the indicator. In the case of 

 the PPI form of display, it is quite common to synchronize the rotation of 

 the deflection coil about the axis of the cathode- ray tube with the azimuth 

 bearing of the antenna through the use of selsyn motors or servo mechanisms. 

 In general, slow-speed sweep deflection problems are associated with bearing 

 coordinate data only. 



The determination of range to the target, on the other hand, requires high- 

 speed scanning whereby the time interval encompassing the time of propaga- 

 tion and return of the radar pulse over the selected range interval must be 

 completely displayed upon the indicator screen. The total time interval 

 available to deflect the beam for range measurement purposes may extend 

 from 2500 microseconds which represents a range measurement of approxi- 

 mately 240 miles down to perhaps 6 microseconds representing an expanded 

 interval of approximately 1000 yards useful in certain fire-control applica- 

 tions. Here, with extremely small times available for deflection purposes, 

 the radar receiver designer is faced with difficult circuit design problems 

 where the usual negligible parasitic circuit elements now severely restrict 

 the circuit performance. In the following discussion, therefore, emphasis 

 will be placed upon the design factors involved in the development of high- 

 speed radar sweep wave forms. 



The radar sweep circuit can be considered as providing the following 

 functions : 



1. Generation of time wave forms. 



2. Generation of display sweep wave forms. 



3. Amplification of sweep wave forms. 



