THE RADAR RECEIVER 781 



going radar pulse and whose duration has been controlled by the limits of 

 deflection desired on the radar indicator tube. This form of sweep circuit 

 is known as a "start-stop" type and has proven extremely satisfactory as 

 employed in a number of military radar ecjuipments designed during the 

 past war period. 



2.53 The Sweep Wave Form Generator 



The sweep wa\'e form generator is required to generate the specitic voltage 

 or current time functions required to properly deflect the electron beam of 

 the cathode-ray display device. The timing of the interval of this sweep 

 wave form is provided by the timing or synchronizing circuits just described. 



In general, it has been required that the range sweep wave form amplitude 

 be essentially a linear function of time over the range interval under observa- 

 tion. During the latter portion of the war, certain airborne applications of 

 radar did require that a specific nonlinear wave form be employed, but the 

 commonly employed displays (A, B, C, and PPI) are usually operated with 

 linear range deflection sweep circuits. 



The basic method of obtaining a sweep voltage wave form which increases 

 with time is illustrated in Fig. 50a. In this circuit Fi is normally operat- 

 ing at little or no bias and, therefore, due to the large voltage drop across the 

 plate resistor R, the plate potential of Vi is considerably lower than the plate 

 supply voltage B. If a negative rectangular pulse is applied to the grid of Vi 

 the tube will be abruptly driven to cut-off and, due to the current flow- 

 through the condenser C, the plate potential will rise exponentially as indi- 

 cated to eventually assume the value of the supply voltage B. At the time 

 of end of the negative driving pulse, I'l will again conduct and the potential 

 at the plate of Fi will be abruptly reduced as shown. 



There are several methods employed in radar sweep circuits to improve 

 the linearity versus time of the fundamental exponential sweep wave form. 

 The first of these takes advantage of the fact that the initial rise of the expo- 

 nential wave form in the limit is a linear function of time. By using only 

 a small portion of the wave form shown and supplying later ampiilication to 

 produce the desired deflection, a simple improvement results. This form 

 of linear sweep generation represents the original and by far the most com- 

 mon of the types employed in military radar systems during the past war. 



Figure 50b illustrates a method of improving the linearity of the sweep 

 wave form whereby the exponential wave form generated by the basic 

 condenser charging operation is modified through the use of feedback. As 

 shown here the asymptotic value of the exponential charging voltage has 

 been increased by a factor of (ju + 1) and the effective time constant of the 

 charging circuit has likewise been increased by the same factor. The use. 

 of an amplifier in the feedback circuit having an effective gain of 50 would 



