THE RADAR RECEIVER 809 



reflex oscillator. Variations in the amplitude of the controlling signal are 

 of less importance by virtue of the biasing action of this d-c amplifier circuit. 

 The performance of this design includes maintenance of the tuning point of 

 this receiver to within ± 0.25 mc of the center of the IF band which in this 

 case is 60 mc. 



2.72 Automatic Gain Control 



Automatic gain control (AGC) of a selected radar signal is often required 

 in military radar systems employed for fire control or aircraft interception 

 purposes. In the case of the common search radar system, AGC is seldom 

 required. The radar receiver AGC function is quite similar to that required 

 of this circuit in the usual radio communication system, i.e., automatic am- 



Fig. 71. — AFC component design as employed in AN/APQ-7 airborne radar system. 



plitude stabilization of the desired signal. For the radar receiver case, 

 however, the desired signal must be selected on a time interval basis. 



In the usual type of automatic tracking radar system, the target is selected 

 by manual alignment of a range and/or bearing "gate". This gating process 

 's essentially a modulation process by which the complete received radar 

 pulse signals are modulated with a rectangular pulse synchronized with 

 the outgoing radar pulse. The modulating pulse or gate has a finite ampli- 

 tude only over the time interval under observation. In this manner all 

 received information, except that occurring during the selected time interval, 

 is effectively rejected and the automatic gain control circuit operation is 

 entirely defined by the data present during this time interval. 



The remainder of the automatic gain control circuit is concerned with 

 the measurement of the amplitude of the selected signal, usually by a peak 

 voltage measurement, the averaging of this measurement over a convenient 

 time interval, and the production of a suitable gain control voltage to be im- 

 pressed upon the radar receiver IF or video amplifier circuit. The detailed 



