M4] 



AUTOMATIC GAIN CONTROL 



417 



caused by target motion which were discussed in Paragraph 8-2 can also be 

 large. These variations in signal strength can seriously interfere with 

 tracking of the target unless steps are taken to protect the receiving system 

 from their effects. This is particularly important in angle tracking where 

 the signal amplitudes in two offset antenna lobes are compared (either 

 sequentially or simultaneously) to generate an angle error. The angle error 

 will normally be directly proportional to differences in the received am- 

 plitudes in the two lobes, so that signal strength variations common to the 

 two lobes must be removed if a usable error signal is to be obtained. 



Regulation of the received signal level is normally accomplished by an 

 automatic gain control (AGC) circuit. This is a feedback loop which adjusts 

 the receiver gain to maintain the average receiver output at a constant 

 reference value. A simplified block diagram of an AGC loop is shown in 

 Fig. 8-15. In operation, the gain of the IF amplifier presented to its input 



IF 

 Input 



Regulated 

 Video Out 



Delay Voltage 

 (Reference) 



Fig. 8-15 Radar Receiver with AGC Loop. 



voltage Ci is automatically adjusted by an AGC bias Cg. This bias is 

 developed as the difference of the video output voltage Co and a reference 

 voltage Cd referred to as the AGC delay. The system basically acts to 

 maintain the output equal to the delay. The degree to which this is done 

 when the input fluctuates is determined by the AGC filter in the feedback 

 path. 



An AGC loop is a nonlinear servo in that the feedback signal Cg is not 

 linearly combined with the input, but acts to modify the gain with which 

 the input is amplified. In a sense, an AGC loop in combination with an 

 angle tracking loop is an example of an adaptive servo system in that 

 the system gain is automatically adjusted to compensate for externally 

 generated variations in the received signal strength. Although basically 

 nonlinear, an approximate linear analysis of the system operation for small 

 deviations of the input from an average operating level is very useful and 

 forms the basis for system design. 



