7-13] CONSIDERATIONS RELATING TO AGC DESIGN 379 



7-13 CONSIDERATIONS RELATING TO AGC DESIGN 



The AGC of the radar may be of two types: (1) a fast AGC which 

 prevents saturation of the receiver or (2) a slow AGC associated with a 

 single target echo. In the radar receiver employed for tracking, AGC 

 circuits of the second type are required. The IF amplifier is one of the 

 limiting factors in the design of a high performance AGC. This subject 

 will be discussed at greater length in Paragraph 8-21. 



In designing the IF amplifier great care must be taken to examine signal 

 distribution in the amplifier as a function of the AGC voltage. The AGC 

 voltage must be applied to the amplifier in a manner that will result in 

 minimum signal distortion and limited degradation of the output signal- 

 to-noise ratio of the receiver. For example when the input signal-to-noise 

 ratio is +90 db, it is necessary to reduce the gain in early stages to minimize 

 distortion, and as a result noise from latter stages becomes significant. A 

 t-vpical design might allow the output signal-to-noise ratio to be +30 db 

 li inimum for +90 db input signal-to-noise ratio. 



For minimum distortion of the modulation on the signal as the gain of an 

 amplifier stage is varied by AGC, it is desirable that the transfer character- 

 istic be a square-law when signal and gain control are applied to the control 

 grid. When sharp cutoff tubes are employed for gain control, considerable 

 distortion is sometimes experienced when gain control is provided for large- 

 signal inputs. Restriction of the gain control to about 10 db per stage in 

 these cases usually results in acceptable signal envelope reproduction. 

 Output stages of the amplifier should operate with linear plate transfer 

 characteristics. This allows the IF signal voltages applied to the last few 

 gain-controlled stages to be small, thereby resulting in less distortion. In 

 addition, wider bandwidths can then be employed in these stages, since 

 filtering of the undesired spectral components of the modulated signal, 

 which result from passing the signal through the nonlinear plate transfer 

 characteristic required for constant incremental gain as a function of AGC 

 voltage, is not required. In a typical case the gains in the IF may be 10 db 

 per stage. Requiring 2 volts rms at the IF envelope detector, the minimum 

 signal voltage at the third from the last stage of the amplifier would be 

 0.2 volt rms if gain control is not applied to the last two stages. The 

 maximum signal on the controlled stage then depends on the gain reduction 

 allowed. By controlling a number of stages the maximum gain reduction 

 required in any one stage can be limited to something on the order of 10 

 to 20 db. It is necessary to examine the signal transmission through each 

 stage for the maximum signal allowed at the input of the stage as a result 

 of the distribution of the AGC control voltage. The AGC and transfer 

 impedance of the stages are then arranged to provide a specified allowable 

 distortion of the modulation on the signal appearing at the amplifier output. 



