-21] 



THE IF AMPLIFIER CONTROL CHARACTERISTIC 



427 



10 

 

 -10 

 -20 

 -30 

 -40 



5 10 20 50 100 200 



ANGULAR FREQUENCY (rad/sec) 



120 

 100 _ 



<o 

 80 H 



Q 

 < 



20 



500 1000 



Fig. 8-22 Closed-Loop Response of AGC, Trial Design. 



8-21 THE IF AMPLIFIER CONTROL CHARACTERISTIC 



The AGC loop must maintain the receiver output constant so that the 

 loop gain of the various tracking loops is negligibly affected by the large 

 variations in input signal power that are encountered. The analysis of 

 Paragraph 8-18 is based on maintaining the average value of the receiver 

 output constant. In receivers which must recover modulation from a PAM 

 signal to obtain an error signal, additional consideration must be given to 

 the distortion of the IF signal as it passes through the amplifier. Ideally 

 it is desirable that gain control be applied to IF tubes which exhibit square- 

 law transfer characteristics. Under these conditions no distortion of the IF 

 signal will be apparent at the demodulated output. Usually the radar 

 receiver must incorporate tubes having good gain bandwidth products. 

 Such tubes are invariably of the sharp cutoff variety and are likely to 

 produce distortion of the IF signal with accompanying excessive variations 

 in the AGC loop gain if proper precautions are not taken. 



To determine the actual distortion through an IF stage, an accurate 

 description of the transfer characteristic is required. In general any 

 characteristic may be expressed as a power series in ^g, the grid-cathode 

 voltage. If the signal input to the IF stage is a modulated signal ei = 

 A sin (xictiX + m cos Wmf) and if an AGC voltage E\ is applied to the number 

 one grid of the tube, then a convenient measure of the distortion is the 

 change in the effective modulation of the signal at the output of the stage. 

 Thus 



1 + 



1^3^2(1 



W) 



a, + £i(2^2 + ^azEi) + la^A^l + ^m'^) 



(8-19) 



where m' is the fractional modulation at the output and <2i, ^2, and a^ are 

 the first three coefficients of the power series expansion for the transfer 

 characteristic. The distortion of the signal is seen to be a function of the 



