-20] 



DESIGN OF AN AGC TRANSFER FUNCTION 



425 



Using the latter approach and 

 referring to Fig. 8-19, 



tan (prr, 



KiG<i 



1 + K,G, 



K,G,. (8-16) 



Fig. 8-19 The Vector [I + K^Giijo:)]. 



If, for example, it is required that 0^ 

 be maintained less than 1.5°, then 

 the gain at the lobing frequency- 

 must comply with 



K2G2 < tan 1.5° = 0.0262 = -31.9 db. (8-17) 



If it is assumed that the lobing frequency is 50 cps, that its regulation is 

 ±5 per cent or ±2.5 cps, and that the angle tracking loop bandwith is 

 1 cps, the open-loop attenuation should be at least 31.9 db between 46.5 

 cps = 292 rad/sec and 53.5 cps = 336 rad/sec in order to maintain the 

 phase shift less than 1.5° over the anticipated range of operating conditions. 



8-20 DESIGN OF AN AGC TRANSFER FUNCTION 



As a trial design, a single time constant RC filter is selected for the AGC 

 filter. This will provide a — 1 slope, which was previously noted as desir- 

 able. The maximum gain is chosen on the basis of the static regulation 

 requirement. In the example of Paragraph 8-16, a static gain of 34 db was 

 required to regulate input variations of 100 db to ±1 db in the output. 

 This requirement is adopted as the loop gain in this example. 



In Paragraph 8-18, it was mentioned that a practical AGC loop design 

 for a system tracking aircraft targets should have a loop gain of 20 db 



5000 



10 20 50 100 200 500 1000 2000 



ANGULAR FREQUENCY (rad/sec) 



Fig. 8-20 Trial Design of AGC Open-Loop Transfer Function. 



