804 



BELL SYSTEM TECHNICAL JOURNAL 



these frequency variations will be somewhat lower with an attendant reduc- 

 tion of the range of operation required of the AFC circuit. 



AFC Circuit Design Considerations 



To obtain a measure of the basic frequency reference for AFC purposes 

 the direct approach is evident. A sufiiaciently attenuated sample of the 

 outgoing radar pulse may be obtained from the transmitter and after sepa- 



(a) 



TO 



BEAT-OSCILLATOR 



REPELLER 



A^^— 1 



TUNING 

 CONTROL 



I +150 VOLTS (b) I -300 VOLTS 



Fig. 68.— Radar AFC system — block diagram and circuit arrangement. 



rate conversion but, under the influence of the regular receiver beat oscil- 

 lator, may be employed as a true sample of the outgoing signal as it exists 

 with the normal receiver IF channel. The separate AFC converter method 

 has been employed in some military radar equipment but has the disad- 

 vantage that additional conversion components are required. A second 

 method is more economical of equipment and has been extensively employed 

 during the past war period, but this later type of AFC circuit has limitations 

 wiiich are imposed on it by the character of the IF signal as normally avail- 

 able in a radar receiver. Figure 68a illustrated the essential elements of 

 such an AFC system for a radar receiver. It will be here observed that a 



