8-3] AUTOMATIC FREQUENCY CONTROL 401 



The effects of the spectral energy distribution of closed-loop angle noise 

 and the contributions of low-frequency amplitude noise modulation of 

 tracking error caused by flight path input information are discussed in 

 Paragraph 8-17. 



8-3 AUTOMATIC FREQUENCY CONTROL 



Automatic frequency control circuits are employed as a means of over- 

 coming tuning tolerance and stability problems. The operating frequency 

 of the receiver is compared to a reference. An error signal, related to the 

 difference between the operating frequency and the reference, is generated. 

 The error signal is then applied to the system in such a manner as to reduce 

 the difference to an acceptable value. 



The general problem of automatic frequency control may be visualized 

 as follows (see Fig. 8-4). 



Fig. 8-4 Automatic Frequency Control in a Pulsed Radar System. 



In the case of radar employing a pulsed oscillator as the transmitter, it is 

 required that the receiver be tuned to the transmitter frequency. As 

 discussed in Chapter 7, this is done by mixing the incoming signal with a 

 local oscillator signal. The resulting intermediate-frequency (IF) output 

 then is amplified by bandpass amplifiers designed to operate at a fixed 

 intermediate frequency. With such an arrangement, the receiver tuning 

 depends upon the ability of the local oscillator to follow variations in the 

 transmitted frequency and thereby maintain the difference frequency (IF) 

 at the value for which the bandpass amplifiers were designed. The auto- 

 matic system employed to accomplish the desired regulation of the IF is 

 called an automatic frequency control (AFC). 



