CONTINUOUS-CORRECTION AFC 



407 



caused by variations in frequency greater than this effective bandwidth are 

 not significantly reduced by the AFC, and their effect on the signal energy 

 at the output of the receiver must be considered. The bandwidth of the 

 receiver IF is then selected so as to maximize the output S /N in the 

 presence of the error resulting from those tuning errors which cannot be 

 removed by the AFC. 



In some applications, transmitter pulling is often the determining factor 

 in the accuracy of receiver tuning. In these cases it is not essential that the 

 static error be extremely small, and a simplification of the AFC can be 

 realized by the use of limit-activated correction (see Paragraph 8-8). 



In tracking applications a further requirement exists that tuning error in 

 the receiver must not produce an error in tracking exceeding a specified 

 amount. Two types of errors arise from the tuning error. The frequency 

 error is converted to amplitude modulation by the IF characteristic. The 

 additional amplitude modulation from this source produces errors in the 

 direction signal of a conical scanning radar. Distortion of the pulse shape 

 also occurs and may produce errors in the measurement of range. 



8-7 CONTINUOUS-CORRECTION AFC 



Continuous-correction AFC constitutes a type of closed-loop operation 

 in which the error continuously tends to be minimized. The residual error 

 is a function of the loop gain. 



A block diagram of a continuous AFC is shown in Fig. 8-9. The input to 

 the AFC is the frequency difference between the transmitting oscillator and 



Fig. 8-9 Continuous AFC. 



the receiver local oscillator. This frequency is measured by the frequency 

 discriminator which is the error detector. A voltage e proportional to the 

 difference between the input frequency and the crossover frequency of the 

 discriminator is applied to the local oscillator through a filter G(s). The 

 output of the filter is a control C which adjusts the receiver local oscillator 

 to minimize e. C may be a mechanical or an electrical output or a com- 



