8-7] 



CONTINUOUS-CORRECTION AFC 



409 



Variation in modulation sensitivity limits the bandwidth that can be 

 employed in the AFC loop. The loop must be designed to have adequate 

 stability with both the highest and lowest gain values. To minimize 

 variations in oscillator sensitivity with tuning, the amplitude of the local 

 oscillator signal is usually made smaller than the amplitude of the sample 

 of the transmitter frequency at the AFC mixer. The IF voltage applied to 

 the discriminator is then proportional to the amplitude of the local oscillator 

 output. The discriminator sensitivity therefore is reduced as the oscillator 

 modulation sensitivity increases. Since loop gain involves the product of 

 the discriminator sensitivity and the oscillator modulation sensitivity, the 

 variation in loop gain that would exist if the IF voltage were maintained 

 constant is reduced. This is shown as an effective modulation sensitivity 

 reduction by the broken curve of Fig. 8-10. 



Pull-in and hold-in performance of the AFC are determined by super- 

 imposing the oscillator control curves on the discriminator characteristic. 

 Fig. 8-11 shows such a curve. Pull-in has been defined previously. Hold-in 



Oscillator 

 Control Curves 



Oscillator 

 Control Curves 



Fig. 8-11 Discriminator Characteristics. 



is the maximum frequency interval over which AFC control is effective. 

 Referring to Fig. 8-11, an error /^i in receiver tuning will occur for a 

 frequency deviation Ja\ of the transmitter frequency. The frequency 

 deviation /d2 corresponds to the hold-in range. An error /« 2 results from 

 an oscillator deviation/d2. The pull-in range corresponds tofdz- The tuning 

 error can be/^s or/es'; only the point /e3 is stable, however. For all devia- 

 tions less than/d3 the tuning error is a stable condition corresponding to the 

 intersection of the discriminator curve and the oscillator control curve. 



