806 



BELL SYSTEM TECHNICAL JOURNAL 



cated. The vector relationships for frequencies above and below this reso- 

 nance, as shown, result in an amplitude change across the rectifier circuit. 

 Figure 69b illustrates a typical rectified voltage versus frequency character- 

 istic of such an array. The location of the actual crossover zero voltage point 

 is determined only by the resonance of the secondary circuit over the limited 

 range under consideration here. The primary resonance contributes essen- 

 tially only to the symmetry of the voltage output versus frequency charac- 

 teristic. The introduction of the time constant elements in the detector 

 output circuit integrates the pulse output and are chosen with due regard 



Fig. 69. — Operation of the AFC circuit — vector relationships in input circuit and out- 

 put voltage vs. frequency characteristic. 



to the maximum frequency rate of change which this circuit must control. 

 The d-c amplifier shown is normally biased to operate at ma.ximum gain 

 consistent with stability, to produce the maximum sensitivity to frequency 

 change and to accordingly achieve the least threshold deviation from the 

 ideal tuning condition. Provision for disabling the AFC circuit is included 

 to enable initial manual adjustment of the beat oscillator repeller potential. 

 With the circuit shown here, failure of the AFC circuit proper will result in 

 the return of the rc[)cller potential to that value originally selected by initial 

 tuning and further manual control may be used. 



It is often convenient to describe the effectiveness of an AFC circuit in 

 terms of its "pull in" range and its "hold in" characteristics. "Pull in" 



