408 



REGULATORY CIRCUITS 



bination of the two, depending on the nature of the control mechanism of 

 the oscillator. 



The characteristics of an AFC in stabilizing the receiver tuning when the 

 transmitter frequency or local-oscillator frequency changes can be expressed 

 as 



£/. 



£/rf 



(8-1) 



1 + KG(s) 



where fei is the frequency error in the receiver IF frequency 



fdi is the frequency error that would result without the AFC 



K is the d-c or zero frequency gain of the AFC. (K = discrimi- 

 nator sensitivity X d-c gain of the filter X modulation sensi- 

 tivity of the oscillator) 



G{s) is the normalized transfer function of the filter. 



Fig. 8-10 shows the control characteristics of a typical klystron oscillator. 

 Referring to Table 8-1, the largest tuning error that might exist in a typical 



5.0 



^ 4.0 



^ 3.0 

 > 



m 2.0 



z 



LlJ 



^ 1.0 



_ 

 -Ih- 



-30 -20 -10 10 20 30 

 CHANGE IN FREQUENCY (Mc) 



Fig. 8-10 Local Oscillator Characteri.stics. 



system is ±15 Mc incident to temperature environment, provided that an 

 initial adjustment is made on the AFC whenever a tube is changed. Fig. 

 8-10 shows that the power output of the local oscillator will vary about 

 1.5 db and the modulation sensitivity will change by a factor of about 2 

 (or 6 db) for such a frequency variation. Variations in the power output of 

 the oscillator affect both receiver sensitivity and signal-handling capability. 

 However, the effect of a 1.5-db change is negligible. In cases where such a 

 change cannot be tolerated or in which the frequency variation between the 

 uncontrolled oscillators exceeds ±15 Mc, an additional feedback loop is 

 sometimes employed. This auxiliary loop measures the oscillator power 

 output and adjusts the klystron cavity for maximum output. 



