402 REGULATORY CIRCUITS 



Automatic frequency control is accomplished by applying the generated 

 difference frequency to an error detector whose reference is the desired 

 IF frequency. Such an error detector is a frequency discriminator. The 

 frequency discriminator provides an output whose magnitude is propor- 

 tional to the error and whose polarity indicates whether the IF frequency 

 is above or below the reference. Since a variation in either the transmitter 

 frequency or the local oscillator frequency produces an error in the IF 

 frequency, these variations can be suppressed by suitable control of the local 

 oscillator frequency if they do not exceed the bandwidth limitation imposed 

 on the feedback control loop by the pulsed data. 



8-4 VARIATION OF TRANSMITTER FREQUENCY WITH 

 ENVIRONMENTAL CONDITIONS 



There are two types of frequency instability which result from the 

 environment in which the transmitter must operate. There are relatively 

 long-term frequency changes which occur incident to the effects of tem- 

 perature, vibration, deterioration, and the like; there are also short-term 

 frequency changes which are the result of a time-varying load impedance 

 connected to the transmitter, and frequency modulation from the heater- 

 supply and power-supply noise. 



Since the reference in an AFC for a conventional radar set is compared 

 with the difference between the transmitter frequency and the local oscil- 

 lator frequency, corresponding variations in the local oscillator frequency 

 occurring at the same time as the transmitter frequency variations are also 

 important. 



Fig. 8-5 shows typical frequency variation of a magnetron and a klystron 

 with ambient temperature. 



Some static and slow frequency differences for typical magnetrons and 

 local oscillators are listed in Table 8-1. 



Table 8-1 FREQUENCY DEVIATIONS OF 

 TYPICAL MAGNETRONS AND KLYSTRON LOCAL OSCILLATORS 



Maximum Diference 



Environmental Factor Frequency {Mc) 

 Scatter of magnetron and 



oscillator frequencies as received from 



manufacturer =^ 50 



Warmup of radar set =^ 1-5 



Temperature =^ 15 



Pressure (0 to 50,000 h) altitude - 2.5 

 Pushing ( ='=10% line-voltage variation) 



5.0 



Aging; ^ 10 



