8-9] THE INFLUENCES OF LOCAL OSCILLATOR CHARACTERISTICS 413 



The dynamic performance of this type of AFC is determined by K and 

 the peak rate of frequency change occurring in the input signal. When the 

 peak rate of the input frequency change is less than K, an error reduction 

 is obtained and the peak errors are of the same order of magnitude as the 

 static error. When the peak rate of the input exceeds K, the error is not 

 reduced. When the input to the AFC is predominantly a single frequency, 

 the required value of K may be determined by 



K = o^f ■ (8-3) 



where co is the angular frequency at which the signal frequency is changing, 

 and / is the peak deviation of the signal frequency. 



8-9 THE INFLUENCES OF LOCAL OSCILLATOR 

 CHARACTERISTICS 



A typical electronic tuning characteristic of the local oscillator is shown in 

 Fig. 8-10. Note that both the power output and the frequency-modulation 

 sensitivity vary with the tuning. A variation in modulation sensitivity 

 means that there will be a variation in the loop gain of the AFC. In the case 

 of a continuous AFC the change in power can be employed to compensate 

 (to some extent) for the variation in modulation sensitivity. However, 

 perfect compensation is not attained. As a result, if a continuous AFC is 

 designed with only electronic tuning capability and it must accommodate 

 dynamic inputs, there is a degradation of the static and dynamic error 

 characteristics at the extremes of the tuning range if the overshoot is 

 selected to be 50 per cent at the middle of the tuning range. As shown in 

 Fig. 8-10, making the local oscillator signal smaller than the transmitter 

 signal reduces the variation in modulation sensitivity, but the variation is 

 still more than 2 to 1 over the tuning range. The static error therefore 

 oscillates when the receiver is tuned at the extremes of its tuning range. 

 A smaller overshoot and somewhat poorer dynamic performance must be 

 accepted if the tuning range is required to exceed 30 Mc >vith a fixed static 

 tuning accuracy. 



In the limit-activated correction AFC there is no benefit from making 

 the local oscillator signal smaller than the transmitter signal since the rate 

 of correction is independent of the magnitude of the error. (A constant 

 rate of control occurs whenever the error is greater than L.) The static 

 error oscillates at all times and will vary over the tuning range of the 

 receiver in accordance with Fig. 8-13 when the change in modulation 

 sensitivity is inserted into the value for K. 



When the system is subjected to severe dynamic error requirements, a 

 double loop is sometimes employed using a low-frequency feedback to the 

 resonator of the klystron and a high-frequency feedback to the reflector. 



