412 REGULATORY CIRCUITS 



8-8 LIMIT-ACTIVATED AFC 



Limit-activated AFC constitutes a type which has a switching action 

 whenever the error exceeds a predetermined value. The static error in such 

 a system is never zero, but oscillates about the correct value with a constant 

 peak error. The static error is determined by the limit level for the error, 

 the interpulse period, and the integrator characteristic. 



Essentially the system is the same as that shown in Fig. 8-9 with G(s) = 

 K /s. The difference between this AFC and the continuous-correction AFC 

 is that the input to G(s) does not produce a change in polarity of the oscil- 

 lator control signal until the frequency error exceeds a certain limit L. The 

 output from the frequency discriminator is applied to an integrator whose 

 output controls the oscillator. A frequency error/ei results in a change of 

 the oscillator frequency at a rate Kfei. An error correction signal can only 

 be obtained in a shortest time T after a signal occurs which causes the 

 oscillator frequency to change. The quantity T is the interpulse period. 

 The output frequency therefore oscillates at a frequency which is some 

 fractional integer of the pulse repetition frequency. Fig. 8-13 shows the 



-1 



5.0 

 2 4.0 



2 3.0 

 in 



I 20 



1.0 







1.0 2.0 3.0 4.0 5.0 



E.O.F.= Error Oscillation 

 Frequency 



Fig. 8-13 Characteristics of a Switched (Limit-Activated) Type Control Loop for 

 Constant Input Conditions. 



relationship between the parameters A', T, and L. It will be observed that 

 multiple modes of oscillation can occur. These result from the fact that the 

 data sample may occur when the frequency error is less than L. The limit 

 level in a practical system is about 50 kc minimum. 



With the continuous AFC employing a 100 per cent pulse lengthener the 

 static accuracy is independent of the PRF, provided that the response time 

 of the discriminator is the same for all pulse lengths. (Normally a pulse- 

 length change would be associated with a PRF change.) In the limit- 

 activated correction type of AFC, the static accuracy increases as the PRF 

 increases. 



