■12] 



INSTANTANEOUS AFC 



iC 



415 



z,= 



l^o(.)|-|gb(s)| 



i(s) 



S(S-S^)(S-Sl)(S-S2) 

 \(S-S^)(S-S;)(S-Se)(S-sl)(S-Sj)\ 



s(s-s)(s-s;)(s-s, 



(S-S^)(S-S;)(S-Se)(S-S;)(S-Sy) 



Fig. 8-14 Frequency Discriminator (Phase Type). 



8-12 INSTANTANEOUS AFC 



When the dynamic inputs are so severe that continuous AFC cannot 

 satisfactorily reduce the tuning error, the possibility exists of using an 

 Instantaneous AFC (I AFC). I AFC is a type in which the error correction 

 is completed before the pulse has ended. Extremely wide bandwidths are 

 required in the AFC-IF amplifier and discriminator in order that negligible 

 time delay may be obtained in these elements. The \FC must include 

 a bidirectional pulse lengthener which is required to have a negligible 

 decrement; the output of the lengthener is the controlled value during the 

 pulse. This type of AFC can potentially provide the best dynamic perform- 

 ance in a sampling-type AFC; however, there are some practical limitations. 

 The discriminator measures the instantaneous frequency and there is 

 negligible lag in the loop, so that if the instantaneous frequency is constant 

 during the pulse the tuning error can be reduced to a value dependent on 

 the gain of the loop. In most cases there are, however, intrapulse frequency 

 changes. In the continuous AFC the average frequency is controlled; in 

 the lAFC the controlled frequency depends on the characteristics of the 

 discriminator and pulse lengthener. The controlled frequency is different 

 from the average when intrapulse frequency variations are large. As a 

 result the static error of an lAFC can be larger than that of a continuous 

 AFC. The wide bandwidth required in the discriminator limits its output 



