416 REGULATORY CIRCUITS 



SO that the reference accuracy is poorer than in a continuous AFC. Another 

 lAFC problem is associated with holding the voltage precisely during the 

 interpulse period. 



The lAFC circuit is not commonly employed because of its limitations 

 and because the dynamic inputs can usually be reduced by proper design of 

 the transmitter and associated circuits so that the continuous AFC is 

 adequate. 



8-13 PROBLEMS OF FREQUENCY SEARCH AND 

 ACQUISITION 



Table 8-1 indicates that in a typical case the transmitter frequency can 

 vary over a greater range than can the electronic tuning of a klystron as 

 shown by Fig. 8-10. It has been noted that it is also not always feasible to 

 utilize a discriminator-IF characteristic which will provide such a wide 

 pull-in range. To cope with this situation it is necessary that a mode of 

 operation be provided which will allow the AFC to search for the trans- 

 mitter frequency, acquire it, and track it. With magnetrons which are 

 tunable it is necessary that two loops be provided for the AFC. A slow- 

 response loop which controls the cavity resonator by thermal or mechanical 

 means is sometimes employed. With fixed-frequency magnetrons, however, 

 the electronic tuning range is usually adequate. In these cases, periodic 

 adjustments can be made to the klystron resonator to accommodate aging 

 or replacement of magnetrons. The slow frequency variations that are then 

 encountered are usually well within the electronic tuning capability of the 

 local oscillator. It is sometimes more economical to provide frequency 

 capture within the electronic tuning range by means of a search sweep of 

 the local oscillator than to provide an IF discriminator which has a pull-in 

 range equal to the maximum frequency difference between the transmitter 

 and local oscillator at the time that the radar set may be energized. 



A typical frequency range over which the local oscillator must search for 

 the transmitter frequency is 40 Mc. The speed at which this search can 

 occur depends on the bandwidth of the IF discriminator, the interpulse 

 period, and the total search range. A typical discriminator might have a 

 pull-in range of 10 Mc and 10 pulses required for acquisition. The maximum 

 search speed is then equal to 1 jT Mc, where T is the interpulse period. 

 Circuits are provided so that the search sweep signal is automatically 

 removed when the transmitter frequency has been captured. 



8-14 AUTOMATIC GAIN CONTROL 



Radar targets act to modulate the amplitude of the reflected signals in 

 several ways. First of all, range variations can produce variations in the 

 received power of more than 100 db. Secondly, amplitude fluctuations 



