410 



REGULATORY CIRCUITS 



The discriminator curve of Fig. 8-1 1 is the characteristic appearing at the 

 output of G(s) in Fig. 8-9. For high-performance systems G(s) is designed 

 with a large zero frequency or d-c gain. Fig. 8-12 shows the resultant discrim- 



.Oscillator Control 

 Characteristic 



Typical Characteristic Where 



D-C Amplifier Follows Discriminator 



Fig. 8-12 Discriminator Characteristics. 



inator curve measured at the oscillator. By increasing the d-c gain for a given 

 IF characteristic the pull-in range is increased. In a number of systems, 

 however, a limited d-c gain follows the IF detectors. To realize a large 

 pull-in range a frequency searcli sweep is applied to the oscillator. The 

 presence of an IF signal in the AFCIF is employed to remove the sweep. 

 A relatively narrow-band IF discriminator can exhibit a large pull-in range 

 by this technique. 



In a typical system the control required on the oscillator may be ±50 

 volts, but this voltage is usually at some bias level, e.g. —150 volts. To 

 obtain maximum performance from a given loop, a d-c voltage is added to 

 the output of the IF discriminator so that the control voltage is at —150 

 considerable energy in modulation sidebands at the IF frequency. 



The AFC mixer of a pulse radar set is operated as a balanced mixer to 

 minimize frequency tuning error caused by discriminator outputs resulting 

 from the modulation spectrum of the transmitted signal. With the usual 

 IF frequencies employed, narrow pulse-length transmitted signals have 

 considerable energy in modulation sidebands at the IF frequency. 



A typical IF discriminator design can provide an output of 2 to 3 volts 

 per megacycle with a peak-to-peak separation of the discriminator of 4 to 5 

 Mc. The output from the discriminator is in the form of video pulses. If 

 these pulses are fed directly into the filter the zero frequency gain required 

 from the filter is 



K/eT 



(8-2) 



