414 REGULATORY CIRCUITS 



A smaller variation in electronic modulation sensitivity is thereby obtained, 

 allowing a design of 50 per cent or greater overshoot in the step response of 

 the electronic tuning loop and also providing a much greater receiver tuning 

 range than can be obtained with electronic tuning alone. 



8-10 RELATION TO RECEIVER IF CHARACTERISTICS 



The dynamic error in the receiver tuning and the received signal IF 

 characteristic are related. The signal at the output of the IF amplifier will 

 contain pulse amplitude modulation arising from the tuning error of the 

 receiver. When the IF response is perfectly symmetrical about the center 

 frequency and the static error is negligibly small, the modulation at the 

 output will be double the frequency of the frequency modulation of the 

 tuning error. 



In conical scanning radars the dominant output incident to such effects is 

 thus at two or four times the lobing frequency. The response of the angle 

 demodulators to these frequencies is greatly attenuated by the use of 

 balanced ring demodulators, and additional noise on the direction signal 

 caused by AFC characteristics is then negligible provided that saturation 

 is not present. If the modulation is large there is of course a loss in signal- 

 to-noise ratio which can be determined from the rms error and Fig. 8-8. 

 To minimize the conversion of the tuning error to amplitude modulation 

 the nose of the receiver selectivity is made as flat as possible consistent 

 with the considerations discussed in Chapter 7. 



8-11 DISCRIMINATOR DESIGN 



There are two types of discriminators employed in pulse AFC — the 

 phase discriminator and the stagger-tuned discriminator. The choice 

 between the two depends on the details of the control circuitry. A slightly 

 higher effective transfer impedance can be realized with the stagger-tuned 

 circuit, but symmetry is difficult to maintain. If a video amplifier is 

 employed after the discriminator but prior to the integration, then the 

 phase discriminator is the more attractive choice. Fig. 8-14 shows a typical 

 phase discriminator circuit, and the form of the transfer impedance. 



In designing the discriminator the network elements can be selected so 

 that j-j = ^5, J-2 = -^7, s ^ = jg, and ^-4 = ^7. The discriminator response is 

 then of the same form as the difference in the envelope response of two 

 stagger-tuned one-pole networks. To obtain the maximum sensitivity from 

 the discriminator the poles are located so that the two equivalent response 

 curves cross at their point of inflection. H is inversely proportional to Ci 

 and C2 and these quaatities are minimized to obtain maximum Z,. 



