492 AUTOMATIC TRACKING CIRCUITS 



9-9 ANGLE TRACKING LOOP MECHANIZATION 



In the preceding paragraphs, desirable transfer functions for the simpli- 

 fied stabilization and tracking loops, shown in Fig. 9-4, have been derived 

 from typical operating conditions and accuracies. Before mechanizing 

 transfer functions such as these, however, it would be desirable to check 

 the theoretical frequency responses and transmit responses for the combined 

 track and stabilization loops. Actually, with the antenna, actuator, 

 amplifier, and networks, the stabilization loop is not the simple function 

 shown in Fig. 9-4. It is much more complex; and although all the closed- 

 loop corner frequencies of the stabilization loop are at considerably larger 

 frequencies than those in the tracking loop, they may cause appreciable 

 phase shift in the region near the tracking bandpass frequency. These 

 calculations should be made as early as possible, perhaps without great 

 precision, because it is not possible to measure many system characteristics 

 accurately and there are usually many transfer characteristics which cannot 

 be defined in a rigorous fashion. Often, the mathematical models are refined 

 to resemble actual system characteristics after preliminary measurements 

 and trials have been made. After the transfer functions have been combined 

 and checked satisfactorily, the mechanization design may be finalized. 



The significant elements in the track loop are depicted in Fig. 9-3. The 

 major component, the radar, has been thoroughly described in earlier 

 chapters. Although it is exceedingly complex, it performs a relatively 

 simple function in the tracking loop: it converts angular space errors into 

 a modulated voltage. If properly designed, the radar is essentially a gain 

 with no appreciable time delays. 



Another major part of the track loop is the closed stabilization loop 

 described in Chapter 8. Although it has a complex transfer function, it 

 provides the track loop with a pole at zero frequency — i.e. a simple 

 integration. This makes the track loop a type I system^'' which reduces 

 steady-state position errors to zero. 



Demodulator and Cross-Coupling Effects. The angle track 

 demodulator Kd in Fig. 9-3 converts the modulated radar output voltage, 

 which represents the total space error, into two voltage components, 

 azimuth and elevation. Mechanically, Kd consists of two ordinary demod- 

 ulators. One has a phase reference which produces an output voltage 

 proportional to the azimuth space error and the other is phased to produce 

 a voltage proportional to the elevation space error. Any of several types 

 of demodulators could be used; but because of the large amount of noise 

 inherent in the radar signal, averaging demodulators are usual. These 

 demodulators have filters on the output to attenuate carrier frequency 



'■'Harold Chestnut and Robert Mayer, Servomechanistn and Regulating System Design, 

 Vol. 1, p. 194, John Wiley & Sons., Inc., 1951. 



