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BELL SYSTEM TECHNICAL JOURNAL 



The closed system formed by the two basic circuits in tandem is of course a 

 feedback loop, the loop transmission characteristic being given by /x/3. 



Any desired form of servo transfer ratio may be obtained by an unlimited 

 number of ju and /3 circuit combinations.^ However, the /3 characteristic, 

 which is usually determined by a passive network or an inherent property 

 of a monitoring device, tends to be more stable with time and varying signal 

 amplitude than that of the ju circuit, which may include vacuum tubes, 

 motors, and other variable components. Consequently, it is desirable to 

 have the servo transfer characteristic largely dependent upon the /3 circuit 

 properties alone. This may be accomplished by making the loop trans- 



la) 



E= 5^(6,-82) 



©IN 

 >OUT— T-jT-QlN 



Fig. 5 — Synchro follow-up system. 



mission ju/3 very large compared to unity over the essential frequency range 

 of the servo input signal /i(,/). Under this condition, equation (6) becomes. 



-/3 



M/3|»l. 



(6.1) 



Thus the external transfer characteristic is set by jtJ.* If, for instance, Fi 

 and F2 are similar or analogous quantities and it is desired to have the servo 

 output a replica of the input, jS may be chosen as —1, yielding F2 c^ Fi . 

 It is not always easy to determine the basic parts n and /3 of a servo by 

 inspection of a schematic diagram of the system. An example is furnished 

 by the synchro follow-up system shown in Fig. 5a. As previously discussed, 

 the characteristic of the synchro comparison circuit is that of a differential 



^ Feedback stability requirements place certain restrictions on the permissible forms of 

 /i/S. This will be discussed in the next section. 



* The error arising from the approximate nature of (6.1) will be discussed in the next 

 section as one type of "servo error." 



