524 BELL SYSTEM TECHXIC.IL JOrRXAL 



quency range. Similarly, if the filament current is changed from 

 1.1 to 1.4 amperes, the average frequency change is 0.03*^0 Changes 

 in the frequency resulting from changes in the load impedance are 

 practically negligible. 



.Such frequency changes as occur in the oscillator referred to above, 

 are due, to a large extent, to variations in the inductance conse- 

 quent upon the \ariations in power level which accompany the par- 

 ticular circuit changes referred to. The stability of the system may, 

 therefore, be increased considerably beyond the limits indicated if 

 the electrical constants of the elements used in the frequenc>' control 

 unit are independent of the power level. The use of an air core coil 

 in place of an iron core coil improves the stability to a very marked 

 extent pro\ided. nf course, that the same time constant is obtained in 

 both cases. 



In oscillators which have been designed primarily for frequencN' 

 stability, it is found that the largest frequency variation is due to 

 the variation in the electrical constants of the frequency' control 

 unit with temperature. When iron core coils are used, the tem- 

 perature coefficient of frequency of the oscillating system is approx- 

 imately 0.01 '^^ for 1° C. Using suitably designed air core coils, 

 the temperature coefficient of the oscillator becomes approximately 

 O.QO'.V'f for 1° C. The change in frequency in this case is due alinost 

 entirely to the change in capacity of the mica condensers used in the 

 frequency control unit. 



.Although the meth(xl of analysis which we have just considered 

 has been discussed largely in terms of the relation of the Irecjuency 

 of an oscillating electrical .system to the constants of the several 

 members of the system, it is by no means limited to such considera- 

 tion. It is, in fact, applicable to practically all types of oscillating 

 svNtenis, including those containing mechanically resonant devices. 

 It should, however, be remembered that while an analytical study of 

 this type may assist materially in furnishing a qualitati\e picture 

 of the conditions existing in some piece of a|)|iaratus, it is by no 

 means a substitute for a rigoious quantitati\e treatment. 



