Constant Frequency Oscillators * 



By F. B. LLEWELLYN 



Summary — ^The manner in which the frequency of vacuum tube oscil- 

 lators depends upon the operating voltages is discussed. The theory of the 

 dependence is derived and is shown to indicate methods of causing the 

 frequency to be independent of the operating voltages. These methods 

 are applied in detail to the more commonly used oscillator circuits. 



Experimental data are cited which show the degree of frequency stability 

 which may be expected as a result of application of the methods outlined in 

 the theory, and also show that the best adjustment is in substantial agree- 

 ment with that predicted by theory. With a carefully built and adjusted 

 oscillator the effects of normal variations in the operating voltages are neg- 

 ligible in comparison with the effects of temperature variations resulting 

 from the changed operating currents. Methods of preventing these latter 

 effects are not discussed in the present paper. 



The appendix contains an analysis of the conditions under which the 

 performance of an oscillator may be represented by the use of linear circuit 

 equations. 



IN recent years the commercial requirements of vacuum tube oscil- 

 lators have grown more rigid. The tremendous increase in the 

 number of radio broadcast stations with the consequent narrowing 

 of frequency band available to each, the analogous demands by the 

 carrier telephone, and the tendency toward higher frequencies where 

 a small percentage frequency change defeats the universal effort to 

 secure better quality, all have united in creating a need for very con- 

 stant frequencies. This need has led to a study of methods for holding 

 the frequency constant. The most notable of these is the piezo-elec- 

 tric crystal. However, it has been known for some time that certain 

 oscillator circuits have the inherent property of maintaining their fre- 

 quency quite constant even though not crystal controlled. Some of 

 these circuits have the additional advantage of combining constant 

 frequency at a given wave-length with the ability to maintain this 

 constancy at other wave-lengths, thus giving a range of available fre- 

 quencies, any one of which may be depended upon to stay constant. 

 The elements which cause the frequency of oscillators w hich are not 

 crystal controlled to vary are such things as vibration, changing tem- 

 perature, Huctuating voltage, and changing load. Vibration and tem- 

 perature affect primarily the inductance and capacity in the circuit 

 which naturally causes the frequency to change. Fluctuating volt- 

 ages change the tube resistance, which in turn affects the frequency. 



* Presented at Sixth Annual Convention of I. R. E., Chicago, Illinois, June 4-6. 

 1931. Published in Proc. I. R. E., Dec, 1931. 



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