4 BELL SYSTEM TECHNICAL JOURNAL 



two groups operate with very little harmonic distortion. The output of 

 oscillators employing such limiters may, therefore, be made quite free from 

 harmonic voltages. Oscillators of this sort are referred to as linear because 

 the tube or tubes serve as simple Class A linear amplifiers. 



IV. Criterion of Self Modulation' 



The block diagram of Fig. 1 is characterized by the fact that the separate 

 elements are connected to each other in the form of an endless ring. The 

 output may be assumed to come from any of the three junctions. It is this 

 fact of closure which complicates the problem of oscillator study. For 

 purposes of analysis it is convenient to open the loop as shown in Fig. 2. 

 For this example it makes no difference where we choose to make the cut, 

 but in actual circuits some caution must be exercised. This matter is dis- 



Fig. 2 — Test for self-modulation in an oscillator. 



cussed more fully later. It is also necessary to choose the impedances of the 

 test generator and test detector so that the operation of the components of 

 the original system is not disturbed. 



If a continuous wave of suitable voltage and frequency is supplied by the 

 test generator it will be found that the terminal voltage of the test detector 

 is identical in magnitude and phase with that of the generator. In this 

 condition the requirements which are fundamental to oscillators are satis- 

 fied. That is, the frequency and level at which oscillation should occur if 

 the circuit were closed as in Fig. 1 have been established. The net phase 

 shift of the system is zero and the net gain is zero. 



Whether the oscillations so produced would be stable or interrupted is now 

 determined by adding amplitude modulation of relatively low frequency and 

 very small magnitude to the test generator. It is clear that this modulation 

 will be transmitted through the amplifier, filter, and limiter to the test 

 detector and that the phase and percentage of the modulation may both be 



