INTERMITTEXT BEHAVIOR IX OSCILLATORS 5 



modilied. By examining the transmission of a lightly modulated wave for 

 various frequencies of modulation it is possible to determine whether or not 

 the normal oscillation will be self modulated when the loop is closed as in 

 Fig. 1. 



The carrier is held constant at the frequency F and ami)litude 1' for which 

 the input and output are identical, and the frequency /of the modulation is 

 varied from zero to infinity. In the following treatment it is assumed that 

 the significant portion of the characteristic is observed for modulation fre- 

 quencies small compared to F. The theory is simplified in this way without 

 l)eing serioush' restricted in usefulness. The percentage of modulation must 

 be held \er}^ low so as not to exceed the normal operating range of the limiter. 

 The criterion is most conveniently stated in terms of the transmission of the 

 modulation envelope which ma}' be considered as a vector quantity. 



Fig. 3 — Nyquist diagram showing magnitude and phase of loop transmission. 

 Legend: U is unstable 



C is conditionally stable 

 S is absolutely stable 



A plot of the vector ratio of output to input modulation for various fre- 

 quencies is prepared as in Fig. 3. The system characterized by curve U is 

 unstable and will generate a self modulated rather than a continuous wave. 

 The system characterized by curve 5 is unconditionally stable and will be 

 free from self modulation. The system characterized by curve C is condi- 

 tionally stable and may generate either a continuous or an interrupted wave 

 depending upon the manner in which the oscillation is started and other 

 factors. 



V. Analogy of the Oscillator to the Feedback Amplifier 



The behavior of oscillators of the t}'pe here considered is entirely de- 

 pendent upon feedback. It is therefore appropriate to review the funda- 

 mental principles which apply to feedback in general. 



In the feedback amplifier, negative feedback is applied to improve the 

 linearity, stability, impedance, or frequency characteristics. Considerable 

 improvements in some or all of the properties may be secured if a consider- 



