INTERMITTENT BEHAVIOR IN OSCILLATORS 



21 



The exact nature of the control which must be added is best ascertained 

 by opening the circuit at the plate of the tube. The loop transmission of a 

 modulation envelope may then be determined, either experimentally or 

 analytically. If instabiUty is found an auxiliary circuit must be designed 

 to produce an over-all system which is stable. In general the elements of 

 the auxiliary circuit are to be chosen so that the loop transmission is con- 

 siderably less than unity in the region of zero phase. < This is ordinarily 

 accomplished by increasing the fmal cutoff frequency at which the over-all 

 loop envelope transmission is negligible. 



-xsmmh 



■*— AyVVW- 



Fig. 27 — Self-modulating oscillator. 



XIII. A Self Modulated Oscillator 



The previous sections have been devoted primarily to the problem of 

 preventing self-modulation in oscillators. Let us now consider an oscillator 

 having envelope instability. The Nyquist diagram indicates that self- 

 modulation will occur and tells the approximate frequency of the envelope 

 wave. More detailed analysis of the circuit is necessary to determine the 

 wave form of the envelope and the manner in which its amplitude is limited. 



If a circuit is to function well as an oscillator the Nyquist diagram for the 

 operating frequency must loop the (1,0) point with considerable margin. 

 This is necessary so that a small loss of gain will not stop oscillation. At the 

 operating level the hmiter reduces the loop transmission to unity. In the 

 region of (1, 0) amplitude stability is favored if the rate of change of gain 



