INTERMITTENT BEHAVIOR IN OSCILLATORS 17 



may be secured by supplying increased inherent gain which is offset by 

 direct negative feedback. 



XI. Design of a Controlled Oscillator 



To clarify the material already presented and to convey some additional 

 concepts an oscillator having a large amount of control will be designed. 

 The block diagram is to be that of Fig. 7 and the circuit is to be similar to 

 that of Fig. 6. 



It may readily be seen that the gain control must satisfy two fundamental 

 requirements. It must deliver a d-c bias which increases rapidly with 

 increase of the level of oscillation and it must not return any appreciable 

 voltage of oscillation frequency. Otherwise the frequency will be affected 

 b>- the elements in the control circuit as well as those in the filter, and the 

 performance will be generally poor. Because of its balance a push-pull 

 rectifier is helpful in meeting the latter requirement. The principal require- 

 ment is achieved by amplification and by the use of a constant counter emf 

 or back bias. No bias is produced until the level of oscillation exceeds some 

 threshold value. .Above this threshold the bias increases approximately 

 volt for volt with the peak value of the signal. The same amplifier which is 

 used to increase the control may be used advantageously as a buffer so 

 that appreciable power outputs may be produced without degrading the 

 frequency or amplitude stability. 



It will be assumed that a () of 100 is available in the coil and that a fre- 

 quency of one megacycle is to be generated. The transmission of a modu- 

 lated wave in terms of the sideband displacement through such a one-circuit 

 filter is shown in Fig. 21. Because the cutoff occurs very slowly it will be 

 convenient to incorporate a rapid cutoff in the auxiliary filter of the gain 

 control, thus avoiding an excessive phase shift at any one frequency. 



The circuit features already discussed are shown in Fig. 22. A basic 

 oscillator with a single tuned coil, a buffer amplifier having little selectivity 

 and therefore contributing very little to the equivalent filter section, a source 

 of biasing voltage, a balanced rectifier, and an auxiliary low-pass filter are 

 shown. The condenser C is only large enough to allow the rectifier to be 

 driven without serious loss at one megacycle. It has relatively little effect 

 upon the modulation performance. 



It is assumed that the buffer-amplifier, rectifier, letc. are so chosen that a 

 modulation of very low frequency of one part per million applied at the 

 plate terminal of the oscillator will result in a modulation of one part in a 

 thousand returned to that point. This is equivalent to saying that the 

 envelope gain is 60 db at low frequencies, and corresponds to 60 db of 

 negative feedback in a conventional amplifier. 



The auxiliary filter will be designed to approximate the attenuation and 



