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BELL SYSTEM TECHNICAL JOURNAL 



XII. Auxiliary Control of Thermally Limited Oscillators 



In the Meacham and certain other oscillator circuits a thermistor is 

 associated with reactive elements in a bridge circuit which functions as both 

 limiter and filter. In these circuits a large increase in the frequency stabihty 

 is observed. This may sometimes be conveniently expressed as a magnifica- 

 tion of the effective Q of the filter. 



The advantages of great frequency stabihty and good amphtude stability 

 of these systems are accompanied by an undesirable tendency toward 

 intermittent operation. The thermal constants of the thermistor are not 

 readily adjustable. Moreover adjustment of the reactances to secure 



Fig. 26 — Meacham circuit with auxiliary control. 



suitable envelope stability is likely to impair the frequency or amplitude 

 stability for which the circuit is chosen. 



This dilemma may be resolved by the addition of an auxiliary network 

 which does not affect the envelope transmission to very low frequencies but 

 does modify the behavior at higher frequencies in such a way as to promote 

 the stability of the system. 



A simple circuit illustrating the principle appears in Fig. 26. It will be 

 noticed that the circuit is so arranged that the average bias applied to the 

 tube is only that due to the cathode resistor. The steady voltage developed 

 across Ci by the rectifier is unable to affect the bias because of the blocking 

 condenser C2. Accordingly the rectifier circuit does not affect the normal 

 operating condition, which is characterized by a bridge loss equal to the 

 amplifier gain. The added elements come into play only if there is a tend- 

 ency toward self -modulation. Then displacement currents of modulation 

 frequency flow through Ci in such a magnitude and phase as to modify the 

 tube gain and compensate the modulation returned from the bridge. 



