PHASE SHIFT OSCILLATORS 607 



the L/C ratio in the oscillator tuned circuit, but only at the expense of 

 frequency stability. 



A simplified schematic of the reactance tube modulated phase shift 

 oscillator is shown in Fig. 2. The mathematical theory of operation is anal- 

 ogous to that of the conventional reactance tube modulated oscillator, and 

 the same methods of analysis may be applied. The 90° phase shift network 

 required in the reactance tube grid circuit is a portion of the feedback net- 

 work and provides half of the 180° phase shift required for oscillation. In 

 this circuit the reactance tube is tightly coupled into the oscillating circuit 

 with minimum loss in the 90° phase shift network. Hence small values of 

 L/C ratio may be employed with a consequent increase in the inherent fre- 

 quency stability. In practice, oscillators comparable in stability to good 

 nonmodulated oscillators may be realized. The direction of deviation is 

 determined by whether the phase of the reactance tube grid voltage leads or 

 lags the reactance tube plate current. The permutations of connections and 

 signs of the 90° phase shift networks are shown on Fig. 3 with the correspond- 

 ing directions of frequency deviation. 



The phase shift networks need not be of the LC lumped constant variety. 

 For example, RC networks or sections of transmission line may be employed 

 to particular advantage at the lower and higher frequencies respectively. 

 A few of the many possible circuit configurations are shown in Figs. 4, 5, 6. 



Experimental Data 



Frequency deviation and output variation curves for some typical oscil- 

 lators are shown in Figs, 7, 8, and 9. 



The oscillator of Fig. 9 which was built by Mr. D. Leed, is shown in 

 Fig. 10. The transmission line is a section of RG59U cable with the shield 

 removed, encased in a copper tube with a slot for bringing out the center 

 tap of the line to the reactance tube grid. The tubes are 6J6's with both 

 sections connected in parallel. 



Conclusion 



Frequency modulated phase shift oscillators of several types have been 

 described. These offer interesting possibilities for applications over a wide 

 range of frequencies wherever stable, simple frequency modulated oscillators 

 are required. With respect to range, linearity, and freedom from amplitude 

 modulation their performance, as shown, is superior to that of conventional 

 circuits and is at least equal to that of the complex circuits employed in the 

 most critical applications. 



