The Bell System Technical Journal 



Vol. XXVIII October, 1949 No. 4 



Reactance Tube Modulation of Phase Shift Oscillators 



By F. R. DENNIS and E. P. FELCH 



This paper describes a l)asic circuit for reactance tul)e modulation of phase 

 shift oscillators. The design of suitable phase shift oscillators for freciuencies from 

 audio through the ultra-high frequencies is discussed. E.xperimental performance 

 data derived from several types of frequency modulated ])hase shift oscillators 

 are presented. 



Introduction 



FREQUENCY modulation of oscillators is finding vvide-s})read use in 

 such diverse fields as FM broadcasting, telemetering systems for 

 guided missiles and measuring apparatus for observing transmission fre- 

 quency characteristics on cathode ray tubes. Design objectives for such 

 oscillators may be listed briefly as: 



1. A wide range of frequency modulation or, alternatively, high modula- 

 tion sensitivity. 



2. A linear relationship between instantaneous values of modulation 

 input voltage and frequency deviation. 



3. Freedom from accompanying amplitude modulation. 



4. Inherent center frequency stability. 



5. Ease and stability of adjustment. 



6. A minimum number of components, none of which should be critical. 



7. Modulation by dc, audio, or video inputs. 



8. Operation anywhere in the frequency spectrum from low audio fre- 

 quencies through the ultra-high frequency region. 



The circuits described in this paper were developed in the course of an 

 investigation of various frequency modulation circuits for use in visual trans- 

 mission measuring systems. The oscillators had to be capable of linear modu- 

 lation at 60 cycles over a ±3 megacycle band at 25 megacycles and over a 

 ±9 megacycle band at 80 megacycles. Existing designs fell short of meeting 

 the requirements with respect to several of the characteristics listed above. 

 The reactance tube modulated phase shift oscillator circuit was found to 

 perform satisfactorily in the transmission set and proved superior in many 

 respects to all the other circuits tried. Tests of the circuit at other frequencies 

 disclosed that the advantages were not peculiar to the frequency range and 

 the following description is presented with the expectation that it may 

 prove useful to others. 



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