Reactance Tube Modulation of Phase Shift Oscillators

By F. R. DENNIS and E. P. FELCH 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 modulation 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 frequencies 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 transmission measuring systems. The oscillators had to be capable of linear modulation 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.