GB707027A - Improvements in or relating to arrangements for supplying phase modulated electric oscillations - Google Patents

Abstract

707,027. Phase and frequency modulation. GENERAL ELECTRIC CO., Ltd., and BRYDEN, J. E. March 12, 1953 [March 24, 1952], No. 7532/52. Class 40 (5) A carrier wave of medium frequency, which may be stabilized by a crystal, is phase modulated by mixing the carrier with a wave of much higher frequency to produce one or both side frequencies, applying the modulation to vary the phase of one or both side frequencies in a travelling-wave tube or a pair thereof, and feeding the outputs to a mixer tube to supply a difference frequency, which is phase-modulated and is of a mean frequency equal to, or determined by, the medium frequency source. In the diagram, Fig. 2, the medium frequency of 30 mc/s is supplied by a crystal controlled source 23 which mixes with, or amplitude-modulates, an oscillation of, say, 4,000 mc/s supplied by a source 22, which may be a magnetron. The resulting upper and lower side frequencies are selected by filters 24, 25 (which may be resonant cavities), and applied to travelling-wave tubes 26, 27 to which the modulating signal is applied in push-pull to effect phase modulation in opposed senses. The modulated outputs are mixed at 29 to produce a phase-modulated wave whose mean frequency is double that of the source 23, and is independent of variations in the frequency of the source 22. In a modified circuit, only one of the side frequencies is used and only one travelling-wave tube, the phase-modulated output of which is reduced to the medium frequency of say 30 mc/s by mixing with the output of the source 22, and selection of the lower sideband, Fig. 3 (not shown). If the audio modulating-signal is fed through a network that deemphasises the higher components, the output wave is in effect frequency-modulated by the signal. Fig. 1 shows a suitable arrangement of a travelling-wave tube modulation. The v.h.f. input is applied through a transverse wave guide 9, is picked up by a wire 7 and traverses the helix 4 to an output wave guide 10. An axial electron beam is set up within the tube 6 by a gun anode 3 and passes through the helix 4 to a collector electrode 5. When a modulating signal is applied at 14 to the anode 3, the velocity of the electrons is varied, and the v.h.f. wave picked up by the output guide 10 is correspondingly shifted in phase.