Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

A CATHODE-RAY FREQUENCY MODULATIONGENERATOR*!ByEOBERT E. SHELBYjVideo Operations Engineer,National Broadcasting Company, Inc.Summary— A device similar to a cathode ray tube is described whichmakes possible conversion from amplitude to phase shift modulation witha maximum phase shift of many times 360°. The modulating signal altersthe rotating electron stream in such a way as to produce phase changes inthe multi-piece anode current. A suitable network in the audio input effectsthe conversion from phase to frequency modulation.y wnjHE device herein described isa means of frequency- or phase-I modulating a carrier signal obtained from an oscillator of con-H- stant frequency and phase, such as a crystal-controlled oscillator.The majority of the known methods for generating frequency orphase modulation involve variation of the frequency or phase of themaster oscillator. There is one well-known method of producing phasemcdulation of a carrier obtained from a constant source which involvesthe addition of two voltages of the same constant frequency havinga constant phase difference of 90°, one voltage having constant amplitudeand the other varying in amplitude with the modulation signal.This method is limited to maximum phase shifts of the order of30°, which means that in order to obtain a large shift at the final carrierfrequency, it is necessary to employ frequency multiplication oflarge ratio. The method proposed here does not have this limitation.Theoretically it should give distortionless phase shift of many times360°.Structurally the device consists of an electron gun, two sets of electrostaticdeflection plates (or magnetic deflection coils), and a targetanode of special design, enclosed in an evacuated container of suitablesize and shape. A typical arrangement is shown schematically inFigure 1. It will be seen that the device illustrated is similar to aconventional cathode ray oscillograph tube. The only fundamental* Decimal Classification: R630.1.t Reprinted from Electronics, February, 1940.J Now Director of Television Engineering Operations, National BroadcastingCo., Inc., New York, N. Y.101