Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

GENERATION AND DETECTION 153mined principally by curvature in the reactance tube characteristic,and isnot a function of audio frequency.Conditions obtaining in phase-modulation systems are quite different,since, for a given frequency deviation, the required peak-phasedeviation varies inversely with audio frequency, i.e.cj> in degrees at carrier freq. =57.3 X freq. dev. c/s at carrier freq.audio-freq. c/sThus, the audio voltage at the phase-modulator grid must varyinversely with audio frequency, toproduce constant frequency deviation.If the distortion in the resultant frequency-modulated signal iscaused by a non-linear relationship between the instantaneous phaseof the generated signal and the instantaneous audio voltage,the distortionwill generally vary in some inverse manner with audio frequency.Another source of distortion lies in the inability of thefrequency multiplier circuits to pass all the required side componentsof the modulated signal. This deficiency will generally produce distortionincreasing with audio frequency.The distortion due to non-linearity in the phase characteristic canbe reduced by employing frequency multiplication from a relativelylow-frequency source of phase-modulated signals, to reduce the requiredphase deviation at the modulator. Side-band clipping distortion effectsare suppressed by employing multiplier circuits having adequate bandwidth.Principle of Operation of ExperimentalPhase-Modulation GeneratorIn most generators employing phase modulation as a nreans for.ultimately producing frequency-modulated signals,audio components or program material ismodulation by theaccomplished at a low valueof signal frequency (generally about 100 kc), which is subsequentlymultiplied and heterodyned toproduce the desired carrier frequency.One system of phase modulation employed experimentally requiresfirst the generation of amplitude-modulated signals at a low signalfrequency, after which the carrier is shifted in phase with respect tothe amplitude-modulated sidebands, to produce a phase-modulated signal.The inherently remanent amplitude modulation present in thisdeveloped signal isthen removed by limiter action.Another system of phase modulation is described in Patent No.2050067, issued August 4, 1936 to Dr. Walter Van B. Roberts. Thissystem makes use of an unique characteristic of a simple parallel tuned