Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

'342 FREQUENCY MODULATION, Volume 1imposed on the desired audio output. When the modulation index isincreased, these peaks occur more and more rapidly, and the harmonicsproduced are redistributed to higher and higher orders. If the receiverhas sufficient band wid^h, a perfect limiter, and a wide-band audio system,the signal-noise ratio does not depend on the modulation index, butis determined solelv by the ratio of the desired signal voltage to theundesired signal voltage.Formulas are given for computing the amplitudes of the harmonicsand cross-modulation frequencies produced by the interference. Theeffect of a de-en phasis network following the discriminator, of alow-passaudio filter, and o f nonlinear phase-shift can be determined by computingthe amplitude of each harmonic that falls within the working range.Each such harmonic is then corrected for amplitude and phase changes inthe audio amplifier and filters, and they are then recombined by superpositionto obtain the filtered audio output.When the modulation index is large, the beat-notes of the noise comevery rapidly, and since this means that the harmonics are then of highorder, most of the distortion will be removed by the audio selectivity.This accounts for the observed noise reduction with wide-band frequencymodulation and shows that it is very important that a de-emphasis networkand low-pass filter be used to obtain maximum performance. Inorder to obtain the r.aximum signal-noise ratio, it