Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

268 FREQUENCY MODULATION, Volume INoise-Reduction CharacteristicsIt has been previously stated that the lock-in oscillator arrangementcan be designed to increase materially the adjacent-channel selectivityof a receiver. This improvement is obtained by restricting the lock-inrange of the oscillator so that it will follow only the frequency variationswhich occur within the desired channel.This restricted lock-inrange is of interest also from the standpoint of the noise-reducingpropertiesof the receiver.In conventional frequency-modulation receivers the discriminatoris designed so that the linear portion of its response characteristic isadequate to accommodate the frequency variations of received signals,with due allowance for mistuning both by the user and that resultingfrom frequency drift of the heterodyne oscillator. The curved portionsof the discriminator characteristic which extend beyond the linearregion just referred to provide an additional frequency range in whichnoise components with wide frequency variations are converted intoPig. 14—Discriminator characteristic.amplitude variations:Figure 14 shows a typical discriminator characteristicin which AB isthe linear region within which the frequencyvariations of received signals are converted into amplitude variations.The sections of the characteristic designated CA and BD are the portionswhich are not useful in the reception of desired signals becauseof the curvature, but which are effective in converting frequencymodulationnoise components into amplitude variations. The upperfigures, indicating deviation, correspond to the discriminator characteristicin a conventional receiver, while the lower figures are the frequencyvalues for the locked-in oscillator discriminator. The restrictedfrequency range of the oscillator in the locked-in oscillator type ofreceiver can be used to limit the portion of the discriminator characteristic,which is utilized in converting the frequency variations of both