Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED -IN OSCILLATOR RECEIVER 273to be considerably more selective with respect to adjacent-channel interferencethan conventional commercial receivers. More distortion was,however, encountered when the locked-in oscillator receivers were tunedso that the signal was received at the edges of the receiver-responsecharacteristic than was obtained with the conventional units.This isdue to the oscillator breakout characteristic and the fact that the voltageat the discriminator remains fixed irrespective of the signal appliedto the oscillator. In general, it can be stated that an increase in distortion,when tuned to one side of a desired signal, goes hand in handwith increased adjacent-channel selectivity in any type of radioreceiver. Some observers felt that this effect assisted in properlytuning the receiver.Observations with respect to noise reduction substantiated thelaboratory measurements which previously have been discussed.Modified-Circuit ArrangementAn experimental receiver was also constructed incorporating themodified arrangement illustrated by Figure 15. Although the tests onthis receiver were not so extensive as those on the receivers in whichthe Figure 16 arrangement was used, they did indicate that the modifiedcircuit possessed superior noise reducing and adjacent-channelselectivity characteristics.ConclusionsA novel method of receiving frequency-modulated signals has beeninvestigated both theoretically and experimentally. The investigationindicates that the system has the following advantages:1. By restricting the lock-in range of the oscillator to follow onlythe frequency variations which occur within the desired-signalchannel, a material improvement in selectivity is obtained.2. An equivalent voltage step-up is secured at a different and lowerfrequency than the intermediate-frequency and a correspondingimprovement in freedom from over-all feedback is secured.3. A constant voltage is applied to the discriminator irrespectiveof the strength of a received signal, and arrangements forminimizing amplitude variations in a received signal are, therefore,not required.4. The frequency-dividing locked-in oscillator receiving systemprovides a means for incorporating, in a frequency-modulationreceiver, a type of selectivity which can be used to discriminatebetween the desired-signal modulation and frequency-modulation-noisecomponents.