Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

A FREQUENCY-DIVIDING LOCKED-IN OSCILLATORFREQUENCY-MODULATION RECEIVERSByG. L. BeersAssistant Director of Engineering in Charge of Advanced Development,RCA Victor Division, Camden, N. J.Summary— A new type of frequency-modulation receiving system isdescribed in which a continuously operating local oscillator is frequencymodulatedby the received signal. In an embodiment of the system whichis described, the oscillator is locked in with the received signal at one fifththe intermediate frequency. With this 5:1 relationship between the intermediatefrequency and the oscillator frequency, an equivalent reduction inthe frequency variations of the local oscillator is obtained. Received signalfrequencyvariations of ±75 kilocycles are reproduced as ± 15-kilocyclevariations in the oscillator frequency. The frequency-modulated signal derivedfrom the oscillator is applied to a discriminator which is designed forthis reduced range of frequencies.The oscillator is designed to lock in only with frequency variationswhich occur within the desired-signal channel. The oscillator is, therefore,prevented from following the frequency variations of a signal on an adjacentchannel. A substantial improvement in selectivity is thus obtained.The voltage required to lock in the oscillator with a weak signal ismapproximately one twentieth of the voltage applied to the discriminator.Since this voltage gain is obtained at a different and lower frequency thanthe intermediate frequency, the stability of the receiver from the standpointof over-all feedback is materially improved.Other performance advantages and the factors affecting the operationof the system are discussed.F^REQUENCY-MODULATION broadcastingis still in its infancyin terms of a nation-wide entertainment service. Until a largenumber of high-powered frequency-modulation broadcast stationsare operating on a commercial basis, the major technical problemswhich are involved in the design of frequency-modulation receiverswill not be fully appreciated.However, the experience which has alreadybeen gained from frequency-modulation broadcasting has indicatedsome of the problems which must be given serious consideration.Probably the most difficult requirement to be met is that of obtainingadequate adjacent-channel selectivity.This problem was emphasizedby a report on "Blanketing of High-Frequency Broadcast Stations"issued in 1941 by the Federal Communications Commission.High sensitivity is necessary in a frequency-modulation receiver to* Decimal classification: R361.111.t Reprinted from Proc. I.R.E., December, 1944.257