Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED-IN OSCILLATOR RECEIVER 269the received signal and noise components into amplitude variations, tothe linear region AB.Another characteristic of the locked-in oscillator which may be usedto advantage in minimizing the effects of noise is the ability to preventthe oscillator from following the frequency variations corresponding tosuperaudible noise components. This is accomplished in the oscillatorarrangement shown inFigure 2 by the proper choice of circuit constants.Modipied-Circuit ArrangementA modification of the frequency-dividing frequency-modulation receiverhas been developed by which its ability to select between desiredsignals or noise is further extended. Figure 15 is a block diagram ofthis modification.The locked-in oscillator used in this arrangement is likewise designedto operate at one fifth of the intermediate frequency. Thenormal lock-in range of the oscillator, however, is restricted to only20 to 35 per cent of the frequency-variation range required for receivedsignals. This very restricted lock-in range is extended by means of aWCONVERTEROSCILLATORFig. 15—Modified-circuit arrangement.reactance-tube arrangement so that the oscillator will follow themaximum frequency variations of received signals. The audio-frequencypotential developed at the discriminator-rectifier combinationis applied through a phase-correcting network to the reactance tubein the proper phase and magnitude to cause the reactance tube toshift the oscillator resonant frequency so that, at any instant its frequencyis such that the limited lock-in range will permit it to lock inwith the received signal. The amplitude of the control potential appliedto the reactance tube is normally kept slightly below the value whichwould shift the oscillator to the correct frequency, assuming that theoscillator had no lock-in range. In other words, for 100 per cent modulationthe reactance tube shifts the oscillator frequency by slightlyless than ±15 kilocycles.Let us consider the merits of this arrangement in connection withnoise impulses and adjacent-channel selectivity. Superaudible fre-