Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED-IN OSCILLATOR RECEIVER 259CONVERTER0WU.ATORREDUCEDMANXIISCRIUIWAUDIOAlPUftERMFig. 1—Block diagram.do not provide sufficient selectivity, a local transmitter on a channeladjacent to the desired signal may produce, at the discriminator, asubstantially greater voltage than is obtained from the desired station.Under these conditions the desired program will not be heard. In thenew receiving system a novel principle is used to provide additionaladjacent-channel selectivity. The oscillator is designed to "lock-in"only with frequency variations which occur within the desired-signalchannel. The oscillator is therefore prevented from following thefrequency variations of a signal on an adjacent channel. A substantialimprovement in selectivity is thus obtained by electronic means.The "locked-in" oscillator arrangement which is used provides,under weak signal conditions, a voltage step up of approximately 20.In other words, the voltage required to lock in the oscillator with aweak signal is approximately one twentieth of the voltage applied tothe discriminator.Since this voltage gain is obtained at a lower frequencythan the intermediate-frequency, the stability of the receiverfrom the standpoint of over-all feedback is materially improved. Thisimprovement is secured without the disadvantage of the additionalimage responses which are obtained with the double-heterodyne typeof superheterodyne receiver.One receiver arrangement is shown in Figure 1. In this diagramthe units which are heavily outlined are those which are peculiar tothe new system.Description op the Locked-In OscillatorThe locked-in oscillator circuit diagram is shown in Figure 2.TheWW—•- TO 0I3C-Fig. 2—The locked-in oscillator.