Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

L0CKED-1N OSCILLATOR RECEIVER 271field. This procedure was repeated with two identical laboratoryreceivers constructed along conventional lines.It should be noted that the locked-in oscillator circuit shown inFigure 16 isrepresentative of the receiving system illustrated by theblock diagram in Figure 1.This arrangement was used in preferenceto the modification illustrated by the block diagram in Figure 15because it was less complicated and, therefore, considered more suitablefor commercial receivers.With the arrangement shown in Figure 16 an intermediate-frequencysignal of about 1volt on the No. 1 grid of the oscillator tubewas required to provide the desired lock-in range of approximately± 110 kilocycles. The frequency range in excess of the ± 75 kilocyclesrequired for the normal modulation of a received signal isprovided totake care of mistuning by the user, frequency drift of the heterodyneoscillator, and over-modulation at the transmitter. The oscillator voltagedeveloped at the discriminator was between 20 and 30 volts. FromI.F AMPLIFIER4,300 KCLOCKED-INOSCILLATOR 860 KCDISCRIMINATOR860 KCOUT.AVCPig. 16—Locked-in oscillator and discriminator.the foregoing, it is apparent that the receiver should be sufficientlysensitive to produce 1 volt on the No. 1 grid of the oscillator to providesatisfactory reception of a desired signal.Improvement in SelectivityThe results of selectivity measurements, made by the two-signalmethod, are shown in Figure 17. In these tests, the receivers weretuned to a desired signal of 100 microvolts, with 400-cycle modulationand a deviation of ± 25 kilocycles. An interfering signal, modulatedwith 1000 cycles, and a deviation of ± 25 kilocycles, was adjusted insignal strength and frequency to give an interference output 30 decibelsbelow the 400-cycle output. A considerable improvement in selectivity,