Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

270 FREQUENCY MODULATION, Volume Iquency-modulation noise components applied to the input circuit of thelocked-in oscillator may appear in the oscillator output circuit. Thephase-correcting network, however, may be designed so that thesecomponents either are not fed back to the reactance tube at allnot fed back inor aresuch phase and amplitude as to permit the oscillatorto follow them. In other words, the receiving system is provided witha circuit which is responsive only to small frequency variations andthis restricted-response range is moved back and forth at a rate whichfollows the desired modulation of received signals but is not movedback and forth at a rate which will follow superaudible noise impulseswhich may be present with the received signals.The effect of the reactance-tube arrangement on adjacent-channelselectivity is also of interest.This can best be Understood by referenceto the discriminator-rectifier—voltage-frequency response characteristicshown in Figure 14. As the output potential of the discriminatorrectifierand hence the potential applied to the reactance tube variesover the useful portion of the discriminator characteristic (the linearportion of the characteristic between the points A and B) the effectof the reactance tube is to shift the oscillator frequency in the samedirection as the frequency changes which give rise to the demodulatorpotentials.If, on the other hand, we assume that a signal on the adjacentchannel could reach the discriminator circuits and produce potentialscaused by frequency variations over the side of the discriminatorcharacteristic as indicated by the portion A-C of the curve, the phaseof the potentials applied to the reactance tube would be such that theeffect of the reactance tube on the oscillator would be to reverse thedirection of the oscillator-frequency change. That is, the reactancetube cannot shift the oscillator frequency so that it will lock in withthe signal on an adjacent channel because the circuit elements are sodesigned that if the frequency of the oscillator were to change beyondthe useful range of the discriminator and toward the adjacent channel,the phase and magnitude of the potential applied to the reactance tubewould shift in such a manner that the oscillator frequency would beshifted away from the adjacent channel frequencies.Experimental ResultsAs a part of an experimental investigation of the new receivingsystem, work was carried on with two identical commercial receivers.One was modified by incorporating the locked-in oscillator and reducedrangediscriminator, shown in Figure 16, in place of the two-tubecascade limiter and the discriminator used in the original construction.The other receiver was used for comparative tests in the laboratory and