Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

GENERATION AND DETECTION 161feUMiMyaM^i^=fe-H--H^-t FBEQVSNCy.novo FuequENCY - erects Kg secoHDFig. 11The distortion at high audio frequencies is independent of modulatorbias, but is very much a function of tuning, even with sufficientcircuit damping. This is particularly the case in the 300-kc triplerfollowing the mixer stage, which must accommodate a band of frequencies15 times as great as that encountered in circuits directlyfollowing the modulator.Attention must be directed toward minimizing amplitude modulationin the frequency-modulated wave, particularly at the lower signalfrequencies, for which the phase deviation is small. As shown inAppendix II, n per cent amplitude modulation can produce n per cent2nd harmonic distortion in the frequency deviation, due to sidebandcancellations, if the phase deviation is sufficiently low to permit therepresentation of the wave by a carrier and a pair of first-ordersidebands.Appendix IAnalysis of Phase Modulator Load Circuit:C^ '/pore)-, c/ecv/r T(/rV£0TO t4t.4 SCC| f/~X~Fig. (a)(1) To demonstrate the constancy of tuned impedance as R isvaried.