Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

286 FREQUENCY MODULATION, Volume IFig. 10—Oscillogram of carrier envelope.The time delay*o ="d3 X 10 8 D= — seconds. (14)If D is the frequency shift required to go from out-of-phase, to inphase,and out-of-phase again, the time delay will be 1/D.Field TestsTo study the distortion encountered in New York City, the NBCtransmitter at the Empire State Building was operated with 300-cyclepure-tone modulation and a deviation of ± 75 kilocycles. A mediumpricedreceiver and an antenna were set up in a room next to the transmitter.The antenna could easily be located to give two-path reception,with a time delay of about seven microseconds. Figures 11 and 12show the carrier envelope and the discriminator output. In Figure 11the two waves canceled each other near one end of the swing. Theresultant amplitude modulation which passed the limiter, and thecoincidental phase modulation gave the result shown in Figure 12.One half of the cycle is relatively free from distortion, since the twosignals did not go out of phase at any part of that swing. The twoirregularities in the audio output correspond to the point of cancellation.This distortion could be eliminated by correct antenna location.In the RCA Building on the side away from the transmitter,theTime£ FrequencyFig. 11—Carrier envelope.Fig. 12—Discriminator output.