Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

IMULTIPATH DISTORTION 279andx =-(5)The first part of this expression (that under the radical sign) givesthe variations in amplitude of the resultant carrier, and the argumentof the sine function shows the variations in phase of the signal.Thereis considerable amplitude modulation introduced, and the frequencymodulatedsignal is also distorted. The resulting carrier envelope willbe considered now, and the distortion in the audio output will be discussedlater.Effect of Interference on the Envelope of the Radio-Frequency WaveThe resultant amplitude of the radio-frequency carrier isgiven by40° 60° 80° ICO' 120° 140" 160° IBO° 200°ZTTflt—*-Fig. 1—Carrier envelope.e l + e 2 I= E l VI + X 2 + 2x COS {Z COS (27Tjui — 7T/ji ) + a>t ). (6)The amplitude is a function of the ratio of the two signal voltages x,the maximum deviation D, the time delay t , and the audio frequency 11.The only effect of the carrier frequency w is to determine the initialphase angle a>t .Figure 1 shows the carrier envelope for a deviation of 60,000 cyclesper second from a mean carrier frequency of 45 megacycles per second.E x was assumed to be one volt and E 2 was taken as 0.9 volt, so thevoltages are nearly equal.The conditions were chosen so that the tworadio-frequency signal voltages were in phase at the undeviated positioncorresponding to 2-nji.t = 90 degrees. The equation of the envelopebecomes| «x + e 2 |= V1.81 + 1.80 cos {4tt cos (2ir/it - 0.18°) }. (7)