Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

128 FREQUENCY MODULATION, Volume Ipeaks beat with it. In the following it is convenient to consider anindividual noise frequency as a separate carrier,many present at any time.Since a combination of two station carriers differing inof which there arefrequencyis equivalent to a station carrier and a single noise voltage, both casesmay be considered at the same time. The effect is most easily shownand understood by means of a simple vector diagram.The desired carrier vector continuously rotates through 360 degreesand is indicated in Figure 15. A weaker carrier, or a noise voltage,rotates around the carrier vector at a frequency which isequal to thedifference between the two. Amplitude modulation is produced asshown. As the undesired vector rotates around the desired vector,AMPLITUDE MODULATIONFig. 15phase modulation also is produced between the limits A and B. Thefaster the undesired vector rotates, or the faster the rate of phasechange becomes, the greater becomes the momentary change in frequencyand, therefore, the greater the frequency modulation becomes,because frequency modulation is a function of the first differential ofphase modulation.Therefore, the amplitude of the F-M noise or beatnote varies directly with beat frequency. This results in a triangularnoise spectrum.In amplitude modulation there is no such effect as this. All noisecomponents combine with the carrier equally, resulting in a rectangularnoise spectrum. The ratio of r-m-s fluctuation noise voltages in FMand AM is, therefore, the ratio between the square root of the squaredordinate areas of these spectrums. This ratio is 1.73 or 4.75 db.