Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

238 FREQUENCY MODULATION, Volume Isponding output of the discriminator is shown in the second column.For the detuned condition characteristic of the oscillograms of Figure5, it can be seen that the greatest noise output occurs for the 180-degree position. This can be partially explained simply on the basis ofamplitude modulation. The carrier alone is producing a d-c output correspondingto peak frequency deviation on the discriminator curve.The reduced amplitude during the pulse reduces the d-c output voltagemomentarily.The third and fourth columns of Figure 5 repeat the conditions ofPhases/n-'/2DiscriminatorOutput45°90°ISO"270°Fig. 5—Oscillograms of noise and discriminator output with carrier tunedto edge of the pass band.the first and second columns except that the signal-to-noise ratio ischanged from 2 to %. A curious effect is obtained in the third columnfor 180-degree phase angle. For this condition the amplitude-modulationcomponent disappears almost completely. In spite of the fact, thenoise output is a maximum for this phase angle. The reason for this isshown in Figure 6. The vector a represents the carrier wave and thevectors b lt b 2, b 3 and & 4represent the successive values assumed by thenoise pulse. The vector c represents the resultant wave (shown in oneposition only). Obviously c makes one complete revolution around theorigin. In other words the resultant wave picks up one extra cycle in