Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY-MODULATION FIELD TEST129The Deviation RatioThe deviation ratio, or modulation index, is obtained by dividingthe maximum carrier deviation by the highest audio frequency transmitted.For an F-M system, the suppression of fluctuation noise isdirectly proportional to the deviation ratio. In Figure 16 the A-Mnoise spectrum corresponds to the total hatched area below 15 kcbecause the i-f and a-f system would cut off there. The FM-75 receiveri-f system actually accepts noise out to 75 Mc and it has the usual F-Mtriangular noise characteristic.ear respond only toHowever, the audio amplifier and thenoise frequencies within the range of audibility,around 15 kc, and reject everything else. Therefore, the FM-75 noisewe actually hear corresponds only to the small cross-hatched triangle.6 9.6SB GAINFig. 16The maximum height of this F-M triangle, corresponding to voltage, isonly one-fifth of the height of an FM-15 triangle or the A-M rectangle.Such being the case, the FM-75 advantage over FM-15 is 5 to 1, or14 db, and over AM it is 1.73 x 5 or 18.75 db.De-emphasisThe use of a 100-microsecond filter to accomplish this high-frequencypre-emphasis and de-emphasis has been adopted as standardpractice in television and u-h-f sound broadcasting by the Radio ManufacturersAssociation and recently by the FCC.It was shown that in FM the noise amplitude decreases as itsfrequency decreases whereas in AM it does not.Therefore, de-emphasisis more effective in FM. This is shown in Figure 17.The full rectangle at the left is the A-M noise spectrum. The fulltriangle at the right is the F-M spectrum. The application of de-