Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

VARIATION OF BANDWIDTH0.0606 4998 7910n = k- 0.8086 1651 7466^--^/k0.0316 7351 0263k(4)A family of curves for modulation indexes from one to twenty isshown by Figure 4. The vertical scale represents the amplitude of thegiven side frequency for each modulation index. The curve of Figure1 can be obtained by cutting a section through the surface for a modulationindex of 10. Contour line A is for the constant value of theBessel function J n {D/n) =0.01. Similarly, the contour B corresponds12 3 0 II 12 13 14 13 16 IT l« |» 20MODULATION ll.'OTCX - RFig. 4—Side-frequency amplitudes.to J„(D/n) = 0.001, and contour C is drawn for J n (D/fi) — 0.0001.Curve D is shown for the order of the Bessel function equal to theargument. If the bandwidth of a frequency-modulated carrier wavewere just equal to twice the deviation, the side frequencies would notextend beyond curve D.It is evident that for a given modulation indexthe bandwidth extends beyond curve D (say to curve A), but that theintensities of the side-frequencies beyond curve D are decreasingrapidly.Curve E is drawn along the top of the first crest and gives the absolutemaximum value of the envelope of the side frequencies for eachmodulation index. This curve is also given by Figure 3. The curves F,