Bandwidth of FM Signals of Spectrum Analyzer

Chapter Bandwidth of FM Signals

Guide to Spectrum and Signal Analysis
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Guide to Spectrum and Signal Analysis

  • | 27Note that the carrier component J0 and the various sidebands JN go to zero amplitude for specific values of m. From these curves we can determine the amplitude of the carrier and the sideband components in relation to the unmodulated carrier. For example, we find for a modulation index of m = 3 the following amplitudes:Carrier J0 = 0.26First order sideband J1 = 0.34 Second order sideband J2 = 0.49 Third order sideband J3 = 0.31The sign of the values we get from the curves is not significant since a spectrum analyzer displays only absolute amplitudes. The exact values for the modulation index corresponding to each of the carrier zeros are listed in the Appendix C.Bandwidth of FM SignalsIn practice, the spectrum of an FM signal is not infinite. The sideband amplitudes become negligible be yond a certain frequency offset from the carrier, depending on the magnitude of m. We can determine the bandwidth required for low distortion transmission by counting the number of significant sidebands. (Significant sidebands usually refers to those sidebands that have a voltage at least 1 percent (40 dB) of that of the unmodulated carrier).Figures 32 and 33 show the analyzer displays of two FM signals, one with m = 0.2, the other with m = 95. Two important facts emerge from these figures:1) For very low modulation indices (m<0.2), we get only one significant pair of sidebands. The required transmission bandwidth in this case is twice fm, as for AM.2) For very high modulation indices (m>100), the transmission bandwidth is twice Δ fpk. For values of m between these margins we have to count the significant sidebands.Figure 32