Frequency Modulation of Spectrum Analyzer

Chapter Frequency Modulation

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

  • 26 | Guide to Spectrum and Signal AnalysisBy definition, the information transmitted by amplitude modulation is carried not by the carrier but via the sidebands. Thus varying the composite AM waveform varies only the sideband amplitude. If the carriers component is suppressed, then the overall power saving improves the efficiency of the transmission system. This type of modulation is called Double Sideband Suppressed Carrier or DSBSC. In order to recover the modulation signal the carrier must be reinserted at the receiver.Furthermore, we could also remove one of the sidebands since the same information is carried by both. This would result in a further power saving and a reduction in the occupied bandwidth of the signal. This type of modulation is called Single Sideband Suppressed Carrier but is usually just called Single Sideband (SSB).Frequency ModulationFrequency modulation, FM, is a form of modulation in which the frequency of a carrier wave is varied above and below its unmodulated value by an amount proportional to the amplitude of a signal wave and at the frequency of the modulating signal. In this case the carrier amplitude remains constant. Frequency modulation differs from amplitude modulation in a number of ways.a) Since the amplitude of the modulated carrier remains constant, regardless of the modulation frequency and amplitude, no power is added to or removed from the carrier wave of an FM signal.b) Frequency modulation of a sinusoidal carrier with a second varying sinusoid yields an infinite number of sidebands separated by the modulation frequency fm.c) The peak-to-peak amplitude of the signal wave determines the maximum frequency deviation of the modulated carrier.The Bessel function curves of Figure 31 show the relationship between the carrier and sideband amplitudes of a frequency modulated wave as a function of the modulation index m.Carrier1st Sideband2nd3rd4th5thCarrier & Sideband AmplitudeDeviation Ratio1.90.80.60.40.200.20.41234567896th7thFigure 31