COMMUNICATION SYSTEMS IChapter IIIDr. Cihat ŞEKERElectrical-Electronics Engineering2020
Dr. Cihat ŞEKER, KBU, EEE, 20202Topics Covered in Chapter 3Amplitude Modulation Fundamentals1.AM Concepts2. Modulation Index and Percentage of Modulation3. Sidebands and the Frequency Domain4. AM Power5. Single-Sideband Modulation6. Classification of Radio Emissions
Dr. Cihat ŞEKER, KBU, EEE, 20203AM ConceptsIn the modulation process, the voice, video, or digital signal modifies another signal called the carrier.Inamplitude modulation (AM)the information signal varies the amplitude of the carrier sine wave.The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequencyvariations of the modulating signal.An imaginary line called theenvelopeconnects the positive and negative peaks of the carrier waveform.Figure 1. Amplitude modulation. (a) The modulating or information signal.
Dr. Cihat ŞEKER, KBU, EEE, 20204AM ConceptsFigure 2. Amplitude modulation. (b) The modulated carrier.
Dr. Cihat ŞEKER, KBU, EEE, 20205AM ConceptsIn AM, it is particularly important that the peak value of the modulating signal be less than the peak value of thecarrier.Vm< VcDistortionoccurs when the amplitude of the modulating signal is greater than the amplitude of the carrier.A modulator is a circuit used to produce AM. Amplitude modulators compute the product of the carrier andmodulating signals.Figure 3. Amplitude modulator showing input and output signals.
Power Gain or AttenuationdB = 10 log Pout/ PinDr. Cihat ŞEKER, KBU, EEE, 20206Modulation Index and Percentage of ModulationThemodulation index (m)is a value that describes the relationship between the amplitude of the modulatingsignal and the amplitude of the carrier signal.m = Vm/ VcThis index is also known as themodulating factororcoefficient, or thedegree of modulation.Multiplying the modulation index by 100 gives the percentage of modulation.Overmodulation and Distortion:The modulation index should be a number between 0 and 1.If the amplitude of the modulating voltage is higher than the carrier voltage,mwill be greater than 1, causingdistortion.If the distortion is great enough, the intelligence signal becomes unintelligible.Distortion of voice transmissions produces garbled, harsh, or unnatural sounds in the speaker.