Am Fundamentals

chapter

3

Amplitude Modulation Fundamentals n the modulation process, the baseband voice, video, or digital signal modifies another, higher-frequency signal called the carrier, which is usually a sine wave. A sine wave carrier can be modified by the intelligence signal through amplitude modulation, frequency modulation, or phase modulation. The focus of this chapter is amplitude modulation (AM).

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Carrier

Amplitude modulation (AM)

Objectives
After completing this chapter, you will be able to:
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Calculate the modulation index and percentage of modulation of an AM signal, given the amplitudes of the carrier and modulating signals. Define overmodulation and explain how to alleviate its effects. Explain how the power in an AM signal is distributed between the carrier and the sideband, and then compute the carrier and sideband powers, given the percentage of modulation. Compute sideband frequencies, given carrier and modulating signal frequencies. Compare time-domain, frequency-domain, and phasor representations of an AM signal. Explain what is meant by the terms DSB and SSB and state the main advantages of an SSB signal over a conventional AM signal. Calculate peak envelope power (PEP), given signal voltages and load impedances.

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3-1 AM Concepts
As the name suggests, in 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 frequency variations of the modulating signal. Figure 3-1 shows a singlefrequency sine wave intelligence signal modulating a higher-frequency carrier. The carrier frequency remains constant during the modulation process, but its amplitude varies in accordance with the modulating signal. An increase in the amplitude of the modulating signal causes the amplitude of the carrier to increase. Both the positive and the negative peaks of the carrier wave vary with the modulating