Electronics · Ch 8 — Modulation and Demodulation
Amplitude modulation
Amplitude modulation
Amplitude modulation (AM) is a modulation process in which the amplitude of a high-frequency carrier wave is varied in accordance with the instantaneous value of the modulating signal, while the frequency and phase of the carrier are kept constant.
Figure 8.2.1 shows how an AM wave is produced when a single-frequency modulating signal acts on a high-frequency carrier. Both the positive and the negative half-cycles of the carrier are varied according to the modulating signal, so the peak value of the carrier's sine wave rises and falls following the shape of the modulating signal. As a result the fast carrier oscillation sits inside a slowly varying outline called the envelope.
Three important observations follow from this picture:
- The AM wave contains all the frequency information of the modulating signal.
- The repetition rate (the rate at which the envelope swells and shrinks) is equal to the frequency of the modulating signal. …
A process in which the amplitude of the carrier wave is varied in accordance with the instantaneous value of the modulating signal, while the carrier frequency and phase remain constant. As a result the carrier's peak value rises and falls following the message, so the fast oscillation sits inside a slowly var …
The slowly varying outline traced by the peaks of the high-frequency carrier in an AM wave. Its shape is identical to that of the modulating signal and it repeats at the modulating-signal frequency, so the …
Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.
Figure 8.2.1 (Principle of Amplitude Modulation): three stacked time-domain panels — a low-frequency modulating sine (), a constant-amplitude high-frequency carrier (), and the resulting AM wave whose amplitude envelope swells and shrinks in step with the modulating sine. A dashed curve marks the envelope and an arr …