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. Fig. 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 signal. An increase or a decrease in
the amplitude of the modulating signal causes a corresponding increase or decrease in both
the positive and the negative peaks of the carrier amplitude.
An imaginary line connecting the positive peaks and negative peaks of the carrier
waveform (the dashed line in Fig. 3-1) gives the exact shape of the modulating
information signal.