ABSTRACT
AMPLITUDE modulation (AM) is a very popular modulation scheme. As we discussed in Chapter 15, AM signals are carried in the envelope of the carrier signal. An AM signal with carrier frequency f c = 550 kHz, message frequency f msg = 5 kHz, and modulation index μ = 0.8, is shown in Figure 16.1. In this figure, the signal’s envelope is clearly sinusoidal. Counting the envelope variations shows that the signal envelope experiences five periods in the displayed time duration of 1 millisecond. The message frequency can now be verified to be f msg = 5/0.001 = 5 kHz. Since the carrier is displayed as solid shading in this figure, it is impossible to precisely determine its exact frequency without rescaling the plot. In fact, the carrier frequency is at f = 550 kHz, which means that this AM signal has 550,000/5,000 = 110 Hz/period. That is, 110 cycles of the carrier occur for every cycle of the message. This represents the worst-case ratio for US-based commercial AM detection, since the minimum authorized carrier frequency is 550 kHz and the maximum allowed message frequency is 5 kHz. We are discussing messages that are sinusoidal in nature. We will use these sinusoidal tones as example messages to illustrate a number of different points. In actual radio systems, the messages will be much more complex and usually will occur at more than one frequency. It is also very common when discussing AM systems to treat the maximum message frequency as the message’s bandwidth. An AM signal in the time domain (<italic>f</italic> <sub> <italic>c</italic> </sub> = 550 kHz, <italic>f</italic> <sub> <italic>msg</italic> </sub> = 5 kHz, and <italic>μ </italic> = 0.8). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315365688/56cbe124-4fa1-45b9-8500-f2fe8f19a986/content/fig16_1.jpg"/> An AM signal shown in the frequency domain (<italic>f</italic> <sub> <italic>c</italic> </sub> = 550 kHz, <italic>f</italic> <sub> <italic>msg</italic> </sub> = 5 kHz, and <italic>μ</italic> = 0.8). These are the only three frequency components in the signal. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315365688/56cbe124-4fa1-45b9-8500-f2fe8f19a986/content/fig16_2.jpg"/>
