ABSTRACT
AMPLITUDE modulation (AM) is a very popular modulation scheme. As we discussedin Chapter 13, AM signals are carried in the envelope of the carrier signal. An AM signal with carrier frequency, fc = 550 kHz, message frequency, fmsg = 5 kHz, and modulation index, µ = 0.8, is shown in Figure 14.1. 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 one millisecond. The message frequency can now be verified to be, fmsg = 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
Figure 14.1: An AM signal in the time domain (fc = 550 kHz, fmsg = 5 kHz, and µ = 0.8).
Figure 14.2: An AM signal shown in the frequency domain (fc = 550 kHz, fmsg = 5 kHz, and µ = 0.8). These are the only three frequency components in the signal.
