Lab 7 Amplitude Modulation (AM) and Demodulation
Dr. Aly El-Osery October 25, 2010
This lab is divided into two parts. In Part I you will learn how to design an amplitude modulator (AM) and inPart II you will be able to demodulate an AM signal.
Modulation is the modiﬁcation of some aspect of a carrier signal. Given a signal of the form xc (t) = A(t) cos[ωc t + φ(t)]where ωc is known as the carrier frequency. If A(t) varies linearly with the modulating signal (your message to be transmitted), then we have linear modulation. If φ(t) varies with the modulatingsignal, then we have angle modulation. Figures 1, 2 and 3, show modulated signals using amplitude modulation, phase modulation and frequency modulation, respectively. As seen in Figure 1, the messagesignal is modifying the amplitude of the carrier signal, hence the name amplitude modulation. In Figure 2, the oscillation increased or decreased based on the slope of the modulating signal. In Figure 3,the amplitude of the signal is modifying the rate of oscillation of the carrier signal. In this lab you will design an AM modulator/demodulator. An AM signal is of the form xc (t) = [A + m(t)] cos(ωct) It is important to choose A so that A + m(t) ≥ 0 you will see why in the lab.
Prelab Part I: Amplitude Modulation
For this lab you will need to create an amplitude modulator. Use fS =48kHz. For your message, have the program generate a sinusoidal signal. You should be able to vary the message frequency dynamically as your code is running. 1. Why do we modulate?
EEDept., New Mexico Tech
4 message transmitted signal 3
Figure 1: Amplitudemodulation
EE Dept., New Mexico Tech
1.5 message transmitted signal
Figure 2: Phase modulation
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