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Last Revised: 10/6/2008
1
EE 200 Lab 5 ‐ Tuned In
1
Introduction
In this lab you will be given a raw data signal representing an AM radio
communication modulating some recorded audio at a particular frequency. You
will perform some analysis on the signal and then attempt to demodulate it to
recover the original sound signal.
2
What you will learn
You will learn to use Fourier analysis (frequency domain) methods and understand
their properties to extract useful information from a signal. In addition, basic
concepts of modulation (in this lab we only focus on amplitude modulation rather
than FM or other methods) will be introduced.
3
Background Information and Notes
Amplitude Modulation
: Amplitude modulation works by taking a signal and
multiplying it with a higher (usually much higher) frequency carrier signal. The
result is that the original signal controls the amplitude or acts as an envelope
function of the modulated frequency. Mathematically, given a signal A(t), AM
modulation at a frequency, f
c
, is achieved by performing the operation:
ܣܯሺݐሻൌܣሺݐሻכcosሺ2ߨ݂
ݐߠሻ
An example is shown below where an 8Hz sinusoid is amplitude modulated at a
carrier frequency of 100 Hz. The resulting modulated signal is then shown.
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Original signal: Sinusoid of f=8 Hz
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Modulation frequency: f=100 Hz
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View Full DocumentEE 200 Lab 5
‐ Tuned In
2
Last Revised: 10/6/2008
Figure 1  a.) Original 8Hz signal.
b.) Carrier Frequency.
c.) Resulting AM signal.
Note: Recall that Matlab uses discrete time signals. Thus we need to sample the
carrier and AM signal at an even higher sampling rate (frequency), F
s
.
Fourier Series Representation
: Any periodic signal can be represented as a Fourier
Series. By using Fourier analysis, we can find the frequency domain representation
of a signal. Given some Fourier spectrum that results from the original signal,
multiplying it by the carrier frequency is equivalent to shifting the frequency
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 Spring '08
 ZADEH
 Frequency

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