Spring 2011
Clark
EE 341
Lab 3: The DFT and Digital Filtering
Due: In your discussion section May 1113
When using a digital computer, frequency analysis and filtering requires that we use
the Discrete Fourier Transform (DFT). In this lab we spend some time becoming
familiar with using the Fast Fourier Transform (FFT) implementation of the DFT to
study the frequency content of a discretetime signal.
1.
MATLAB function FFT
In this problem you will learn how to use the MATLAB command
fft
. First, use the
help
feature in MATLAB to learn the syntax of the
fft
function. The FFT function
computes the Discrete Fourier Transform (DFT) of a sequence. In general the FFT of
a sequence will be a complex function so you will need to look at the magnitude and
phase separately. The MATLAB commands
abs
and
angle
are useful for obtaining
the magnitude and phase of a complex valued sequence. Also, since the FFT only has
values at discrete frequencies, it may be useful to do the plots with
stem
to reinforce
that idea, but continuous frequency plots (i.e. using
plot
) are often used since they
approximate the DTFT that you are ultimately interested in. You may use either one
for your report.
The FFT outputs a sequence over the frequency range
02
. You are probably
more familiar with seeing the spectrum plotted over the range
(or
normalized
11
f
). The
fftshift
function can be used for this purpose.
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 Spring '09
 Digital Signal Processing, Frequency, DFT, Normalized Frequency, Matlab function

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