PROBLEM 7.1
KNOWN:
E(t) = 5sin 2
t mV
FIND:
Convert to a discrete time series and plot
SOLUTION
The signal is converted to a discrete time series for using N = 8 and sample time
increments of 0.125, 0.30, and 0.75 s and plotted below. The time increments of 0.125 and
0.30 s produce discrete series with a period of 1s or frequency of 1 Hz. The series created
with a time increment of 0.75 s, which fails the Sampling Theorem criterion, portrays a
signal with a different frequency content; this frequency is the alias frequency.
-6
-4
-2
0
2
4
6
0
0.2
0.4
0.6
0.8
1
1.2
TIME [s]
DISCRETE SERIES
dt = 0.125 s

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-6
-4
-2
0
2
4
6
0
0.5
1
1.5
2
2.5
3
TIME [s]
DISCRETE SERIES
dt = 0.30 s
-6
-4
-2
0
2
4
6
0
1
2
3
4
5
6
TIME [s]
DISCRETE SERIES
dt =0.75 s

PROBLEM 7.2
KNOWN:
Repeat Problem 7.1 using N = 128 points.
FIND:
The discrete Fourier transform for each series.
SOLUTION
The DFT for the time series representations of E(t) using N = 128 and
t
= 0.125, 0.30, and 0.75 s, respectively was executed using a DFT algorithm
(any such algorithm on the companion software or using the approach described in Chapter 2
will work) and shown below. The DFT returns an exact Fourier transform of the discrete time
series but not necessarily the time signal from which it is based. Whether this DFT exactly
represents E(t) depends on the criteria:
(1)
f
s
= 1/
t
> 2f
(2)
m/f
1
= N
t
m = 1,2,3, ...
With f = 1 Hz and f
1
= f: (a)
t
= 0.125 s and N = 128:
f
s
= 1/0.125s = 8 Hz > 2 Hz
m/1 Hz = 128/0.125
or
m = 16 an exact integer value.
Another way to look at this second criterion: the DFT resolution
f
= 1/N
t
=
0.0625 Hz, to which 1 Hz is an exact multiple.
Both criteria are met. Therefore, this DFT will exactly represent E(t) in
both frequency and amplitude, as shown below.
(b)
t
= 0.3 s
and N = 128
f
s
= 1/0.3s = 3.3 Hz > 2 Hz
m/1 Hz = 128/0.3
or
m = 38.4
not an exact integer value.
Criterion (1) is met but Criterion (2) is not met. Therefore, an alias frequency will not
appear. But this DFT will not exactly represent E(t) in amplitude and spectral leakage will
occur, as seen below. So we find an amplitude less than 5 at a frequency centered at 1 Hz.

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