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7/27/2019 Ee602 Fourier Series
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EE602 CIRCUIT ANALYSIS
The Fourier Series
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A Fourier series is an expansion of a
periodicfunctionf(t) in terms of an infinite sum
ofcosines and sines
Introduction
1
0 )sincos()(
n
nn tnbtnaatf
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In other words, any periodicfunction can be
resolved as a summation of
constant value and cosine and sine functions:
10 )sincos()(
nnn tnbtnaatf
)sincos( 11 tbta 0a
)2sin2cos( 22 tbta
)3sin3cos( 33 tbta
dc ac
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The computation and study of Fourierseries is known as harmonic analysis and
is extremely useful as a way to break up
an arbitrary periodic function into a set of
simple terms that can be plugged in,
solved individually, and then recombined
to obtain the solution to the original
problem or an approximation to it towhatever accuracy is desired or practical.
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=
+ +
+ + +
Periodic Function0a
ta cos1
ta 2cos2
tb sin1
tb 2sin2
f(t)
t
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1000 )sincos()(
nnn tnbtnaatf
where frequenclFundementa2
0 T
Tt
tn dttntfTa
0
00cos)(
2
Tt
tdttf
Ta
0
0
)(1
0
Tt
t
n dttntf
T
b0
0
0sin)(2
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Notice that
componentdc
valueAverage
periodaovergraphbelowArea
)(1 0
00
T
dttfT
aTt
t
[*The right sign of the area below graph must be
obeyed. +ve sign for area above x-axis & ve sign
for area below x-axis]
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Symmetry Considerations
Symmetry functions:
(i) even symmetry
(ii) odd symmetry
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Even symmetry
Any functionf(t) is even if its plot is
symmetrical about the vertical axis, i.e.
)()( tftf
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Even symmetry (cont.)
The examples ofeven functions are:
2)( ttf
t t
t
||)( ttf
ttf cos)(
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Even symmetry (cont.)
The integral of an even function from A to
+A is twice the integral from 0 to +A
t
AA
Adttfdttf
0
eveneven )(2)(A +A
)(even tf
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Odd symmetry
Any functionf(t) is odd if its plot is
antisymmetrical about the vertical axis, i.e.
)()( tftf
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Odd symmetry (cont.)
The examples ofodd functions are:
3)( ttf
t t
t
ttf )(
ttf sin)(
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Odd symmetry (cont.)
The integral of an odd function from A to
+A is zero
t 0)(odd
A
A
dttfA +A
)(odd
tf
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Even and odd functions
(even)(even) = (even)
(odd)(odd) = (even)
(even)(odd) = (odd) (odd)(even) = (odd)
The product properties ofeven and odd
functions are:
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Symmetry consideration
From the properties ofeven and odd
functions, we can show that:
foreven periodic function;
2/
0
cos)(4
T
n tdtntfT
a 0nb
forodd periodic function;
2/
0
sin)(4
T
n tdtntfT
b 00 naa
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How?? [Even function]
2
T
2
T
2/
0
2/
2/
cos)(4
cos)(2
TT
T
n tdtntfT
tdtntfT
a
(even)
(even)
| |
(even)
0sin)(2
2/
2/
T
T
n tdtntfT
b
(even) (odd)
| |
(odd)
)(tf
t
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How?? [Odd function]
2
T
2
T
2/
0
2/
2/
sin)(4
sin)(2
TT
T
n tdtntfT
tdtntfT
b
(odd)
(odd)
| |
(even)
0cos)(22/
2/
T
T
n tdtntfT
a
(odd) (even)
| |
(odd)
)(tf
t
0)(2
2/
2/
0
T
T
dttfT
a
(odd)
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The amplitude-phase form
is known as the sine-cosine form
We can also express the Fourier series in
the cosine form only, that is
This form is called as the amplitude-phase
form
1
000 )sincos()(n
nn tnbtnaatf
100 )cos()(
nnn tnAatf
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From this form, we can plot the amplitude
spectrum, vs. n and the phasespectrum, vs. n.
It can be shown that the combination of
cosine and sine function can be expressedas a cosine function only:
Comparing both sides of eqn:
n
A
n
xAxA
xxA
xAxbxa
sin)sin(cos)cos(
)sinsincos(cos
)cos(sincos
.....(1cos aA .....(2)sin bA
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22222
22222
222222
)sin(cos
sincos
baAbaA
baA
baAA
a
b
a
ba
b
A
A
1tantan
cos
sin
:)2()1( 22
:
)1(
)2(
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Hence
)cos(sincos 000 nnnn tnAtnbtna
where
22
amplitude, nnn baA
n
n
na
b1tanphase,
Or in phasor/complex form:
n
nnnnnnn
a
bbajbaA 122 tan
00 aA
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Example 1
Determine the Fourier series of the following
waveform. Obtain the amplitude and phase
spectra.
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SolutionFirst, determine the period & describe the one period
of the function:
T= 2
21,0
10,1)(
t
ttf )()2( tftf
T
2
0We find that
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Then, obtain the coefficients a0, an and bn:
2
1)01(2
1012
1)(2
1
)(1
2
1
1
0
2
0
0
0
dtdtdttf
dttfT
aT
Or
21
211graphbelowArea
0 T
a
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n
n
n
tndttdtn
tdtntf
T
an
sinsin0cos1
cos)(2
1
0
2
1
1
0
2
0
0
Notice that n is integer which leads ,
since
0sin n03sin2sinsin
Therefore, .0na
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n
n
n
tndttdtn
tdtntfT
bn
cos1cos0sin1
sin)(2
1
0
2
1
1
0
2
00
15cos3coscos 16cos4cos2cos
Notice that
Therefore,
even,0
odd,/2)1(1
n
nn
nb
n
n
orn
n )1(cos
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ttt
tnn
tn
n
tnbtnaatf
nn
n
n
n
nn
5sin5
23sin
3
2sin
2
2
1
sin2
2
1
sin)1(1
2
1
)sincos()(
odd1
1
1
000
Finally,
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)905cos(5
2
)903cos(3
2)90cos(
2
2
1
)90cos(221
sin2
2
1)(
odd1
odd1
t
tt
tnn
tnn
tf
nn
nn
In amplitude-phase form,
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Amplitude spectrum: Phase spectrum:
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Some helpful identities
Forn integers,n
n )1(cos 0sin n02sin n 12cos n
xx sin)sin(
xx cos)cos(
)90cos(sin xx
)90cos(sin xx
)180cos(cos xx
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The sum of the Fourier series terms can
evolve (progress) into the original
waveform
From Example 1, we obtain
ttttf
5sin
5
23sin
3
2sin
2
2
1)(
It can be demonstrated that the sum will
lead to the square wave:
Notes:
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tttt
7sin7
25sin
5
23sin
3
2sin
2ttt
5sin
5
23sin
3
2sin
2
tt
3sin3
2sin
2t
sin
2
(a) (b)
(c) (d)
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ttttt
9sin9
27sin
7
25sin
5
23sin
3
2sin
2
ttt
23sin23
23sin
3
2sin
2
2
1
(e)
(f)
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Example 2
Given ,)( ttf 11 t
)()2( tftf
Sketch the graph off(t) such that .33 t
Then compute the Fourier series expansion off(t).
Plot the amplitude and phase spectra until the forthharmonic.
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Solution
The function is described by the following graph:
T= 2
T
20We find that
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nnn
n
n
n
ntn
nn
dtn
tn
n
tnt
tdtnttdtntfT
b
nn
n
1
22
1
0
22
1
0
1
0
1
0
1
0
0
)1(20
)1(2sin2cos2
sin2cos2
cos2
cos2
sin2
4sin)(
4
Then we compute the coefficients:
00 naa sincef(t) is an odd function.
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)904cos(2
1)903cos(
3
2
)902cos(1
)90cos(2
4sin2
13sin
3
22sin
1sin
2
sin)1(2
)sincos()(
1
1
1
000
tt
tt
tttt
tn
n
tnbtnaatf
n
n
n
nn
Hence,
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Amplitude spectrum: Phase spectrum:
0.64
0.32
0.210.16
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Example 3
Given
42,0
20,2)(
t
tttv
)()4( tvtv
(i) Sketch the graph ofv (t) such that .120 t
(ii) Compute the trigonometric Fourier series ofv (t).
(iii) Express the Fourier series ofv (t) in the
amplitude-phase form. Then plot the amplitude
and phase spectra until the forth harmonic.
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Solution
(i) The function is described by the following graph:
T= 4
2
20
T
We find that
0 2 4 6 8 10 12t
v (t)
2
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(ii) Then we compute the coefficients:
21
22
41)2(
41
0)2(4
1)(
1
2
0
22
0
4
2
2
0
4
0
0
ttdtt
dtdttdttvT
a
Or
2
1
4
2221
0
a
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odd,/4
even,0])1(1[2)cos1(2
2
2cos1cos
2
10
sin
2
1sin)2(
2
1
0cos)2(21cos)(2
222222
2
0
2
0
2
0
2
0
2
0
2
0 0
0
2
00
0
4
2
2
0
0
4
0
0
nn
n
nn
n
n
n
n
tn
dt
n
tn
n
tnt
tdtnttdtntvT
a
n
n
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nnnn
n
n
tn
n
dtn
tn
n
tnt
tdtnttdtntvT
bn
212
2sin1
sin
2
11
cos
2
1cos)2(
2
1
0sin)2(2
1sin)(
2
0
2
0
2
0
0
2
0
2
0
2
0
0
2
0 0
0
2
00
0
4
2
2
00
4
00
since 0sin2sin 0 nn
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1
00
122
1
000
)cos(
2sin
2
2cos
])1(1[2
2
1
)sincos()(
n
nn
n
n
n
nn
tnAa
tn
n
tn
n
tnbtnaatv
Hence,
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odd,2
tan142
even,902
odd,22
even,2
1
22n
n
nn
n
n
njnn
nn
j
jbaA nnnn
(iii) Where
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5.5775.011 A
0.7822.033 A
9032.022 A
9016.044 A
5.000 aA
We obtained that
902cos16.00.782
3cos22.0
90cos32.05.572cos75.05.0
)cos()(1
00
tt
t
t
tnAatvn
nn
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Amplitude spectrum: Phase spectrum:
0.5
0.75
0.32
0.22
0.16
0 /2 3/2 2
/2 3/2 2
57.5
78.090 90
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Example 4
Given
21,1
11,
12,1
)(
t
tt
t
tf
)()4( tftf
Sketch the graph off(t) such that .66 t
Then compute the Fourier series expansion off(t).
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Solution
The function is described by the following graph:
T= 4
2
2
T
We find that
046 2 4 6t
f(t)
2
1
1
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Then we compute the coefficients. Sincef(t) is
an odd function, then
0)(2
2
2
0
dttfT
a
0cos)(2
2
2
tdtntf
T
an
and
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2222
1
0
22
2
1
1
0
1
0
2
1
1
0
2
0
2
2
)2/sin(4cos2sin2cos
cos2cossincos
coscoscos
sin1sin4
4
sin)(4
sin)(2
n
n
n
n
n
n
n
nn
nn
n
tn
n
n
ntndt
ntn
ntnt
tdtntdtnt
tdtntfT
tdtntfT
bn
since 0sin2sin
nn
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1
1
1
1
0
2sin)1(2
2sin
cos2
)sincos(2
)(
n
n
n
n
nn
tnn
tn
n
n
tnbtnaa
tf
Finally,
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Example 5
Compute the Fourier series expansion off(t).
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Solution
The function is described by
T= 3
3
220
Tand
32,1
21,2
10,1
)(
t
t
t
tf
)()3( tftf
T= 3
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Then we compute the coefficients.
3
81
2
32)01(
3
421
3
4)(
4)(
22/3
1
1
0
2/3
0
3
0
0
dtdtdttfT
dttfT
a
3
8)23()12(2)01(
3
2121
3
2)(
2 3
2
2
1
1
0
3
0
0
dtdtdtdttfTa
Or, sincef(t) is an even function, then
Or, simply
3
84
3
2
periodain
graphbelowareaTotal2)(
23
0
0
T
dttfT
a
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3
2sin
2
3
2sinsin2
2
sin2
3sin2
3
4
sin2
3sin2sin
3
4
sin2
3
4sin
3
4
cos2cos13
4
cos)(4
cos)(2
2/3
1
1
0
2/3
1
1
0
2/3
0
3
0
n
n
nn
n
nn
n
nn
nn
n
tn
n
tn
tdtntdtn
tdtntfT
tdtntfT
an
;3
2
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1
1
1
0
3
2cos
3
2sin
12
3
4
3
2cos
3
2sin
2
3
4
)sincos(2
)(
n
n
n
nn
tnn
n
tnn
n
tnbtnaa
tf
Finally,
and 0nb sincef(t) is an even function.
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Parsevals Theorem
Parservals theorem states that the
average power in a periodic signal is equal
to the sum of the average power in its DCcomponent and the average powers in its
harmonics
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=
+ +
+ + +
ta cos1
ta 2cos2
tb sin1
tb 2sin2
f(t)
t
PavgPdc
Pa1 Pb1
Pa2 Pb2
0a
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For sinusoidal (cosine or sine) signal,
R
V
R
V
R
VP
2
peak
2
peak2
rms
2
12
For simplicity, we often assumeR= 1,
which yields2
peak
2
rms21VVP
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And since ,
Therefore, the total power of all harmonics
is
2
2
2
2
2
1
2
1
2
0
dcavg
2
1
2
1
2
1
2
1
2211
babaa
PPPPPP baba
1
22
0
1
222
0avg2
1)(
2
1
n
n
n
nn AabaaP
2rm sVP
1
22
0
1
222
0rms
2
1)(
2
1
n
n
n
nn AabaaV
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Circuit applications
Steps for applying Fourier series:
1. Express the excitation as a Fourier
series
2. Transform the circuit from the time
domain to the frequency domain
3. Find the response of the dc and ac
components in the Fourier series
4. Add the individual dc and ac responses
using the superposition principle
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The signal in Figure 6.1 is applied to the circuit in
Figure 6.2.
(i) Find the Fourier series of is(t).(ii) Plot the amplitude spectrum for the dc
component and the first three non-zero
harmonics ofis(t).
(iii) Find the load current iL(t).(iv) Find the average power dissipated in the
resistorRL.
Example 6
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)(tis
)(tis
H1
1 1
LR
)(tiL
Figure 6.2
Figure 6.1
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Solution
(i) The current source is described by
and
0,
0,
,||)(
tt
tt
tttis
1,2 0 T
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Compute the Fourier series. Since is(t) is an even
function,
or
22
1
2
2)(
2
0
2
0
2/
0
0
ttdtdtti
Ta
T
s
22
11
0over
graphbelowArea10
t
a
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even,0
odd,/4]1)1[(2
)1(cos2
cos2sin2
sin2sin2cos
2
4
cos)(4
2
2
2
0
2
000
2/
0
0
n
nn
n
n
n
nnt
nn
dtn
nt
n
nttntdtt
tdtntiT
a
n
T
sn
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Therefore, the Fourier series ofis
(t) is
0nb
)1805cos(
25
4
)1803cos(94)180cos(4
2
)180cos(4
2
cos4
2)(
odd1
2
odd1
2
t
tt
ntn
ntn
ti
nn
nn
s
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nsI nsI
n n
1.5711.273
0.141
0.051
(ii) Hence,
,5,3,1,18042 nn
Ins 20
sI
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(iii) To compute iL(t), separate to dc and ac analysis:
dc analysis (n = 0)
S/c the inductor, hence
0sI
11
42
2/
2
0
0
s
L
II
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1802
tantan4
14
180
4
2
1
,)(
11
2
2
2
2
0
nn
n
n
n
njn
jn
nnILjRR
LjRI nsnL
nL nn
s
L
L ILjRR
LjRI)(
ac analysis (n 1)
or
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)49.1905cos(048.0)26.1953cos(124.0
)43.198cos(805.0785.0)(
tt
ttiL
Hence,
49.190048.0
,26.195124.0,43.198805.0
,785.0
5
31
0
L
LL
L
I
II
I
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W949.0)048.0124.0805.0(2
1785.0
2
1
2222
1
22
2)(
0
L
n
LL
LrmsL
RII
RIP
n
(iv) Average power absorbed byRl is
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The signal in Figure 7.1 is applied to the circuit in
Figure 7.2.
(i) Find the Fourier series of vs(t).
(ii) Find the output voltage vo(t).(iii) Plot the amplitude spectrum for the dc
component and the first three non-zero harmonics
of the output voltage vo(t).
(iv) Find the r.m.s value of vo(t) .
Example 7
)(tv
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)(tvsF1
1
)(tvo
Figure 7.2
Figure 7.1
1
)(tvs
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Solution
(i) The voltage source is described by
and
01,1
10,1)(
t
ttv
s
0,2T
Compute the Fourier series. Since vs(t) is an odd
function,
00 naa
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even,0
odd,/4])1(1[2
)cos1(2
cos2sin
2
4
sin)(4
1
0
1
0
2/
00
n
nn
n
n
n
n
tntdtn
tdtntvT
b
n
T
sn
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Therefore, the Fourier series ofvs(t) is
)905cos(5
4
)903cos(3
4)90cos(
4
)90cos(4sin4)(
odd1
odd1
t
tt
tnn
tnn
tv
nn
nn
s
Hence,
,5,3,1,904
nn
Vn
s
0
0
sV
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(ii) To compute vo(t), separate to dc and ac analysis:
dc analysis (n = 0)
O/c the capacitor, hence
00 sV
1
00 oV
1
0I
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902tantan41
14
90
4
21
1
,21
1
11
22
22
0
nnn
n
n
nnj
jn
nnVRCj
RCjV nsn
no nn
sso VRCjRCjV
CjRRCjRV
211
)]/1([)/1(
ac analysis (n 1)
or
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)82.915cos(128.0
)02.933cos(213.0)61.98cos(660.0)(
t
tttvo
Hence,
82.91128.0,02.93213.0,61.98660.0
,0
5
31
0
o
oo
o
VVV
V
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noV
noV
n
n
0.660
0.213
0.128
98.61
(iii)
93.02
91.82
(iv)
V705.0)128.0213.0660.0(2
1
2
1
222
1
22
)rms( 0
nooo nVVV
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Exponential Fourier series
Recall that, from the Eulers identity,
xjxe jx sincos
yields
2
cosjxjx ee
x
2
sin
j
eex
jxjx and
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Then the Fourier series representation becomes
11
0
1
0
1
0
1
0
1
0
222
222
222
222
)sincos(2
)(
n
tjnnn
n
tjnnn
n
tjnnntjnnn
n
tjntjn
n
tjntjn
n
n
tjntjn
n
tjntjn
n
n
nn
ejba
ejbaa
ejbaejbaa
eejb
eea
a
j
eeb
eea
a
tnbtnaatf
jb jb
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Here, let we name
11
0
222)(
n
tjnnn
n
tjnnn ejba
ejbaa
tf
2
nnn
jbac
,
2
nnn
jbac
Hence,
n
tjn
n
n
tjn
n
n
tjn
n
n
tjn
n
n
tjn
n
n
tjn
n
n
tjn
n
ececcec
ececc
ececc
1
0
1
11
0
11
0
and .20
0
a
c
c0 cncn
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Then, the coefficient cn can be derived from
T
tjn
T
TT
TT
nnn
dtetfT
dttnjtntfT
tdtntfjtdtntfT
tdtntfT
jtdtntf
T
jbac
0
0
00
00
)(1
]sin)[cos(
1
sin)(cos)(1
sin)(2
2cos)(
2
2
1
2
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In fact, in many cases, the complex
Fourier series is easier to obtain rather
than the trigonometrical Fourier series
In summary, the relationship between the
complex and trigonometrical Fourier series
are:
nnnn
n Ajbac
21
2 2
nnn
jbac
T
dttfT
ac0
00 )(1
T
tjn
n dtetfT
c0
)(1
nn ccor
22
22
nnn
n
Abac
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The average power and the rms value in
the term of Fourier complex coefficient are
1
22
0
2
1 2n
n
n
n cccP
1
22
0
2
rms 2n
n
n
n cccF
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Example 8
For the given function,
(i) Obtain the complex Fourier series
(ii) Plot the amplitude and the phase spectra
of the complex Fourier series for5
n
5(iii) Calculate the average power and the rms value
of the signal
2
4
4
2
0
2e
1
)(tv
t
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(i) Since , . Hence
Solution
2
1
2
1
2
1
)(1
22
0
2
0
0
0
ee
dte
dttv
T
c
t
t
T
10 2T
T
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)1(21
)1(21
)1(21
12
1
2
1
2
1
)(1
222)1(2
2
0
)1(
2
0
)1(
2
0
0
0
jne
jnee
jne
jn
e
dtedtee
dtetvT
c
njjn
tjn
tjnjntt
T
tjn
n
since 1012sin2cos2 njne nj
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jnt
nn
tjn
n ejn
eectv
)1(2
1)(
2
0
Therefore, the complex Fourier series ofv(t) is
0
2
0
2
0 2
1
)1(2
1c
e
jn
ec
nnn
*Notes: Even though c0 can be found by substituting
cn with n = 0, sometimes it doesnt works (as shown
in the next example). Therefore, it is always better to
calculate c0 alone.
ne2 1
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nn
n
n
ec nn
1
2
1
2
tan1
85
1tan0
12
1
(ii)
The complex frequency spectra are
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kW19.20
)7.166.209.26381.60(285 222222
2
1
n
ncP
(ii) The average power is (assumeR= 1 )
and since 2rm s1 VP
V09.142k19.20rms V
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Example 9
Obtain the complex Fourier series of the function in
Example 1. Then plot the complex frequency spectra
for4 n4 and calculate the rms value of the signal.
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Solution
2
11
2
1)(
11
00
0 dtdttfTc
T 0
)1(22
1
012
1)(
1
1
0
2
1
1
00
jntjn
tjn
T
tjn
n
enj
jne
dtedtetfT
c
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)1(2
jn
n en
jc
Butnjn nnjne )1(cossincos
Thus,
even,0
odd,/]1)1[(
2 n
nnj
nj n
Therefore,
odd
0
2
1)( 0
n
nn
tjn
n
tjn
n en
jectf
*Here notice that .00 cc nn
00c For even
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2
10 c
0,0 nnc Forneven,
Fornodd,
0,90
0,90,1
n
n
nc nn
0.5
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692.0)11.032.0(25.0
222
2
rms
nncF
The average rms value is
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Summary
Sine-cosine form
Amplitude-phase form
Exponential (or complex) form
1
000 )sincos()(n
nn tnbtnaatf
,cos)(2
00
T
n
tdtntfT
a
T
n tdtntf
T
b0
0sin)(2
,)(1
00
T
dttfT
a,2
0
T
1
00 )cos()(n
nn tnAatf
n
tjn
nectf0)(
nnnn
jbaA
,00
ac ,2
nnn
jbac
*nn cc
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Prob.17.37, pg.803
If the sawtooth waveform in Fig.17.9 is the
voltage source vs(t) in the circuit of
Fig.17.22, find the response vo(t).
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Prob.17.37, pg.803
If the periodic current waveform in
Fig.17.73(a) is applied to the circuit in
Fig.17.73(b), find vo.
3
)(tis
0 1 2 3
1
3
t
(a) (b)
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Problem
1 2 30 4
1
5
2
3
6
1oR
1sR F1C
)(tvo
)(tvs
)(tvs
t
(i) Compute the output voltage vo(t).(ii) Plot the amplitude and phase spectra ofvo(t)
for the first three nonzero harmonics.
(ii) Calculate the average power dissipated in
the resistor Ro.
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QUIZ 1
)(tvs
)(tvo
H1
H1
1
1
Given the Fourier series expansion of thevoltage signal vs(t) is
Find vo
(t) in the amplitude-phase form.
1
sin10
5)(
n
s nt
n
tv
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Quiz1
Determine the Fourier series of the following
waveform.
)(tv
4
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QUIZ 2
Find i(t) ifv(t) is the signal as given in QUIZ 1.
odd1odd1
)90cos(2
2
1sin
2
2
1)(
nnnn
tn
n
tn
n
tv
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QUIZ 3
Find the output voltage vo(t) if the currentsource is(t) is given by
1
)90cos(2
1)(ns
ntn
ti
)(tis
)(tvo5 F5.0
3
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QUIZ 4
Using time differentiation technique, find theFourier transform off(t).