engineering circuit analysis
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Engineering Circuit Analysis. CH8 Fourier Circuit Analysis. 8.1 Fourier Series 8.2 Use of Symmetry. Ch8 Fourier Circuit Analysis. 8.1 Fourier Series. Most of the functions of a circuit are periodic functions - PowerPoint PPT PresentationTRANSCRIPT
Engineering Circuit AnalysisEngineering Circuit Analysis
CH8 Fourier Circuit AnalysisCH8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series8.2 Use of Symmetry8.2 Use of Symmetry
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
- Most of the functions of a circuit are periodic functions
- They can be decomposed into infinite number of sine and cosine functions that are harmonically related.
- A complete responds of a forcing function =
Partial response to each harmonics. erpositonsup
Harmonies: Give a cosine function
- : fundamental frequency ( is the fundamental wave form)
- Harmonics have frequencies
0
0
01
21:
2:
cos2
wfTT
wff
twtv
0w tv1
tnwatv nn 0cos
Amplitude of the nth harmonics(amplitude of the fundamental wave form)
,4,3,2, 0000 wwww
Freq. of the 1st harmonics (=fund. freq)
Freq. of the 2nd harmonics
Freq. of the nth harmonics
Freq. of the 3rd harmonics
Freq. of the 4th harmonics
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
8.1 Fourier Series8.1 Fourier SeriesExample Fundamental: v1 = 2cosw0t
v3a = cos3w0t v3b = 1.5cos3w0t
v3c = sin3w0t
Ch8 Fourier Circuit Analysis
- Fourier series of a periodic function
Given a periodic function
can be represented by the infinite series as
( ) ( ) ( )Ttftftf +=:
tf
1000
0201
02010
sincos
2sinsin
2coscos
nnn tnwbtnwaa
twbtwb
twatwaatf
?
?
?0
n
n
b
a
a
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
Example 12.1
3.01.0,0
1.01.0,5cos
t
ttVtV m
mVa 0
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
Given a periodic function
It is knowing
21mVa
1n
12cos2
2
n
nVa mn
00b
It can be seen , we can evaluate 50
3
22
mVa 03a 15
24
mVa 05a 35
26
mVa
tV
tV
tV
tVV
tV
m
mmmm
30cos35
2
20cos15
210cos
3
25cos
2
-Review of some trigonometry integral observations
(a)
(c)
(d)
0sin0 0 dttnwT
0cos0 0 dttnwT
(b)
(e)
0sinsin2
1cossin
0 000 00 dttwnktwnkdttnwtkwTT
nkif
nkifT
dttwnktwnk
dttnwtkw
T
T
,0
,2
coscos2
1
sinsin
0 00
0 00
nkif
nkifT
dttwnktwnk
dttnwtkw
T
T
,0
,2
coscos2
1
cossin
0 00
0 00
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
-Evaluations of nn baa ,,0
0a
T
nnn
TTdttnwbtnwadtadttf
01
000 00sincos
Based on (a) (b)
0sincos0
100
T
nnn dttnwbtnwa
Tdttf
Ta
00
1
0a ( is also called the DC component of ) tf
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
na
Based on (b)
T
nn
T
nn
TT
dttkwtnwb
dttkwtnwadttkwatdtkwtf
01
00
01
000 000 0
cossin
coscoscoscos
T
n
n
T
nn
tdtnwtfT
a
aT
dttkwtnwa
0 0
01
00
cos2
2coscos
0cossin0
100
T
nn dttkwtnwb
0cos0 00 dttkwaT
Based on (c)
Based on (e)
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
When k=n
nb
Based on (a)
T
nn
T
nn
TT
dttkwtnwb
dttkwtnwadttkwatdtkwtf
01
00
01
000 000 0
sinsin
sincossinsin
T
n
n
T
nn
tdtnwtfT
b
bT
dttkwtnwb
0 0
01
00
sin2
2sinsin
0sincos0
100
T
nn dttkwtnwa
0sin0 00 dttkwaT
Based on (c)
Based on (d)
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
When k=n
nnnnn tnwbatnwbtnwa 022
00 cossincos
)(Hzf
22nnn bav
Harmonic
amplitude
7v
07 f06 f05 f04 f03 f02 f0f
5v6v4v3v2v1v
Phase spectrum
0f 02 f 03 f 04 f 05 f 06 f 07 f)(Hzf
n
n
nn a
b 1tan
Ch8 Fourier Circuit Analysis
8.1 Fourier Series8.1 Fourier Series
- Depending on the symmetry (odd or even), the Fourier series can be further simplified.
Even Symmetry
Observation: rotate the function curve along axis, the curve will overlap with the curve on the other half of .
Example :
Odd Symmetry
Observation: rotate the function curve along the axis, then along the axis, the curve will overlap with the curve on the other half .
Example :
tftf
tftf
wttf cos
tf tf
wttf sin
t tf
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry
Symmetry Algebra
(a) odd func. =odd func. × even func.
Example:
(b) even func. =odd func. × odd func.
Example:
(c) even func. =even func. × even func.
Example:
tωtωtω
cossin=2
2sin
tωtωtω
cos×cos=2
1+2cos
tωtωtω
sin×sin=2
2cos-1
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry
(d) even func. =const. +∑ even func. (No odd func.)
Example:
(e) odd func. =∑odd func.
Example:
tωtωtω 22 sin2-1=1-cos2=2cos
( ) φtωφtωφtω sincos+cossin=+sin
odd func. odd func.
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry
Apply the symmetry algebra to analyze the Fourier series.
If is an even function
If is an odd function
1
000 sincosn
nn tnwbtnwaatf
100 cos
0
nn
n
tnwaatf
b
10
0
sin
00
nn
n
tnwbtf
aa
tf
tf
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry
Half-wave symmetry f(t) = -f(t - ) or f(t) = -f(t + )2
T2
T
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry
evenisn
oddisntdtnwtfTb
evenisn
oddisntdtnwtfTa
T
n
T
n
0
sin4
0
cos4
0
0
0
0
Fourier series:
Ch8 Fourier Circuit Analysis
8.2 Use of Symmetry8.2 Use of Symmetry