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

  • 7/27/2019 Ee602 Fourier Series

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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)

  • 7/27/2019 Ee602 Fourier Series

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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).