Module -7
Properties of Fourier Series and Complex Fourier Spectrum.
Objective:To understand the change in Fourier series coefficients due to different signal
operations and to plot complex Fourier spectrum.
Introduction:
The Continuous Time Fourier Series is a good analysis tool for systems with
periodicexcitation. Understanding properties of Fourier series makes the work simple in
calculating the Fourier series coefficients in the case when signals modified by some basic
operations.
Graphical representation of a periodic signal in frequency domain represents Complex
Fourier Spectrum.
Description:
Properties of continuous- time Fourier series
The Fourier series representation possesses a number of important properties
that are useful for various purposes during the transformation of signals from one form to other .
Some of the properties are listed below.
[x1(t) and x2(t)] are two periodic signals with period T and with Fourier series
coefficients Cn and Dn respectively.
1) Linearity property
The linearity property states that, if
x1(t)πΉπ Cn and x2 (π‘)
πΉπ Dn
then Ax1(t)+Bx2(t) πΉπ ACn+BDn
proof: From the definition of Fourier series, we have
FS[Ax1(t)+Bx2(t)]=1
π [π΄π₯1 π‘ + π΅π₯2 π‘ ]πβπππ0π‘ππ‘π‘0+π
π‘0
=π΄(1
π π₯1 π‘ π
βπππ0π‘ππ‘π‘0+π
π‘0)+ π΅(
1
π π₯2 π‘ π
βπππ0π‘ππ‘)π‘0+π
π‘0
=ACn+BDn
or proved
2) Time shifting property
The time shifting property states that,if
x(t)πΉπ Cn
then x(t-t0)πΉπ πβπππ0π‘0Cn
proof: From the definition of Fourier series, we have
x(t)= πΆπβπ=ββ πππ π0π‘
x(t-t0)= πΆπβπ=ββ πππ π0(π‘βπ‘0)
= [πΆπβπ=ββ πβπππ0π‘]πππ π0π‘
=FS-1
[Cnπβπππ0π‘]
Ax1(t)+Bx2(t)πΉπ ACn+BDn
x(t-t0)πΉπ πβπππ0π‘0Cn
Or proved
3) Time reversal property
The time reversal property states that,if
x(t)πΉπ Cn
then x(-t)πΉπ C-n
proof: From the definition of Fourier series, we have
x(t)= πΆπβπ=ββ πππ π0π‘
x(-t)= πΆπβπ=ββ πππ π0(βπ‘)
substituting n = -p in the right hand side, we get
x(-t)= πΆβπββπ=β ππ (βπ)π0(βπ‘) = πΆβπ
βπ=ββ πππ π0π‘
substituting p = n , we get
x(-t)= πΆβπβπ=ββ πππ π0π‘ = FS
-1[C-n]
proved
4) Time scaling property
The time scaling property states that , if
x(t)πΉπ Cn
then x(πΌt)πΉπ Cn with Ο0βπΌΟ0
proof: From the definition of Fourier series, we have
x(t)= πΆπβπ=ββ πππ π0π‘
x(πΌt)= πΆπβπ=ββ πππ π0πΌπ‘= πΆπ
βπ=ββ πππ (π0πΌ)π‘= FS
-1[Cn]
where Ο0βπΌΟ0.
proved.
5) Time differential property
The time differential property states that, if
x(t)πΉπ Cn
then ππ₯ (π‘)
ππ‘
πΉπ jnΟ0Cn
proof: From the definition of Fourier series, we have
x(t)= πΆπβπ=ββ πππ π0π‘
Differentiating both sides with respect to t, we get ππ₯ (π‘)
ππ‘= πΆπ
π(π ππ π0π‘)
ππ‘βπ=ββ = πΆπ(jnΟ0)β
π=ββ πππ π0π‘
= πΆπ(jnΟ0)βπ=ββ πππ π0π‘ = FS
-1[jnΟ0Cn]
Proved.
6) Time integration property
x(-t)πΉπ C-n
x(πΌt)πΉπ Cn with fundamental frequency of πΌΟ0
ππ₯ (π‘)
ππ‘
πΉπ jnΟ0Cn
The time integration property states that,
if x(t)πΉπ Cn
then π₯ π πππ‘
ββ
πΉπ
πΆπ
ππ π0 (if C0 = 0)
proof: From the definition of Fourier series, we have
x(t)= πΆπβπ=ββ πππ π0π‘
π₯ π πππ‘
ββ = [ πΆπ
βπ=ββ πππ π0π]ππ
π‘
ββ
Interchanging the order of integration and summation, we get
π₯ π πππ‘
ββ = πΆπ
βπ=ββ πππ π0πππ
π‘
ββ
= πΆπβπ=ββ [
π ππ π0π
ππ π0 ] ββ
π‘
= (πΆπ
ππ π0)β
π=ββ πππ π0π‘ = FS-1
[πΆπ
ππ π0]
Proved.
7) Convolution theorem or property
The convolution theorem or property states that , The Fourier series of the
convolution of two time domain functions x1(t) and x2(t) is equal to the multiplication of their
Fourier series coefficients, i.e.βConvolution of two functions in time domain is equivalent to
multiplication of their Fourier coefficients in frequency domainβ.
The convolution property states that,if
x1(t)πΉπ Cn and x2 (π‘)
πΉπ Dn
then x1(t)*x2(t) πΉπ TCnDn
proof: From the definition of Fourier series, we have
FS[x1(t)*x2(t)]=1
π [x1(t) β x2(t)]π‘0+π
π‘0πβπππ0π‘ππ‘ =
1
π [x1(t) β x2(t)]π
0πβπππ0π‘ππ‘
But from the convolution integral for a periodic signal, we have
x1(t) β x2(t)= x1(Ο) β x2(t β Ο)π
0ππ or x1(t) β x2(t)= x1(t β Ο) β x2(Ο)
π
0ππ
Substituting back, we get
FS[x1(t)*x2(t)] =1
π ( x1(Ο) β x2(t β Ο)
π
0ππ)
π
0πβπππ0π‘ππ‘
Interchanging the order of integration, we get
FS[x1(t)*x2(t)] =1
π x1(Ο)[ x2(t β Ο)
π
0
π
0πβπππ0π‘ππ‘] ππ
Substituting t-π =t' in RHS, we have dt=dt'. Substituting back, we get
FS[x1(t)*x2(t)] =1
π x1(Ο)[ x2(tβ²)
πβΟ
βΟ
π
0πβππ π0(π‘ β² +Ο)ππ‘β²] ππ
=T(1
π x1(Ο)πβπππ0π‘πππ
0) (
1
π x1(tβ²)πβπππ0π‘β²πtβ²π
0)
=T(Cn)(Dn)
0 to T or βπ to T-π or t0 to t0+T will have the same period.
Proved.
π₯ π πππ‘
ββ
πΉπ
πΆπ
ππ π0 , C0 = 0
x1(t)*x2(t)πΉπ TCnDn
8) Modulation or Multiplication property
The Modulation or Multiplication property states that,if
x1(t)πΉπ Cn and x2 (π‘)
πΉπ Dn
then x1(t)x2(t) πΉπ πΆππ·πβπ
βπ=ββ
proof: From the definition of Fourier series, we have
FS[x1(t)x2(t)]= 1
π x1 t x2 t π‘0+π
π‘0πβπππ0π‘ππ‘
=1
π x1 t ( πΆππ
ππ π0π‘βπ=ββ )
π‘0+π
π‘0πβπππ0π‘ππ‘
=1
π x1 t ( πΆππ
βπ (πβπ)π0π‘βπ=ββ )
π‘0+π
π‘0ππ‘
Interchanging the order of integration and summation, we have
FS[x1(t)x2(t)]= πΆπβπ=ββ (
1
π x1 t πβπ (πβπ)π0π‘π‘0+π
π‘0ππ‘) = πΆππ·πβπ
βπ=ββ
Proved.
9) Conjugation and Conjugate symmetry property
If x(t)πΉπ Cn
Then the Conjugation property states that
x*(t)
πΉπ πΆβπ
β [for complex x(t)]
proof: Conjugation property
From the definition of Fourier series, we have
FS[x*(t)]=
1
π x β (t)π‘0+π
π‘0πβπππ0π‘ππ‘=(
1
π x(t)π‘0+π
π‘0πππ π0π‘ππ‘)*
=(1
π x(t)π‘0+π
π‘0πβπ (βπ)π0π‘ππ‘)*
=[C-n]*
Proved.
10) Parsevalβs Relation or Theorem or property
If x1(t)πΉπ Cn [for complex x1(t)]
And x2 (π‘)πΉπ Dn [for complex x2(t)]
Then, the parsevals relation states that 1
π x1 t x2
β t π‘0+π
π‘0ππ‘= Cn
βπ=ββ π·π
β [for complex x1(t) and x2(t)]
And parsevalβs identity states that 1
π βΉx(t)βΈ2π‘0+π
π‘0ππ‘ = βΉCnβΈ
2βπ=ββ if x1(t) = x2(t) = x(t)
Proof: parsevals relation
LHS =1
π x1 t x2
β t π‘0+π
π‘0ππ‘ =
1
π ( πΆππ
ππ π0π‘βπ=ββ )
π‘0+π
π‘0π₯2βππ‘
Interchanging the order of integration and summation in the RHS , we have 1
π x1 t x2
β t π‘0+π
π‘0ππ‘ = πΆπ
βπ=ββ (
1
π x2
β t πππ π0π‘π‘0+π
π‘0ππ‘)
= πΆπβπ=ββ (
1
π x2 t πβπππ0π‘π‘0+π
π‘0ππ‘)*
= Cnβπ=ββ [Dn]β
x1(t)x2(t)πΉπ πΆππ·πβπ
βπ=ββ
x*(t)
πΉπ πΆβπ
β
Proved
Parsevalβs identity
If x1(t) = x2(t) = x(t), then the relation changes to
1
π x t xβ t π‘0+π
π‘0ππ‘= Cn
βπ=ββ πΆπ
β
Since βΉx(t)βΈ2= x t xβ t and βΉCnβΈ2= CnπΆπ
β , substituting these values in above equation, we get
Proved.
Complex Fourier Spectrum
The Fourier spectrum of a periodic signal x(t) is a plot of its Fourier coefficients versus
frequency Ο. It is in two parts : (a) Amplitude spectrum and (b) phase spectrum. The plot of the
amplitude of Fourier coefficients verses frequency is known as the amplitude spectra, and the
plot of the phase of Fourier coefficients verses frequency is known as phase spectra. The two
plots together are known as Fourier frequency spectra of x(t).This type of representation is also
called frequency domain representation. The Fourier spectrum exists only at discrete frequencies
nΟo, where n=0,1,2,β¦.. Hence it is known as discrete spectrum or line spectrum. The envelope
of the spectrum depends only upon the pulse shape, but not upon the period of repetition.
The trigonometric representation of a periodic signal x(t) contains both sine and cosine
terms with positive and negative amplitude coefficients (an and bn) but with no phase angles.
The cosine representation of a periodic signal contains only positive amplitude
coefficients with phase angle ΞΈn. Therefore, we can plot amplitude spectra(An versus Ο)and phase
spectra (ΞΈn versus Ο). Since, in this representation, Fourier coefficients exist only for positive
frequencies, this spectra is called single-sided spectra.
The exponential representation of a periodic signal x(t) contains amplitude coefficients
Cn Which are complex. Hence, they can be represented by magnitude and phase. Therefore, we
can plot two spectra, the magnitude spectrum(βΉCnβΈversus Ο) and phase spectrum ( πΆπ versus Ο).
The spectra can be plotted for both positive and negative frequencies.Henceit is called two-sided
spectra.
The below figure (a) represents the spectrum of a trigonometric Fourier series extending
from 0 toβ,producing a one-sided spectrum as no negative frequencies exist here. The figure (b)
represents the spectrum of a complex exponential Fourier series extending from -
βπ‘πβ,producing a two-sided spectrum.
The amplitude spectrum of the exponential Fourier series is symmetrical Fourier series is
symmetrical about the vertical axis. This is true for all periodic functions.
1
π x1 t x2
β t π‘0+π
π‘0ππ‘= Cn
βπ=ββ π·π
β
1
π βΉx(t)βΈ2π‘0+π
π‘0ππ‘ = βΉCnβΈ
2βπ=ββ
Fig: Complex frequency spectrum for (a) Trigonometric Fourier series and (b) complex
exponential Fourier series.
If Cn is a general complex number, then
Cn = βΉCnβΈππ ππ
C-n = βΉCnβΈπβπππ
Cn = βΉC-nβΈ
The magnitude spectrum is symmetrical about the vertical axis passing
through the origin, and the phase spectrum is antisymmetrical about the vertical axis passing
through the origin. So the magnitude spectrum exhibits even symmetry and phase spectrum
exhibits odd symmetry.
When x(t) is real , then C-n=πΆπβ, the complex conjugate of Cn.
Solved Problems:
Problem 1: Show that the magnitude spectrum of every periodic function is symmetrical about
the vertical axis passing through the origin and the phase spectrum is antisymmetrical about the
vertical axis passing through the origin.
Solution: The coefficient Cn of exponential Fourier series is given by
Cn = 1
π π₯ π‘ πβπππ0π‘ππ‘
T
2
βπ
2
)
And C-n = 1
π π₯ π‘ πππ π0π‘ππ‘
T
2
βπ
2
)
It is evident from these equations that the coefficients Cn and C-n are complex conjugate of each
other , that is
Cn= πΆπβ
Hence βΉCnβΈ=βΉCβnβΈ
It, therefore, follows that the magnitude spectrum is symmetrical about the vertical axis
passing through the origin, and hence is an even function of Ο.
If Cn is real, then C-n is also real and Cn is equal to C-n. is complex, let
Cn=βΉCnβΈejΞΈn
then C-n=βΉCnβΈeβjΞΈn
The phase of Cn is ΞΈn ;however , the phase of C-n isβΞΈn .Hence , it is obvious that the phase
spectrum is antisymmetrical, and the magnitude spectrum is symmetrical about the vertical axis
passing through the origin.
Problem 2: With regard to Fourier series representation, justify the following statement: odd
functions have only sine terms.
Solution: We know that the trigonometric Fourier series of a periodic function x(t) in any
interval t0 to t0+T or 0 to T or -π
2 to
π
2 is given by
x(t)= π0+ (ππ cosππ0π‘βπ=1 + ππ sinπΟ0t)
where π0 = 1
π π₯ π‘ ππ‘
T
2
βπ
2
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
2
βπ
2
andππ = 2
π π₯ π‘ sin πΟ0tππ‘
T
2
βπ
2
For an odd function
x(t) = -x(-t)
Also even part is zero, i.e. xe(t) = 0 and x(t) = x0(t)
π0 = 1
π π₯ π‘ ππ‘
T
2
βπ
2
= 1
π π₯0 π‘ ππ‘
T
2
βπ
2
= 0
Since the integration of an odd function,over one cycle is always zero.
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
2
βπ
2
= 2
π π₯0 π‘ cosππ0π‘ ππ‘
T
2
βπ
2
Here π₯0(t) is odd and cos ππ0π‘ is even. So π₯0 π‘ cosππ0π‘, i.e. the product if an odd function
and even function is odd. So integration over one cycle is zero. Therefore,
ππ = 0
Now, ππ = 2
π π₯ π‘ sin πΟ0tππ‘
T
2
βπ
2
= 2
π π₯0 π‘ sin πΟ0tππ‘
T
2
βπ
2
Now, π₯0(t) is odd sinπΟ0t is also odd. Therefore, the product π₯0(t)sinπΟ0t is even. So we
have to evaluate the integral:
ππ = 2
π π₯ π‘ sinπΟ0tππ‘
T
2
βπ
2
= 4
π π₯ π‘ sinπΟ0tππ‘
T
20
x(t)= ππ sinπΟ0tβπ=1
Thus, the Fourier series of odd functions contains only sine terms.
Problem3: With regard to Fourier series representation, justify the following statement: Even
functions have no sine terms.
Solution: We know that the trigonometric Fourier series of a periodic function x(t) in any
interval t0 to t0+T or 0 to T or -π
2 to
π
2 is given by
x(t)= π0+ (ππ cosππ0π‘βπ=1 + ππ sinπΟ0t)
where π0 = 1
π π₯ π‘ ππ‘
T
2
βπ
2
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
2
βπ
2
and ππ = 2
π π₯ π‘ sin πΟ0tππ‘
T
2
βπ
2
For an even function
x(t) = x(-t)
Also odd part is zero, i.e. x(t) = xe(t)
π0 = 1
π π₯ π‘ ππ‘
T
2
βπ
2
= 1
π π₯π π‘ ππ‘
T
2
βπ
2
= 2
π π₯ π‘ ππ‘
T
20
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
2
βπ
2
= 2
π π₯π π‘ cosππ0π‘ ππ‘
T
2
βπ
2
π₯π(t) is even and also cos ππ0π‘ is even. So, the product π₯π(t)cosππ0π‘ is even. So we have to
evaluate the integral:
ππ = 2
π π₯π π‘ cosππ0π‘ ππ‘
T
2
βπ
2
= 4
π π₯ π‘ cosππ0π‘ ππ‘
T
20
ππ = 2
π π₯ π‘ sinπΟ0tππ‘
T
2
βπ
2
= 2
π π₯π π‘ sin πΟ0tππ‘
T
2
βπ
2
Here π₯π(t) is even sinπΟ0tand is odd. So, the product π₯π π‘ sin πΟ0t is odd. Thus, the integral
over one complete cycle is zero.
ππ = 0
x(t)= π0+ ππ cosππ0π‘βπ=1
Thus, the Fourier series of even functions contains no sine terms.
Problem 4: With regard to Fourier series representation, justify the following statement:
Functions with half wave symmetry have only odd harmonics.
Solution: We know that the trigonometric Fourier series of a periodic function x(t) in any
interval t0 to t0+T or 0 to T or -π
2 to
π
2 is given by
x(t)= π0+ (ππ cosππ0π‘βπ=1 + ππ sinπΟ0t)
where π0 = 1
π π₯ π‘ ππ‘
T
2
βπ
2
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
2
βπ
2
and ππ = 2
π π₯ π‘ sin πΟ0tππ‘
T
2
βπ
2
For a half wave symmetric function,
x(t) = -x(tΒ±π
2)
Therefore, π0 = 0
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
0
= 2
π[ π₯ π‘ cosππ0π‘ ππ‘
T
20
+ π₯ π‘ cosππ0π‘ ππ‘Tπ
2
]
To have the limits of second integration also from 0 to T/2 , change the variable t by t + (T/2) in
the second integration.
ππ =2
π[ π₯ π‘ cosππ0π‘ ππ‘
T
20
+ π₯ π‘ +T
2 cosππ0(π‘ +
T
2) ππ‘
T
20
]
But x(t) = -x(tΒ±π
2)
ππ = 2
π[ π₯ π‘ cosππ0π‘ ππ‘
T
20
+ βπ₯ π‘ cos(ππ0π‘ + πΟ) ππ‘T
20
]
= 2
π[ π₯ π‘ cosππ0π‘ β π₯ π‘ cos(ππ0π‘ + πΟ)] ππ‘
T
20
ππ = 0 πππ ππ£ππ π
4
π π₯ π‘ cosππ0π‘
T
20
πππ πππ π
Now,
ππ = 2
π π₯ π‘ sinπΟ0tππ‘
T
20
= 2
π[ π₯ π‘ sinπΟ0tππ‘
T
20
+ π₯ π‘ sin πΟ0tππ‘Tπ
2
]
To have the limits of second integration also from 0 to T/2 change the variable t by t + (T/2) in
the second integration.
ππ = 2
π[ π₯ π‘ sinπΟ0tππ‘
T
20
+ π₯ π‘ +π
2 sinπΟ0(t +
T
2)ππ‘
T
20
]
But x(t) = -x(tΒ±π
2)
ππ = 2
π[ π₯ π‘ sinπΟ0tππ‘
T
20
+ βπ₯ π‘ sin(πΟ0t + nΟ)ππ‘T
20
]
= 2
π[ π₯ π‘ sinπΟ0tππ‘
T
20
- π₯ π‘ sinπΟ0t cos nΟ) ππ‘T
20
]
ππ = 0 πππ ππ£ππ π
4
π π₯ π‘ sinπΟ0t
T
20
πππ πππ π
Thus, the Fourier series of half wave symmetric function consists of only odd harmonics.
Problem 5: Draw the complex fourier spectrum of given figure.
Solution: It is a plot between Cn and nΟ0 = Ο. As Cn is real, only amplitude plot is sufficient.
The amplitude versus frequency plot, called the amplitude spectrum.
To draw the frequency spectrum,
x(t) = 2π΄
π +
A
Ο[ β1 n
2n+1+
β1 n +1
2nβ1]ej2ntβ
π=ββπβ 0
exponential Fourier series coefficients
C0 = 2π΄
π
C1= 2π΄
3π
C2= 2π΄
15π
C3= 2π΄
35π
Problem 6: For the periodic gate function shown in below figure, plot the magnitude and phase
spectra.
Solution: Amplitude and phase spectra are shown in below figures respectively.
Example 7: Find the complex exponential Fourier series and the trigonometric Fourier series of
unit impulse train Ξ΄T(t) shown in bellow figure.
Solution: The periodic waveform shown in above figure with period T can be expressed as
x(t) = Ξ΄T(t) = πΏ(π‘ β ππ)βπ=ββ
Let t0= -π
2
So t0+T = - π
2
In one period, only πΏ(t) exists.
Fundamental frequency Ο0 = (2π/T)
Cn =1
π π₯(π‘)πβπππ0π‘
π‘0+π
π‘0
ππ‘
=1
π πΏ(t) πβππ π0π‘ππ‘
T
2
βπ
2
= 1
πe0 =
1
π
Cn= 1
π
The exponential Fourier series is
x(t)= πΆπβπ=ββ πππ π0π‘ =
1
πβπ=ββ πππ (
2Ο
T)π‘
=1
π πππ
2Ο
T π‘β
π=ββ
To get the trigonometric Fourier coeifficients , we have
π0 = C0= 1
π
ππ = 2
π π₯ π‘ cosππ0π‘ ππ‘
T
0 =
2
π Ξ΄ π‘ cosππ0π‘ ππ‘
T
0 =
2
π
ππ = 2
π π₯ π‘ sinπΟ0tππ‘
T
0 =
2
π π₯ π‘ sinπΟ0tππ‘
T
0 = 0
Therefore , the trigonometric Fourier series is
x(t) +π0 + ππcosππ0π‘βπ=1 + ππ sinπΟ0t =
1
π +
2
π cosππ0π‘
βπ=1
The complex Fourier series coefficients are
Cn= 1
π for all n
The magnitude and phase spectrum are
βΉCnβΈ= βΉ1
πβΈfor all n
πΆπ = 0o for all n
The frequency spectra are plotted in bellow figure.
Problem 8: Find the Fourier series of the signal x(t)=e
-t with T = 1 sec as shown in bellow
figure. Draw its magnitude and phase spectra.
πΏ(t)= 1 at t = 0
= 0 elsewhere
Solution: Given signal x(t)=e
-t
Period T = 1 sec
Fundamental frequency Ο0 =2π
π =
2π
1 =2π
The exponential Fourier series is
Cn =1
π π₯(π‘)πβπππ0π‘π
0ππ‘ =
1
1 eβtπβππ 2ππ‘1
0ππ‘
= πβ(1+ππ 2π)π‘1
0ππ‘ = [
πβ(1+ππ 2π )π‘
β(1+ππ 2π) ]0
1
= πβ(1+ππ 2π )β1
β(1+ππ 2π) =
1βπβ1πβππ 2π
(1+ππ 2π) =
1βπβ1
(1+ππ 2π)
x(t)= πΆπβπ=ββ πππ π0π‘ = (
1βπβ1
1+ππ 2π)β
π=ββ πππ 2ππ‘
The spectra are shown in bellow figure.
Problem 9: Obtain the exponential Fourier Series for the wave form shown in below figure.
Also draw the frequency spectrum.
Solution: The periodic waveform shown in fig with a period T= 2Ο can be expressed as:
x(t) = A 0 β€ t β€ Ο
βA Ο β€ t β€ 2Ο
Let
t0 = 0, t0+T = 2Ο
and Fundamental frequency β΅0=2Ο
T =
2Ο
2Ο =1
Exponential Fourier series
C0 = 1
T x(t)
T
0 dt
= 1
2Ο AΟ
0 dt +
1
2Ο βA
2Ο
Ο dt = 0
Cn = 1
T x(t)eβjnβ΅0tT
0 dt
= 1
2Ο AeβjntΟ
0 dt +
1
2Ο βAeβjnt2Ο
Ο dt
= βA
j2nΟ (β1)n β 1 β 1 β (β1)n = βj
A
2nΟ
Cn = βj
2A
Οn for odd n
0 for even n
β΄ x(t) = C0+ Cnejnβ΅0tβn=ββ
nβ 0 = βj
A
2nΟejntβ
n=ββ
C0 = 0, C-1 = C1 = 2π΄
π, πΆβ3 = C3 =
2π΄
3π, πΆβ5 = C5=
2π΄
5π
The frequency spectrum is shown in the below figure.
Problem10: Find the exponential Fourier series and plot the frequency spectrum for the full
wave rectified sine wave given in below figure
Solution: The waveform shown in fig can be expressed over one period(0 to Ο) as:
x(t) = A sin β΅t where β΅= 2Ο
2Ο =1
because it is part of a sine wave with period = 2Ο
x(t) = A sin β΅t 0β€ t β€ Ο
The full wave rectified sine wave is periodic with period T = Ο
Let
t0 = 0, t0+T = 0+Ο = Ο
and Fundamental frequency β΅0=2Ο
T =
2Ο
Ο = 2
The exponential Fourier series is
x(t) = Cnejnβ΅0tβn=ββ = Cnej2ntβ
n=ββ
where Cn = 1
T x(t)eβjnβ΅0tT
0 dt
= 1
Ο A sin t eβj2ntΟ
0 dt =
A
Ο sin t eβj2ntΟ
0 dt
= A
j2Ο
ej 1β2n tβe0
j 1β2n β
eβj 1β2n tβe0
βj 1β2n
β΄Cn = 2A
Ο(1β4n2)
C0 = 1
T x(t)
T
0 dt
= 1
Ο AΟ
0 sin t dt =
A
Ο β cos π‘ 0
π = 2A
Ο
The exponential Fourier series is given by
x(t) = 2A
Ο(1β4n2)ej2ntβ
n=ββ = 2A
Ο +
2A
Ο β
n=ββnβ 0
ej2nt
1β4n2
C0 = 2π΄
π, C-1 = C1 =
2π΄
3π, πΆβ2 = C2 = β
2π΄
15π, πΆβ3 = C3 = β
2π΄
35π
The frequency spectrum is shown in the below figure.
Assignment Problems:
1. Find the trigonometric Fourier series for the periodic signal x(t) shown in figure below. Also
draw the frequency spectrum.
2. Find the trigonometric Fourier series for the periodic signal x(t) shown in figure below. Also
draw the frequency spectrum.
3. Compute the exponential Fourier series of the signal shown in figure below. Also draw the
frequency spectrum.
4. Find the exponential series of the signal shown in the figure below. Also draw the frequency
spectrum.
5. Find the trigonometric Fourier series of x(t) =t2 over the interval (-1, 1).
6. Find the Fourier series for the periodic signal x(t) =t2 for 0 β€ π‘ β€ 1, so that it repeats every 1
sec.
7. Determine the time signal corresponding to the magnitude and phase spectra shown in below
figure with β΅o = π.
Figure (a) Magnitude and (b) Phase spectra
8. Determine the Fourier series representation for x(t) = 2 sin(2ππ‘ β 3)+sin6ππ‘. 9. Find the complex exponential Fourier series coefficient of the signal x(t) = 3cos 4β΅0t. Also
draw the frequency spectrum.
10. Find the complex exponential Fourier series coefficient of the signal x(t) = 2cos 3β΅0t. Also
draw the frequency spectrum.
Simulation:
MATLAB can be used to find and plot the exponential Fourier series of a periodic function.
Using the symbolic math toolbox, we can perform the integrations necessary to find the
coefficients Fn.
Exponential Fourier Series
For the waveform, f(t), shown below:
we can write the exponential Fourier Series expansion as:
π π‘ = πΉππππ ππ π‘
β
π=ββ
Where
The Fn are complex coefficients. Thus, when we plot them we need to plot the magnitude
andphase separately. In Matlab we use the βabsβ function to find the magnitude and βangleβ to
findthe phase angle. The following Matlab statements show one method for obtaining these
plots:
Program
clc;clear all; close all; n=-10:10; Fn=(1./(j*n*2*pi)).*(4*exp(j*n*2*pi/7)+ 2*exp(-j*n*12*pi/7)-6*exp(-
j*n*6*pi/7)); Fn(find(n==0))=10/7; stem(n,abs(Fn)) xlabel('n-->'); ylabel('|Fn|-->'); title('Magnitude spectrum'); figure, stem(n,angle(Fn)) xlabel('n-->');
ylabel('L Fn -->'); title('Phase spectrum');
Since the value of Fn is different at n = 0, we assign that value using the find command.
Thestatement: find(n==0) will locate the index of the n vector that contains a zero. In this case
thatindex is 6, since the zero is found in the sixth position in n. That value is then used to index
Fn,so the value of Fn(6) is set to 10/7. This corresponds to the value for n = 0. These
commandsresult in the following magnitude and phase plots:
Note that the magnitude spectrum is even and the phase spectrum is odd as expected.
References:
[1] Alan V.Oppenheim, Alan S.Willsky and S.Hamind Nawab, βSignals & Systemsβ, Second
edition, Pearson Education, 8th
Indian Reprint, 2005.
[2] M.J.Roberts, βSignals and Systems, Analysis using Transform methods and MATLABβ,
Second edition,McGraw-Hill Education,2011
[3] John R Buck, Michael M Daniel and Andrew C.Singer, βComputer explorations in Signals
and Systems using MATLABβ,Prentice Hall Signal Processing Series
[4] P Ramakrishna rao, βSignals and Systemsβ, Tata McGraw-Hill, 2008
[5] Tarun Kumar Rawat, βSignals and Systemsβ, Oxford University Press,2011
[6] A.Anand Kumar, βSignals and Systemsβ , PHI Learning Private Limited ,2011