Prof. Nizamettin AYDIN
http://www.yildiz.edu.tr/~naydin
Advanced Digital Signal Processing
1
Introduction to the Fourier Introduction to the Fourier TransformTransform
2
• Review– Frequency Response– Fourier Series
• Definition of Fourier transform
Relation to Fourier Series
• Examples of Fourier transform pairs
dtetxjX tj )()(
Everything = Sum of Sinusoids
• One Square Pulse = Sum of Sinusoids– ???????????
• Finite Length
• Not Periodic
• Limit of Square Wave as Period infinity– Intuitive Argument
Fourier Series: Periodic x(t)
x(t) x(t T0 )
T0 2T0 T0 2T00 t
x(t) akej 0k t
k
ak 1
T0
x(t)e j0ktdt T0/ 2
T0 / 2
Fundamental Freq.
0 2 / T0 2f0
Fourier Synthesis
Fourier Analysis
Square Wave Signal
ak e j0kt
j0kT0 T0 /4
T0 / 4
e jk / 2 e jk / 2
j2k
sin(k / 2)
k
x(t) x(t T0 )
T0 2T0 T0 2T00 t
4/
4/0
0
0
0)1(1
T
T
tkjk dte
Ta
Spectrum from Fourier Series
,4,20
,3,1,00)2/sin(
k
k
k
kak
What if x(t) is not periodic?
• Sum of Sinusoids?– Non-harmonically related sinusoids – Would not be periodic, but would probably be
non-zero for all t.
• Fourier transform– gives a “sum” (actually an integralintegral) that involves
ALLALL frequencies– can represent signals that are identically zero for
negative t. !!!!!!!!!
Limiting Behavior of FS
T0=2T
T0=4T
T0=8T
Limiting Behavior of Spectrum
T0=2T
T0=4T
T0=8T
)(Plot
0 kaT
FS in the LIMIT (long period)
Fourier Synthesis
Fourier Analysis
dejXtxeaTtx tj
Ttkj
kkT )()()( 2
1202
10
0
0
dtetxjXdtetxaT tjT
T
tkjTk
)()()(2/
2/
0
0
0
0
0
d
TT
0
2lim0
k
TT 0
2lim0
)(lim 00
jXaT kT
Fourier Transform Defined
• For non-periodic signals
Fourier Synthesis
Fourier Analysis
dtetxjX tj )()(
dejXtx tj)()( 21
Example 1: x(t) e atu(t)
X( j) 1
a j
X( j ) e at
0
e j tdt 0
e (a j )tdt
X( j )e ate j t
a j0
1
a j
a 0
Frequency Response
• Fourier Transform of h(t) is the Frequency Response
jjHtueth t
1
1)()()(
)()( tueth t
Magnitude and Phase Plots
jajH
1)(
ajH
1tan)(
22
11
aja
)()( jHjH
X( j) sin(T / 2)
/ 2
Example 2:x(t)
1 t T / 2
0 t T / 2
X( j ) e j t
j T / 2
T /2
e jT / 2 e jT /2
j
X( j ) (1)e jtdt T / 2
T /2
e jtdt T / 2
T /2
x(t)
1 t T / 2
0 t T / 2
X( j ) sin(T / 2)
/ 2
Example 3:
b
bjX
0
1)(
t
ttx b
)sin(
)(
b
b
dedejXtx tjtj
12
1)(
2
1)(
jt
ee
jt
etx
tjtjtj bbb
b
2
1
2
1)(
b
bb jX
t
ttx
0
1)(
)sin()(
Example 4:
X( j ) (t)e jtdt
1
Shifting Property of the Impulse
)()( 0tttx
0)()( 0tjtj edtettjX
x(t) (t) X( j) 1
Example 5: X( j ) 2 ( 0 )
x(t) 1
22 ( 0 )e jtd
e j0t
x(t) 1 X( j ) 2 ( )
x(t) e j0 t X( j) 2 ( 0 )
x(t) cos(0t)
X( j) ( 0 ) ( 0 )
x(t) cos(0t)
X( j) ( 0 ) ( 0 )
Table of Fourier Transforms
x(t) e atu(t) X( j ) 1
a j
x(t) 1 t T / 2
0 t T / 2
X( j ) sin(T / 2)
/ 2
x(t) sin(0t)
t X( j )
1 0
0 0
x(t) (t t0 ) X( j ) e jt0
x(t) e j0 t X( j ) 2 ( 0 )
Fourier TransformFourier Transform
PropertiesProperties
25
• The Fourier transform
• More examples of Fourier transform pairs• Basic properties of Fourier transforms
– Convolution property
– Multiplication property
dtetxjX tj )()(
Fourier Transform
Fourier Analysis(Forward Transform)
dtetxjX tj )()(
Fourier Synthesis(Inverse Transform)
dejXtx tj)(2
1)(
)()(
Domain-FrequencyDomain-Time
jXtx
WHY use the Fourier transform?
• Manipulate the “Frequency Spectrum”
• Analog Communication Systems– AM: Amplitude Modulation; FM
– What are the “Building Blocks” ?• Abstract Layer, not implementation
• Ideal Filters: mostly BPFs
• Frequency Shifters– aka Modulators, Mixers or Multipliers: x(t)p(t)
Frequency Response
• Fourier Transform of h(t) is the Frequency Response
jjHtueth t
1
1)()()(
)()( tueth t
2/
)2/sin()(
2/0
2/1)(
T
jXTt
Tttx
b
bb jX
t
ttx
0
1)(
)sin()(
0)()()( 0tjejXtttx
00 t
Table of Fourier Transforms
)(2)()( ctj jXetx c
0)()()( 0tjejXtttx
b
bb jX
t
ttx
0
1)(
)sin()(
2/
)2/sin()(
2/0
2/1)(
T
jXTt
Tttx
jjXtuetx t
1
1)()()(
)()()()cos()( ccc jXttx
Fourier Transform of a General Periodic Signal
• If x(t) is periodic with period T0 ,
0
00
00
)(1
)(T
tjkk
k
tjkk dtetx
Taeatx
)(2 since Therefore, 00 ke tjk
k
k kajX )(2)( 0
Square Wave Signal
x(t) x(t T0 )
T0 2T0 T0 2T00 t
ak e j0kt
j0kT0 0
T0 / 2
e j 0kt
j0kT0 T0 /2
T0
1 e jk
jk
ak 1
T0
(1)e j0 ktdt 1
T0
( 1)e j 0ktdtT0 / 2
T0
0
T0 / 2
Square Wave Fourier Transform
X( j ) 2 ak( k0 )k
x(t) x(t T0 )
T0 2T0 T0 2T00 t
Table of Easy FT Properties
ax1(t) bx2 (t) aX1( j ) bX2 ( j )
x(t td ) e jtd X( j )
x(t)e j0t X( j( 0 ))
Delay Property
Frequency Shifting
Linearity Property
x(at) 1|a | X( j(a ))
Scaling
Scaling Property
expands)(shrinks;)2( 221 jXtx
)(
)()(
1
)/(
aa
adajtj
jX
exdteatx
)()( 1aa
jXatx
Scaling Property
)()( 1aa
jXatx
)2()( 12 txtx
Uncertainty Principle
• Try to make x(t) shorter– Then X(j) will get wider– Narrow pulses have wide bandwidth
• Try to make X(j) narrower– Then x(t) will have longer duration
• Cannot simultaneously reduce time Cannot simultaneously reduce time duration and bandwidthduration and bandwidth
Significant FT Properties
x(t)h(t) H( j )X( j )
x(t)e j0t X( j( 0 ))
x(t)p(t) 1
2X( j )P( j )
dx(t)
dt ( j)X( j)
Differentiation Property
Convolution Property
• Convolution in the time-domain
corresponds to MULTIPLICATIONMULTIPLICATION in the frequency-
domain
y(t) h(t) x(t) h( )
x(t )d
Y( j ) H( j )X( j )
y(t) h(t) x(t)x(t)
Y( j ) H( j )X( j )X( j )
Convolution Example
• Bandlimited Input Signal– “sinc” function
• Ideal LPF (Lowpass Filter)– h(t) is a “sinc”
• Output is Bandlimited– Convolve “sincs”
Ideally Bandlimited Signal
1000
1001)(
)100sin()( jX
t
ttx
100b
Convolution Example
sin(100 t)
t
sin(200t)
t
x(t)h(t) H( j )X( j )
sin(100 t)
t
Cosine Input to LTI System
Y ( j) H( j )X( j)
H( j )[( 0 )( 0)]
H( j0 ) ( 0 ) H( j0 ) ( 0 )
y(t) H (j0 ) 12 e j0t H( j0 ) 1
2 e j 0t
H( j0 ) 12 e j0t H *( j 0)
12 e j0t
H( j0 ) cos( 0t H( j0 ))
Ideal Lowpass Filter
Hlp( j )
co co
y(t) x(t) if 0 co
y(t) 0 if 0 co
Ideal Lowpass Filter
y(t) 4
sin 50t 4
3sin 150t
fco "cutoff freq."
H( j ) 1 co
0 co
Signal Multiplier (Modulator)
• Multiplication in the time-domain corresponds to convolution in the frequency-domain.
Y( j ) 1
2X( j )P( j )
y(t) p(t)x(t)
X( j)
x(t)
p(t)
Y( j ) 1
2X( j )
P( j( ))d
Frequency Shifting Property
x(t)e j0t X( j( 0 ))
y(t) sin 7t
te j 0 t Y ( j )
1 0 7 07
0 elsewhere
))((
)()(
0
)( 00
jX
dtetxdtetxe tjtjtj
y(t) x(t)cos(0t)
Y( j ) 12
X( j( 0 )) 12
X( j( 0 ))
x(t)
Differentiation Property
dx(t)
dt
d
dt1
2X( j )e j td
1
2( j )X( j )e j td
d
dte atu(t) ae atu(t) e at (t)
(t) ae atu(t)
ja j
Multiply by j