lecture # ch2-3 complex exponentials & complex numbers
TRANSCRIPT
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DSP-First, 2/e
LECTURE # CH2-3
Complex Exponentials
& Complex Numbers
Aug 2016 1© 2003-2016, JH McClellan & RW Schafer
TLH MODIFIED
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Aug 2016 © 2003-2016, JH McClellan & RW Schafer 2
READING ASSIGNMENTS
▪ This Lecture:
▪ Chapter 2, Sects. 2-3 to 2-5
▪ Appendix A: Complex Numbers
▪ Complex Exponentials
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2020 © 2003-2016, JH McClellan & RW Schafer 3
Dr. Van Veen Strikes Again
3. Complex Numbers Review (Wouldn’t hurt to review) 10:22Review of how to work with complex numbers in rectangular and polar coordinates.
https://www.youtube.com/watch?v=UAn9uah7puU&list=PLGI7M8vwfrFNO-gQ1xoJmN3bJy2-wp2J3
4. Complex Sinusoid Representations for Real Sinusoids 13:21
https://www.youtube.com/watch?v=Tm3gI6PQOYo&feature=youtu.be
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LECTURE OBJECTIVES
▪ Introduce more tools for manipulating
complex numbers Euler Eq.
▪ Conjugate
▪ Multiplication & Division
▪ Powers
▪ N-th Roots of unity
Aug 2016 4© 2003-2016, JH McClellan & RW Schafer
re j = rcos() + jrsin()
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LECTURE OBJECTIVES
▪ Phasors = Complex Amplitude
▪ Complex Numbers represent Sinusoids
▪ Take Real or Complex part
}){()cos( tjj eAetA =+
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Phasors Not Phasers
Aug 2016 © 2003-2016, JH McClellan & RW Schafer 7
Captain Kirk &
Sally Kellerman STAR TREK
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WHY? What do we gain?
▪ Sinusoids are the basis of DSP,
▪ but trig identities are very tedious
▪ Abstraction of complex numbers
▪ Represent cosine functions
▪ Can replace most trigonometry with algebra
▪ Avoid (Most) all Trigonometric
manipulations
Aug 2016 8© 2003-2016, JH McClellan & RW Schafer
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COMPLEX NUMBERS
▪ To solve: z2 = -1
▪ z = j
▪ Math and Physics use z = i
▪ Complex number: z = x + j y
x
y z
Cartesian
coordinate
system
Aug 2016 9© 2003-2016, JH McClellan & RW Schafer
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PLOT COMPLEX NUMBERS
}{zx =
}{zy =
Real part:
Imaginary part:}05{5 j+−=−
5}52{ =+ j
2}52{ =+ j
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COMPLEX ADDITION = VECTOR Addition
26
)53()24(
)52()34(213
j
j
jj
zzz
+=
+−++=
++−=
+=
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*** POLAR FORM ***
▪ Vector Form
▪ Length =1
▪ Angle =
▪ Common Values▪ j has angle of 0.5p
▪ −1 has angle of p
▪ − j has angle of 1.5p
▪ also, angle of −j could be −0.5p = 1.5p −2p
▪ because the PHASE is AMBIGUOUS
Aug 2016 12© 2003-2016, JH McClellan & RW Schafer
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POLAR <--> RECTANGULAR
▪ Relate (x,y) to (r,) r
x
y
Need a notation for POLAR FORM
( )x
y
yxr
1
222
Tan−=
+=
sin
cos
ry
rx
=
=Most calculators do
Polar-Rectangular
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Euler’s FORMULA
▪ Complex Exponential
▪ Real part is cosine
▪ Imaginary part is sine
▪ Magnitude is one
re j = rcos() + jrsin()
)sin()cos( je j +=
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Cosine = Real Part
▪ Complex Exponential
▪ Real part is cosine
▪ Imaginary part is sine
)sin()cos( jrrre j +=
)cos(}{ rre j =
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Common Values of exp(j)
▪ Changing the angle
p njj eej 200110 ==+=→=
4/21 pjej =
ppp )2/12(2/2/ +==→= njj eej
ppp )12(011 +==+−=−→= njj eej
pppp )2/12(2/2/32/3 −− ===−→= njjj eeej
Aug 2016 16© 2003-2016, JH McClellan & RW Schafer
𝑎2 + 𝑏2 = 𝑐2 θ = arctan(𝑏
𝑎)
IF 𝑎 ≥ 0
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COMPLEX EXPONENTIAL
▪ Interpret this as a Rotating Vector
▪ = t
▪ Angle changes vs. time
▪ ex: =20p rad/s
▪ Rotates 0.2p in 0.01 secs
)sin()cos( tjte tj +=
)sin()cos( je j +=
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Cos = REAL PART
}{
}{)cos( )(
tjj
tj
eAe
AetA
=
=+ +So,
Real Part of Euler’s
}{)cos( tjet =
General Sinusoid
)cos()( += tAtx
Aug 2016 18© 2003-2016, JH McClellan & RW Schafer
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COMPLEX AMPLITUDE
}{)cos()( tjj eAetAtx =+=
General Sinusoid
)}({}{)( tzXetx tj ==
Sinusoid = REAL PART of complex exp: z(t)=(Aejf)ejt
Complex AMPLITUDE = X, which is a constant
tjj XetzAeX == )(when
Aug 2016 19© 2003-2016, JH McClellan & RW Schafer
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POP QUIZ: Complex Amp
▪ Find the COMPLEX AMPLITUDE for:
▪ Use EULER’s FORMULA:
p5.03 jeX =
)5.077cos(3)( pp += ttx
}3{
}3{)(
775.0
)5.077(
tjj
tj
ee
etx
pp
pp
=
= +
Aug 2016 20© 2003-2016, JH McClellan & RW Schafer
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POP QUIZ-2: Complex Amp
▪ Determine the 60-Hz sinusoid whose
COMPLEX AMPLITUDE is:
▪ Convert X to POLAR:
33 jX +=
)3/120cos(12)( pp += ttx
}12{
})33{()(
1203/
)120(
tjj
tj
ee
ejtx
pp
p
=
+=
Aug 2016 21© 2003-2016, JH McClellan & RW Schafer
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Note atan (3/√3) –is atan(√3) = π/3
Remember 60 degree angle
Cos = 1/2 Sin = √3/2
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COMPLEX CONJUGATE (z*)
▪ Useful concept: change the sign of all j’s
▪ RECTANGULAR: If z = x + j y, then the
complex conjugate is z* = x – j y
▪ POLAR: Magnitude is the same but angle
has sign change
z = re j z* = re− j
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COMPLEX CONJUGATION
▪ Flips vector
about the real
axis!
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USES OF CONJUGATION
▪ Conjugates useful for many calculations
▪ Real part:
▪ Imaginary part:
}{2
)()(
2
*zx
jyxjyxzz==
−++=
+
}{2
2
2
*zy
j
yj
j
zz===
−
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Inverse Euler Relations
▪ Cosine is real part of exp, sine is imaginary part
▪ Real part:
▪ Imaginary part:
)cos(2
}{,
}{2
*
=+
==
=+
− jjjj ee
eez
zzz
)sin(2
}{,
}{2
*
=−
==
==−
−
j
eeeez
zyj
zz
jjjj
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Mag & Magnitude Squared
▪ Magnitude Squared (polar form):
▪ Magnitude Squared (Cartesian form):
▪ Magnitude of complex exponential is one:
22 ||))((* zrrerezz jj === −
|e j |2= cos2 ( )+ sin2 ( )=1
z z* = (x + jy) (x − jy) = x2 − j2y2 = x2 + y2
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COMPLEX MULTIPLY =
VECTOR ROTATION
▪ Multiplication/division scales and rotates
vectors
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POWERS
▪ Raising to a power N rotates vector by Nθ
and scales vector length by rN
zN = re j( )N
= rNe jN
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MORE POWERS
▪
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ROOTS OF UNITY
▪ We often have to solve zN=1
▪ How many solutions?
kjNjNN eerz p 21===
N
kkNr
pp
22,1 ===
1,2,1,0,2 −== Nkez Nkj p
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ROOTS OF UNITY for N=6
▪ Solutions to zN=1
are N equally
spaced vectors on
the unit circle!
▪ What happens if
we take the sum
of all of them?
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Sum the Roots of Unity
▪ Looks like the answer is zero (for N=6)
▪ Write as geometric sum
e j2p k /N
k= 0
N−1
= 0?
rk
k= 0
N−1
=1− rN
1− rthen let r = e j2p /N
01)(11Numerator 2/2 =−=−=− pp jNNjN eer
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▪ Needed later to describe periodic signals
in terms of sinusoids (Fourier Series)▪ Especially over one period
j
ee
j
ede
jajbb
a
b
a
jj −
==
0110/2
0
/2 =−
=−
= jj
eedte
jTTjT
Ttjp
p
Integrate Complex Exp
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BOTTOM LINE
▪ CARTESIAN: Addition/subtraction is most
efficient in Cartesian form
▪ POLAR: good for multiplication/division
▪ STEPS:
▪ Identify arithmetic operation
▪ Convert to easy form
▪ Calculate
▪ Convert back to original form
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Review Appendix A and
TLH Ch2 on Course Website
Aug 2016 © 2003-2016, JH McClellan & RW Schafer 44
Harman Chapter 2 Pages 55-60 Complex Numbers
and
MATLAB Complex Numbers