551
CHAPTER
30
h '
&gt 2
2
% v t
Complex Numbers
Complex numbers are an extension of the ordinary numbers used in everyday math. They have
the unique property of representing and manipulating two variables as a single quantity. This fits
very naturally with Fourier analysis, where the frequency domain is composed of two signals, the
real and the imaginary parts. Complex numbers shorten the equations used in DSP, and enable
techniques that are difficult or impossible with real numbers alone. For instance, the Fast Fourier
Transform is based on complex numbers. Unfortunately, complex techniques are very
mathematical, and it requires a great deal of study and practice to use them effectively. Many
scientists and engineers regard complex techniques as the dividing line between DSP as a tool,
and DSP as a career. In this chapter, we look at the mathematics of complex numbers, and
elementary ways of using them in science and engineering. The following three chapters discuss
important techniques based on complex numbers: the complex Fourier transform, the Laplace
transform, and the z-transform. These complex transforms are the heart of theoretical DSP. Get
ready, here comes the math!
The Complex Number System
To illustrate complex numbers, consider a child throwing a ball into the air.
For example, assume that the ball is thrown straight up, with an initial
velocity of 9.8 meters per second. One second after it leaves the child's
hand, the ball has reached a height of 4.9 meters, and the acceleration of
gravity (9.8 meters per second2) has reduced its velocity to zero. The ball
then accelerates toward the ground, being caught by the child two seconds
after it was thrown. From basic physics equations, the height of the ball at
any instant of time is given by:
The Scientist and Engineer's Guide to Digital Signal Processing 552
t ' 1± 1&h/4.9
where h is the height above the ground (in meters), g is the acceleration of
gravity (9.8 meters per second2), v is the initial velocity (9.8 meters per
second), and t is the time (in seconds).
Now, suppose we want to know when the ball passes a certain height.
Plugging in the known values and solving for t:
For instance, the ball is at a height of 3 meters twice: (going up) t ' 0.38
and seconds (going down). t ' 1.62
As long as we ask reasonable questions, these equations give reasonable
answers. But what happens when we ask unreasonable questions? For
example: At what time does the ball reach a height of 10 meters? This
question has no answer in reality because the ball never reaches this height.
Nevertheless, plugging the value of into the above equation gives two h ' 10
answers: and . Both these answers contain t ' 1% &1.041 t ' 1& &1.041
the square-root of a negative number, something that does not exist in the world
as we know it. This unusual property of polynomial equations was first used
by the Italian mathematician Girolamo Cardano (1501-1576). Two centuries
later, the great German mathematician Carl Friedrich Gauss (1777-1855)
coined the term complex numbers, and paved the way for the modern
understanding of the field.
Every complex number is the sum of two components: a real part and an
imaginary part. The real part is a real number, one of the ordinary
numbers we all learned in childhood. The imaginary part is an imaginary
number, that is, the square-root of a negative number. To keep things
standardized, the imaginary part is usually reduced to an ordinary number
multiplied by the square-root of negative one. As an example, the complex
number: , is first reduced to: , and then to t ' 1% &1.041 t ' 1% 1.041 &1
the final form: . The real part of this complex number is 1, t ' 1% 1.02 &1
while the imaginary part is . This notation allows the abstract term, 1.02 &1
, to be given a special symbol. Mathematicians have long used i to denote &1
. In comparison, electrical engineers use the symbol, j, because i is used &1
to represent electrical current. Both symbols are common in DSP. In this book
the electrical engineering convention, j, will be used.
For example, all the following are valid complex numbers: , , 1% 2j 1& 2j
, , , etc. All ordinary numbers, such as: &1% 2 j 3.14159% 2.7183 j (4/3)% (19/2) j
2, 6.34, and -1.414, can be viewed as a complex number with zero for the
imaginary part, i.e., , , and . 2% 0 j 6.34% 0j &1.414% 0j
Just as real numbers are described as having positions along a number line,
complex numbers are represented by locations in a two-dimensional display
called the complex plane. As shown in Fig. 30-1, the horizontal axis of the
Chapter 30- Complex Numbers 553
Real axis
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
2 + 6 j
-4 - 1.5 j
3 - 7 j
8j
7j
6j
5j
4j
3j
2j
1j
0j
-1j
-2j
-3j
-4j
-5j
-6j
-7j
-8j
FIGURE 30-1
The complex plane. Every complex number
has a unique location in the complex plane,
as illustrated by the three examples shown
here. The horizontal axis represents the real
part, while the vertical axis represents the
imaginary part.
Imaginary axis
A ' 2 % 6j
B ' &4 & 1.5j
C ' 3 & 7j
Re A = 2 Im A = 6
Re B = -4 Im B = -1.5
Re C = 3 Im C = -7
complex plane is the real part of the complex number, while the vertical axis
is the imaginary part. Since real numbers are those complex numbers that have
an imaginary part equal to zero, the real number line is the same as the x-axis
of the complex plane.
In mathematical equations, a complex number is represented by a single
variable, even though it is composed of two parts. For example, the three
complex variables in Fig. 30-1 could be written:
where A, B, & C are complex variables. This illustrates a strong advantage
and a strong disadvantage of using complex numbers. The advantage is the
inherent shorthand of representing two things by a single symbol. The disadvantage
is having to remember which variables are complex and which
variables are ordinary numbers.
The mathematical notation for separating a complex number into its real and
imaginary parts uses the operators: and . For example, using the Re( ) Im( )
above complex numbers:
The Scientist and Engineer's Guide to Digital Signal Processing 554
(a%bj ) % (c%dj ) ' (a%c ) % j (b%d)
(a%bj ) & (c%dj ) ' (a&c ) % j (b&d)
(a%bj ) (c%dj ) ' (ac& bd) % j (bc% ad)
(a%bj )
(c%dj )
' ac% bd
c 2% d 2
% j
bc & ad
c 2% d 2
EQUATION 30-1
Addition of complex numbers.
EQUATION 30-2
Subtraction of complex numbers.
EQUATION 30-3
Multiplication of complex numbers.
EQUATION 30-4
Division of complex numbers.
AB ' BA EQUATION 30-5
Commutative property.
EQUATION 30-6
Associative property.
EQUATION 30-7
Distributive property.
(A% B)% C ' A% (B% C)
A(B%C) ' AB% AC
Notice that the value returned by the mathematical operator, , does not Im ( )
include the j. For example, is equal to 4, not . Im(3% 4j ) 4j
Complex numbers follow the same algebra as ordinary numbers, treating the
quantity, j, as a constant. For instance, addition, subtraction, multiplication and
division are given by:
Two tricks are used when manipulating equations such as these. First,
whenever a term is encountered, it is replaced by -1. This follows from the j 2
definition of j, that is: . The second trick is a way to j 2 ' ( &1 )2 ' &1
eliminate the j term from the denominator of a fraction. For instance, the left
side of Eq. 30-4 has a denominator of . This is handled by multiplying c % dj
the numerator and denominator by the term , cancelling all the c & jd
imaginary terms from the denominator. In the jargon of the field, switching
the sign of the imaginary part of a complex number is called taking the
complex conjugate. This is denoted by a star at the upper right corner of the
variable. For example, if , then . In other words, Eq. 30- Z' a % bj Zt ' a & bj
4 is derived by multiplying both the numerator and denominator by the complex
conjugate of the denominator.
The following properties hold even when the variables A, B, and C are
complex. These relations can be proven by breaking each variable into its real
and imaginary parts and working out the algebra.
Chapter 30- Complex Numbers 555
M ' (Re A)2 % (Im A)2
2 ' arctan Im A
Re A
Re A ' M cos (2)
Im A ' M sin (2)
EQUATION 30-8
Rectangular-to-polar conversion. The
complex variable, A, can be changed from
rectangular form: Re A & Im A, to polar
form: M & 2.
EQUATION 30-9
Polar-to-rectangular conversion. This is
changing the complex number from M &
2 to Re A & Im A.
Real axis
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
2 + 6 j or
M = % 40
2 = arctan (6/2)
3 - 7 j or
M = % 58
2 = arctan (-7/3)
-4 - 1.5 j or
M = % 18.25
2 = arctan (-1.5/-4)
8j
7j
6j
5j
4j
3j
2j
1j
0j
-1j
-2j
-3j
-4j
-5j
-6j
-7j
-8j
FIGURE 30-2
Complex numbers in polar form. Three
example points in the complex plane are
shown in polar coordinates. Figure 30-1
shows these same points in rectangular
form.
Imaginary axis
Polar Notation
Complex numbers can also be expressed in polar notation, besides the
rectangular notation just described. For example, Fig. 30-2 shows three
complex numbers in polar form, the same ones previously presented in Fig.
30-1. The magnitude is the length of the vector starting at the origin and
ending at the complex point, while the phase angle is measured between
this vector and the positive x-axis. Complex numbers can be converted
between rectangular and polar notation by the following equations (paying
attention to the polar notation nuisances discussed in Chapter 8):
This brings up a giant leap in the mathematics. (Yes, this means you should
pay extra attention). A complex number written in rectangular notation
The Scientist and Engineer's Guide to Digital Signal Processing 556
EQUATION 30-10
Rectangular and polar complex numbers.
The left side is the rectangular form of a
complex number, while the expression on
the right is the polar representation. The
conversion between: M & 2 and a & b, is
given by Eqs. 30-8 and 30-9.
a%jb ' M ( cos2 % j sin 2 )
EQUATION 30-11
Euler's relation. This is a key equation
for using complex numbers in science
and engineering.
e jx ' cos x % j sin x
e jx ' j4
n' 0
( j x)n
n!
' j4
k' 0
(&1)k x 2k
(2k)!
% j j4
k' 0
(&1)k x 2k%1
(2k%1)!
is in the form: . The information is carried in the variables: , but a % bj a & b
the proper complex number is the entire expression: . In polar form, the a % bj
key information is contained in M & 2, but what is the full expression for the
proper complex number?
The key to this is Eq. 30-9, the polar-to-rectangular conversion. If we start
with the proper complex number, , and apply Eq. 30-9, we obtain: a % bj
The expression on the left is the proper rectangular description of a complex
number, while the expression on the right is the proper polar description.
Before continuing with the next step, let's review how we arrived at this point.
First, we gave the rectangular form of a complex number a graphical
representation, that is, a location in a two-dimensional plane. Second, we
defined the terms M & 2 to be consistent with our previous experience about
the relationship between polar and rectangular coordinates (Eq. 30-8 and 30-9).
Third, we followed the mathematical consequences of these actions, arriving at
what the correct polar form of a complex number must be, i.e.,
. Even though this logic is straightforward, the result is M(cos2% j sin2)
difficult to see with "intuition." Unfortunately, it gets worse.
One of the most important equations in complex mathematics is Euler's
relation, named for the clever and very prolific Swiss mathematician,
Leonhard Euler (1707-1783; Euler is pronounced: "Oiler"):
If you like such things, this relation can be proven by expanding the
exponential term into a Taylor series:
The two bracketed terms on the right of this expression are the Taylor series
for and . Don't spend too much time on this proof; we aren't going cos(x) sin(x)
to use it for anything.
Chapter 30- Complex Numbers 557
EQUATION 30-12
Exponential form of complex numbers.
The rectangular form, on the left, is
equal to the exponential polar form, on
the right.
a%jb ' M e j 2
M1 e j21 M2 e j22 ' M1M2 e j (21
% 22 ) EQUATION 30-13
Multiplication of complex numbers.
EQUATION 30-14
Division of complex numbers.
M1 e j21
M2 e j22
'
M1
M2
e j( 21
&22 )
Rewriting Eq. 30-10 using Euler's relation results in the most common way of
expressing a complex number in polar notation, a complex exponential:
Complex numbers in this exponential form are the backbone of DSP
mathematics. Start your understanding by memorizing Eqs. 30-8 through 30-
12. A strong advantage of using this exponential polar form is that it is very
simple to multiply and divide complex numbers:
That is, complex numbers in polar form are multiplied by multiplying their
magnitudes and adding their angles. The easiest way to perform addition
and subtraction in polar form is to convert the numbers to rectangular form,
perform the operation, and reconvert back into polar. Complex numbers are
usually expressed in rectangular form in computer routines, but in polar
form when writing and manipulating equations. Just as and Re( ) Im( )
are used to extract the rectangular components from a complex number, the
operators and are used to extract the polar components. Mag( ) Phase( )
For example, if , then and . A ' 5e jB/7 Mag(A) ' 5 Phase(A) ' B/7
Using Complex Numbers by Substitution
Let's summarize where we are at. Solutions to common algebraic equations
often contain the square-root of a negative number. These are called
complex numbers, and represent solutions that cannot exist in the world as
we know it. Complex numbers are expressed in one of two forms: a % bj
(rectangular), or (polar), where j is a symbol representing . Using Me j 2 &1
either notation, a single complex number contains two separate pieces of
information, either a & b, or M & 2. In spite of their elusive nature, complex
numbers follow mathematical laws that are similar (or identical) to those
governing ordinary numbers.
This describes what complex numbers are and how they fit into the world of
pure mathematics. Our next task is to describe ways they are useful in science
The Scientist and Engineer's Guide to Digital Signal Processing 558
and engineering problems. How is it possible to use a mathematics that has no
connection with our everyday experience? The answer: If the tool we have is
a hammer, make the problem look like a nail. In other words, we change the
physical problem into a complex number form, manipulate the complex
numbers, and then change back into a physical answer.
There are two ways that physical problems can be represented using complex
numbers: a simple method of substitution, and a more elegant method we will
call mathematical equivalence. Mathematical equivalence will be discussed
in the next chapter on the complex Fourier transform. The remainder of this
chapter is devoted to substitution.
Substitution takes two real physical parameters and places one in the real part
of the complex number and one in the imaginary part. This allows the two
values to be manipulated as a single entity, i.e., a single complex number.
After the desired mathematical operations, the complex number is separated
into its real and imaginary parts, which again correspond to the physical
parameters we are concerned with.
A simple example will show how this works. As you recall from elementary
physics, vectors can represent such things as: force, velocity, acceleration, etc.
For example, imagine a sailboat being pushed in one direction by the wind, and
in another direction by the ocean current. The resulting force on the boat is the
vector sum of the two individual force vectors. This example is shown in Fig.
30-3, where two vectors, A and B, are added through the parallelogram law,
resulting in C.
We can represent this problem with complex numbers by placing the east/west
coordinate into the real part of a complex number, and the north/south
coordinate into the imaginary part. This allows us to treat each vector as a
single complex number, even though it is composed of two parts. For instance,
the force of the wind, vector A, might be in the direction of 2 parts to the east
and 6 parts to the north, represented as the complex number: . Likewise, 2 % 6j
the force of the ocean current, vector B, might be in the direction of 4 parts to
the east and 3 parts to the south, represented as the complex number: . 4 & 3j
These two vectors can be added via Eq. 30-1, resulting in the complex number
representing vector C: . Converting this back into a physical meaning, 6 % 3j
the combined force on the sailboat is in the direction of 6 parts to the north and
3 parts to the east.
Could this problem be solved without complex numbers? Of course! The
complex numbers merely provide a formalized way of keeping track of the two
components that form a single vector. The idea to remember is that some
physical problems can be converted into a complex form by simply adding a j
to one of the components. Converting back to the physical problem is nothing
more than dropping the j. This is the essence of the substitution method.
Here's the rub. How do we know that the rules and laws that apply to
complex mathematics are the same rules and laws that apply to the original
Chapter 30- Complex Numbers 559
Real axis
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
B
A+B=C
North
South
8j
7j
6j
5j
4j
3j
2j
1j
0j
-1j
-2j
-3j
-4j
-5j
-6j
-7j
-8j
C
A FIGURE 30-3
Adding vectors with complex numbers. The
vectors A & B represent forces measured
with respect to north/south and east/west.
The east/west dimension is replaced by the
real part of the complex number, while the
north/south dimension is replaced by the
imaginary part. This substitution allows
complex mathematics to be used with an
entirely real problem.
Imaginary axis
West
East
physical problem? For instance, we used Eq. 30-1 to add the force vectors in
the sailboat problem. How do we know that the addition of complex numbers
provides the same result as the addition of force vectors? In most cases, we
know that complex mathematics can be used for a particular application
because someone else said it does. Some brilliant and well respected
mathematician or engineer worked out the details and published the results.
The point to remember is that we cannot substitute just any problem into a
complex form and expect the answer to make sense. We must stick to
applications that have been shown to be applicable to complex analysis.
Let's look at an example where complex number substitution does not work.
Imagine that you buy apples for $5 a box, and oranges for $10 a box. You
represent this by the complex number: . During a particular week, you 5 % 10j
buy 6 boxes of apples and 2 boxes of oranges, which you represent by the
complex number: . The total price you must pay for the goods is equal 6 % 2j
to number of items multiplied by the price of each item, that is,
. In other words, the complex math indicates you (5 % 10j ) (6 % 2 j ) ' 10 % 70 j
must pay a total of $10 for the apples and $70 for the oranges. The problem
is, the answer is completely wrong! The rules of complex mathematics do not
follow the rules of this particular physical problem.
Complex Representation of Sinusoids
Complex numbers find a niche in electronics and signal processing because
they are a compact way to represent and manipulate the most useful of all
waveforms: sine and cosine waves. The conventional way to represent a
sinusoid is: or , in polar and rectangular M cos(Tt % N) Acos(Tt ) % Bsin (Tt )
The Scientist and Engineer's Guide to Digital Signal Processing 560
Acos (Tt) % Bsin (Tt) W a% jb
(conventional representation) (complex number)
M cos(Tt % N) W Me j 2
(conventional representation) (complex number)
notation, respectively. Notice that we are representing frequency by T, the
natural frequency in radians per second. If it makes you more comfortable,
you can replace each T with 2Bf to make the expressions in hertz. However,
most DSP mathematics is written using the shorter notation, and you should
become familiar with it. Since it requires two parameters to represent a single
sinusoid (i.e., A & B, or M & N), the use of complex numbers to represent
these important waveforms is a natural. Using substitution, the change from
the conventional sinusoid representation to a complex number is straightforward.
In rectangular form:
where , and . Put in words, the amplitude of the cosine wave AWa B W&b
becomes the real part of the complex number, while the negative of the sine
wave's amplitude becomes the imaginary part. It is important to understand
that this is not an equation, but merely a way of letting a complex number
represent a sinusoid. This substitution also can be applied in polar form:
where , and . In words, the polar notation substitution leaves the M WM 2W&N
magnitude the same, but changes the sign of the phase angle.
Why change the sign of the imaginary part & phase angle? This is to make the
substitution appear in the same form as the complex Fourier transform
described in the next chapter. The substitution techniques of this chapter gain
nothing from this sign change, but it is almost always done to keep things
consistent with the more advanced methods.
Using complex numbers to represent sine and cosine waves is a common
technique in electrical circuit analysis and DSP. This is because many (but not
all) of the rules and laws governing complex numbers are the same as those
governing sinusoids. In other words, we can represent the sine and cosine
waves with complex numbers, manipulate the numbers in various ways, and
have the resulting answer match the way the sinusoids behave.
However, we must be careful to use only those mathematical operations that
mimic the physical problem being represented (sinusoids in this case). For
example, suppose we use the complex variables, A and B, to represent two
sinusoids of the same frequency, but with different amplitudes and phase shifts.
When the two complex numbers are added, a third complex number is
produced. Likewise, a third sinusoid is created when the two sinusoids are
Chapter 30- Complex Numbers 561
added. As you would hope, the third complex number represents the third
sinusoid. The complex addition matches the physical system.
Now, imagine multiplying the complex numbers A and B, resulting in another
complex number. Does this match what happens when the two sinusoids are
multiplied? No! Multiplying two sinusoids does not produce another sinusoid.
Complex multiplication fails to match the physical system, and therefore cannot
be used.
Fortunately, the valid operations are clearly defined. Two conditions must be
satisfied. First, all of the sinusoids must be at the same frequency. For
example, if the complex numbers: and represent sinusoids at the 1%1j 2%2j
same frequency, then the sum of the two sinusoids is represented by the
complex number: . However, if and represent sinusoids with 3%3j 1%1j 2%2j
different frequencies, there is nothing that can be done with the complex
representation. In this case, the sum of the complex numbers, , is 3%3j
meaningless.
In spite of this, frequency can be left as a variable when using complex
numbers, but it must be the same frequency everywhere. For instance, it is
perfectly valid to add: and , to produce: . These 2T%3Tj 3T%1 j 5T% (3T%1) j
represent sinusoids where the amplitude and phase vary as frequency changes.
While we do not know what the particular frequency is, we do know that it is
the same everywhere, i.e., T.
The second requirement is that the operations being represented must be linear,
as discussed in Chapter 5. For instance, sinusoids can be combined by addition
and subtraction, but not by multiplication or division. Likewise, systems may
be amplifiers, attenuators, high or low-pass filters, etc., but not such actions as:
squaring, clipping and thresholding. Remember, even convolution and Fourier
analysis are only valid for linear systems.
Complex Representation of Systems
Figure 30-4 shows an example of using complex numbers to represent a
sinusoid passing through a linear system. We will use continuous signals
for this example, although discrete signals are handled the same way. Since
the input signal is a sinusoid, and the system is linear, the output will also
be a sinusoid, and at the same frequency as the input. As shown, the
example input signal has a conventional representation of: , 3cos(Tt % B/4)
or the equivalent expression: . When 2.1213 cos(Tt ) & 2.1213 sin(Tt )
represented by a complex number this becomes: or . 3e &jB/4 2.1213% j 2.1213
Likewise, the conventional representation of the output is: , 1.5cos(Tt & B/8)
or in the alternate form: . This is represented 1.3858 cos(Tt ) % 0.5740sin (Tt )
by the complex number: or . 1.5e j B/8 1.3858& j 0.5740
The system's characteristics can also be represented by a complex number. The
magnitude of the complex number is the ratio between the magnitudes
The Scientist and Engineer's Guide to Digital Signal Processing 562
Time
0 1 2 3 4 5
-4
-3
-2
-1
0
1
2
3
4
Time
0 1 2 3 4 5
-4
-3
-2
-1
0
1
2
3
4
Linear
System
Input signal Output signal
or or
or or or
1.5e jB/8
2.1213 % j 2.1213
3e &jB/4
3cos(Tt % B/4)
2.1213cos(Tt ) & 2.1213sin (Tt )
1.3858 & j 0.5740 0.1913 & j 0.4619
0.5e j3B/8
1.5cos(Tt & B/8)
1.3858cos(Tt ) % 0.5740sin (Tt )
FIGURE 30-4
Sinusoids represented by complex numbers. Complex numbers are popular in DSP and electronics because
they are a convenient way to represent and manipulate sinusoids. As shown in this example, sinusoidal input
and output signals can be represented as complex numbers, expressed in either polar or rectangular form. In
addition, the change that a linear system makes to a sinusoid can also be expressed as a complex number.
Conventional Complex
representation
Amplitude
Amplitude
of the input and output (i.e., ). Likewise, the angle of the complex Mout /Min
number is the negative of the difference between the input and output angles
(i.e., ). In the example used here, the system is described by the & [Nout
& Nin ]
complex number, . In other words, the amplitude of the sinusoid is 0.5 e j 3B/8
reduced by 0.5, while the phase angle is changed by . &3B/8
The complex number representing the system can be converted into rectangular
form as: , but we must be careful in interpreting what this 0.1913& j 0.4619
means. It does not mean that a sine wave passing through the system is
changed in amplitude by 0.1913, nor that a cosine wave is changed by -0.4619.
In general, a pure sine or cosine wave entering a linear system is converted into
a mixture of sine and cosine waves.
Fortunately, the complex math automatically keeps track of these cross-terms.
When a sinusoid passes through a linear system, the complex numbers
representing the input signal and the system are multiplied, producing the
complex number representing the output. If any two of the complex numbers are
known, the third can be found. The calculations can be carried out in either
polar or rectangular form, as shown in Fig. 30-4.
In previous chapters we described how the Fourier transform decomposes a
signal into cosine and sine waves. The amplitudes of the cosine waves are
called the real part, while the amplitudes of the sine waves are called the
imaginary part. We stressed that these amplitudes are ordinary numbers, and
Chapter 30- Complex Numbers 563
I ×Z ' V
the terms real and imaginary are just names used to keep the two separate.
Now that complex numbers have been introduced, it should be quite obvious
were the names come from. For example, imagine a 1024 point signal being
decomposed into 513 cosine waves and 513 sine waves. Using substitution, we
can represent the spectrum by 513 complex numbers. However, don't be misled
into thinking that this is the complex Fourier transform, the topic of Chapter
31. This is still the real Fourier transform; the spectrum has just been placed
in a complex format by using substitution.
Electrical Circuit Analysis
This method of substituting complex numbers for cosine & sine waves is called
the Phasor transform. It is the main tool used to analyze networks composed
of resistors, capacitors and inductors. [More formally, electrical engineers
define the phasor transform as multiplying by the complex term: and taking e jTt
the real part. This allows the procedure to be written as an equation, making
it easier to deal with in mathematical work. “Substitution” achieves the same
end result, but is less elegant].
The first step is to understand the relationship between the current and voltage
for each of these devices. For the resistor, this is expressed in Ohm's law:
, where i is the instantaneous current through the device, v is the v ' iR
instantaneous voltage across the device, and R is the resistance. In contrast,
the capacitor and inductor are governed by the differential equations:
, and , where C is the capacitance and L is the i ' C dv/dt v ' L di /dt
inductance. In the most general method of circuit analysis, these nasty
differential equations are combined as dictated by the circuit configuration, and
then solved for the parameters of interest. While this will answer everything
about the circuit, the math can become a real mess.
This can be greatly simplified by restricting the signals to be sinusoids. By
representing these sinusoids with complex numbers, the difficult differential
equations can be directly replaced with much simpler algebraic equations.
Figure 30-5 illustrates how this works. We treat each of these three
components (resistor, capacitor & inductor) as a system. The input to the
system is the sinusoidal current through the device, while the output is the
sinusoidal voltage across its two terminals. This means we can represent the
input and output of the system by the two complex variables: I (for current) and
V (for voltage), respectively. The relation between the input and output can
also be expressed by a complex number. This complex number is called the
impedance, and is given the symbol: Z. This means:
In words, the complex number representing the sinusoidal voltage is equal to
the complex number representing the sinusoidal current multiplied by the
impedance (another complex number). Given any two, the third can be
The Scientist and Engineer's Guide to Digital Signal Processing 564
Time
V
I
Time Time
V
V
I
I
Resistor Capacitor Inductor
V
I
V
I V
I
Amplitude
Amplitude
Amplitude
FIGURE 30-5
Definition of impedance. When sinusoidal voltages and currents are represented by complex numbers, the ratio
between the two is called the impedance, and is denoted by the complex variable, Z. Resistors, capacitors and
inductors have impedances of R, , and , respectively. &j/TC jTL
found. In polar form, the magnitude of the impedance is the ratio between the
amplitudes of V and I. Likewise, the phase of the impedance is the phase
difference between V and I.
This relation can be thought of as Ohm's law for sinusoids. Ohms's law
( ) describes how the resistance relates the instantaneous current and v ' iR
voltage in a resistor. When the signals are sinusoids represented by
complex numbers, the relation becomes: . That is, the impedance V ' IZ
relates the current and voltage. Resistance is an ordinary number, since it
deals with two ordinary numbers. Impedance is a complex number, since
it relates two complex numbers. Impedance contains more information than
resistance, because it dictates both the amplitudes and the phase angles.
From the differential equations that govern their operation, it can be shown that
the impedance of the resistor, capacitor, and inductor are: R, , and , &j /TC jTL
respectively. As an example, imagine that the current in each of these
components is a unity amplitude cosine wave, as shown in Fig. 30-5. Using
substitution, this is represented by the complex number: . The voltage 1%0 j
across the resistor will be: . In other words, a V ' I Z ' (1%0 j )R ' R%0j
cosine wave of amplitude R. The voltage across the capacitor is found to be:
. This reduces to: , a sine wave of V ' IZ ' (1%0j )(&j /TC ) 0&j /TC
amplitude, . Likewise, the voltage across the inductor can be calculated: 1/TC
. This reduces to: , a negative sine wave of V ' IZ ' (1%0j ) ( jTL ) 0%jTL
amplitude, . TL
The beauty of this method is that RLC circuits can be analyzed without having
to resort to differential equations. The impedance of the resistors, capacitors,
Chapter 30- Complex Numbers 565
Vin
Vout
Z1
Z2
Z3
FIGURE 30-6
RLC notch filter. This circuit removes a
narrow band of frequencies from a signal.
The use of complex substitution greatly
simplifies the analysis of this and similar
circuits.
Vout
Vin
' Z2% Z3
Z1% Z2% Z3
Vout
Vin
'
jTL & j
TC
R % jTL & j
TC
and inductors is treated the same as resistance in a DC circuit. This includes
all of the basic combinations, such as: resistors in series, resistors in parallel,
voltage dividers, etc.
As an example, Fig. 30-6 shows an RLC circuit called a notch filter, used to
remove a narrow band of frequencies. For instance, it could eliminate 60 hertz
interference in an audio or instrumentation signal. If this circuit were
composed of three resistors (instead of the resistor, capacitor and inductor), the
relationship between the input and output signals would be given by the voltage
divider formula: . Since the circuit contains vout / vin
' (R2%R3) / (R1%R2%R3)
capacitors and inductors, the equation is rewritten with impedances:
where: Vout, Vin, Z1, Z2, and Z3 are all complex variables. Plugging in the
impedance of each component:
Next, we crank through the algebra to separate everything containing a j,
from everything that does not contain a j. In other words, we separate the
equation into its real and imaginary parts. This algebra can be tiresome and
long, but the alternative is to write and solve differential equations, an
The Scientist and Engineer's Guide to Digital Signal Processing 566
Frequency (MHz)
0.0 0.5 1.0 1.5 2.0
-2
-1
0
1
2
b. Phase
Frequency (MHz)
0.0 0.5 1.0 1.5 2.0
0.0
0.5
1.0
1.5
a. Magnitude
Phase (radians)
Amplitude
FIGURE 30-7
Notch filter frequency response. These curves are for the component values: R = 50S, C
= 470DF, and L = 54 µH.
Vout
Vin
' k 2
R 2% k 2
% j
Rk
R 2% k 2
where: k ' TL& 1/TC
Mag ' TL &1/TC
R 2% [TL &1/TC ] 2
1/2
Phase ' arctan R
TL& 1/TC
even nastier task. When separated into the real and imaginary parts, the
complex representation of the notch filter becomes:
Lastly, the relation is converted to polar notation, and graphed in Fig. 30-7:
The key point to remember from these examples is how substitution allows
complex numbers to represent real world problems. In the next chapter we will
look at a more advanced way to use complex numbers in science and
engineering, mathematical equivalence.