gazine, Jan, 2000, p. 18


What is Characteristic Impedance?

Eric Bogatin
Bogatin Enterprises
913-393-1305
eric@bogent.com

www.bogatinenterprises.com




Introduction

Perhaps the most important and most common electrical issue that keeps
coming up in high speed design is controlled impedance boards and the
characteristic impedance of the traces. Yet, it is also one of the most un-intuitive
and confusing topics for non electrical engineers. And, in my experience teaching
over 2000 engineers, its confusing to many electrical engineers as well. In this
brief note, we will walk through a simple and intuitive explanation of characteristic
impedance, the most fundamental quality of a transmission line.


Whats a transmission line
Lets start with whats a transmission line. All it takes to make a transmission line
is two conductors that have length. One conductor acts as the signal path and
the other is the return path. (Forget the word ground and always think return
path). In a multilayer board, EVERY trace is part of a transmission line, using the
adjacent reference plane as the second trace or return path. What makes a trace
a good transmission line is that its characteristic impedance is constant
everywhere down its length. What makes a board a controlled impedance
board is that the characteristic impedance of all the traces meets a target spec
value, typically between 25 Ohms and 70 Ohms. But whats this quality,
characteristic impedance?




Page - 2



BOGATIN ENTERPRISES
26235 W. 110
th
Terr. Olathe, KS 66061
(v) 913-393-1305; (f) 913-393-1306; (e
)
eric@bogent.com



www.bogatinenterprise.com


Be the Signal
The easiest way to think about characteristic impedance is to look at what a
signal sees when it travels down a line. First, lets consider a transmission line
that just happens to look like a microstrip and has the same cross section as we
move down the line. An example of a microstrip is shown in Figure 1.



Figure 1. Cross section of a typical microstrip transmission line.

Suppose we launch a step voltage wave of 1 volt into the line. For example, we
connect to the front end of the line, between the signal path and the return path,
a 1 volt battery. The instant we make the connection, this voltage wave signal
begins to move down the line at the speed of light in the dielectric medium,
typically about 6 inches/nsec. (Why it moves so fast and is not the speed of the
electrons, which is closer to 1 cm/sec, might be the topic of a future article). The
signal, of course, is really the voltage difference between the signal line and the
return path, measured between any point along the signal line and the adjacent
point on the return path. The voltage edge moving down the line is illustrated in
Figure 2.


Signal trace
Return path trace
dielectric


Page - 3



BOGATIN ENTERPRISES
26235 W. 110
th
Terr. Olathe, KS 66061
(v) 913-393-1305; (f) 913-393-1306; (e
)
eric@bogent.com



www.bogatinenterprise.com



Figure 2. Signal moving down transmission line, frozen in time. Courtesy of ICE

In a Zen sort of way, be the signal, and travel down this transmission line at 6
inches/nsec. What do you see? In the first 10 psec, you have walked about
0.06 inches down the line. Lets freeze time and look at the line. Behind you, you
have left a wake of a constant voltage of 1 volt between the signal line and an
adjacent point on the return path. This means there is some excess + charge
on the trace behind you and some excess charge on the return path behind
you. Its this excess charge difference that sustains the 1 volt signal between
these two conductors that make up a capacitor.

Ahead of you, the line doesnt know you are coming and the voltage between the
signal line and the return path is still zero. In another 10 psec, with your next
step, you will transform the voltage in the 0.06 inch section under your foot from
0 volts to 1 volt. To do this, you must add some + charge to the signal line and
add some charge to the return path line. For each 0.06 inch step you take,
you are adding more + charge to the signal line and more - charge to the
return path. Every 10 psec, another section of the transmission line gets charged
up and the signal propagates down the line. But where does this charge come
from?

Answer: it comes from the source- the battery we attached to the front of the
transmission line to launch the initial signal. As we, the signal, march down the
transmission line, we are charging up successive sections of the line as we go,
leaving behind a 1 volt different between the signal and return path lines. Every
10psec, we take a step and draw another small charge, Q, from the battery.
This constant amount of charge, Q, flowing out of the battery, in constant time
interval, t, is a constant current. There is + current flowing into the signal line
and at the same time, - current flowing into the return path.


Page - 4



BOGATIN ENTERPRISES
26235 W. 110
th
Terr. Olathe, KS 66061
(v) 913-393-1305; (f) 913-393-1306; (e
)
eric@bogent.com



www.bogatinenterprise.com



The - current flowing into the return path is really the same as + current
flowing out of the return path. And, right at the wave front of the signal, the (AC)
current flows through the capacitance between the top line to the bottom line,
closing the loop. This is illustrated in Figure 3.


Figure 3. Current paths in a transmission line as the signal propagates. Note that the
current returns to the source where ever the signal voltage is changing
.



Impedance of the Line
From the batterys perspective, it sees that as soon as you connected its leads to
the front of the transmission line, there is a constant draw of current, for its
constant applied voltage. It you were to ask it, what sort of circuit element
behaves this way- has a constant current draw for constant applied voltage, of
course, it would say a resistor.

To the battery, while the signal is moving down the transmission line, (emphasis
added) charging up successive 0.06 inch sections every 10 psec, drawing a
constant current from the source, the transmission line looks like a resistor and
has a constant value of resistance, which we call the surge impedance of the
line.

Likewise, for you as the signal, walking down the line, with each step you take,
you are constantly probing the electrical environment of the line and asking what
is the impedance of my next step? As you put your foot down on the line, you
are electrically asking, what is the current required to charge up this footstep to
1v, in the 10 psec I have until my next step? This is the instantaneous impedance
you are probing for.

As we saw with the battery, if you move down the line at a steady pace, and the
line has the same cross section, then taking each step requires the same amount
of charge in 10 psec, for the same signal voltage. With every step you take, you
signal


Page - 5



BOGATIN ENTERPRISES
26235 W. 110
th
Terr. Olathe, KS 66061
(v) 913-393-1305; (f) 913-393-1306; (e
)
eric@bogent.com



www.bogatinenterprise.com


will draw the same current from your foot (and the battery), at the same voltage,
and you too will see the same instantaneous impedance as you walk down the
line.

If the line has the same signal velocity down its length and it has the same
capacitance per length down its length then with every step you, the signal, take
you will see the same instantaneous impedance as you move down the line.
Because this impedance is constant along the line, we give it the special name
that says it is characteristic for this particular transmission line. We call it the
characteristic impedance of the line. Characteristic impedance is the
instantaneous impedance a signal sees as it moves down the line. If, as the
signal moves down the line, it sees the same characteristic impedance with every
step, we can label the line as a controlled impedance line.


Whats so Important about the Characteristic Impedance of a
Transmission Line?
The instantaneous impedance, or characteristic impedance, is an important
quality for a signal. If the impedance of the next step is the same as the last step,
life will be wonderful and you will continue on as you were. If the next step is
different than the last and the impedance changes, you will reflect a little energy
back and what continues gets a little distorted.

For optimal signal quality, the goal in interconnect design is to keep the
impedance the signal sees as constant as possible. This means primarily, keep
the characteristic impedance of the line constant. Hence the growing importance
in manufacturing controlled impedance boards. All the other tricks like minimize
stub lengths, terminate the ends, daisy chain rather than branch, are all designed
to keep the instantaneous impedance the signal sees constant.


Calculating Characteristic Impedance
From this simple model we can derive a value for the characteristic impedance,
which is the instantaneous impedance the signal sees as it walks down the line.
The impedance seen in each step, Z, is just the basic definition of impedance:

Z = V/I

The voltage is the voltage of the signal launched