Digital Signal Processing Reference
In-Depth Information
CHAPTER 6
Transmission Line Fundamentals
6.1 Introduction
In this chapter we use a circuit analysis approach to introduce fundamental trans-
mission line concepts. This approach is more intuitive than field theory, but [1-3]
should be helpful for those interested in exploring transmission lines in that way.
We begin by defining a transmission line and its return path and developing a
transmission line circuit model. We will restrict our study here to transmission lines
having low losses, because this lets us develop an intuitive understanding without
the mathematics necessary when losses cannot be ignored. Lossy transmission lines
are discussed in Chapter 8. We discuss return paths in some detail because return
path management is an important aspect of signal integrity analysis. We conclude
the chapter by developing rules of thumb that define when a conductor acts as a
simple lumped circuit or when the circuit elements are distributed.
6.2
What Is a Transmission Line?
As illustrated in Figure 6.1, a transmission line is formed with two or more con-
ductors. The signal conductor carries the signal energy from a generator to a load,
and the second conductor (the return) completes the circuit by returning the signal
current back to the generator. The signal conductor could be a microstrip trace and
the return path the underlying ground plane. The generator is any signal launching
device, such as an ASIC I/O driver.
6.2.1 What Is the Signal Return Path?
Frequently, signal integrity engineers speak of the “signal return path,” but often
it is taken for granted and not fully understood. Studying the return path is impor-
tant because it determines the trace's capacitance, inductance, resistance, and noise
coupling.
Because capacitance is often more intuitive than inductance, we will ignore
inductance for now and focus on the importance of the return path in determin-
ing the capacitance. Resistance is also affected by the return path, but we will wait
until Chapters 7 and 8 to discuss this important topic.
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