AN-108 Datasheet, PDF (1/8 Pages) National Semiconductor (TI) – Transmission Line Characteristics

Transmission Line
Characteristics
National Semiconductor
Application Note 108
Bill Fowler
July 1986
INTRODUCTION
Digital systems generally require the transmission of digital
signals to and from other elements of the system. The com-
ponent wavelengths of the digital signals will usually be
shorter than the electrical length of the cable used to connect
the subsystems together and, therefore, the cables should
be treated as a transmissions line. In addition, the digital sig-
nal is usually exposed to hostile electrical noise sources
which will require more noise immunity than required in the
individual subsystems environment.
The requirements for transmission line techniques and noise
immunity are recognized by the designers of subsystems
and systems, but the solutions used vary considerably. Two
widely used example methods of the solution are shown in
Figure 1. The two methods illustrated use unbalanced and
balanced circuit techniques. This application note will delin-
eate the characteristics of digital signals in transmission
lines and characteristics of the line that effect the quality, and
will compare the unbalanced and balanced circuits perfor-
mance in digital systems.
nated at one end of the cable is near to a driver at the same
end, as shown in Figure 3. Some noise may be induced from
relay circuits which have very large transient voltage swings
compared to the digital signals in the same cable. Another
source of induced noise is current in the common ground
wire or wires in the cable.
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FIGURE 2. External Noise Sources
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NOISE
The cables used to transmit digital signals external to a sub-
system and in route between the subsystem, are exposed to
external electromagnetic noise caused by switching tran-
sients from actuating devices of neighboring control sys-
tems. Also external to a specific subsystem, another sub-
system may have a ground problem which will induce noise
on the system, as indicated in Figure 2.
The signals in adjacent wires inside a cable may induce
electromagnetic noise on other wires in the cable. The in-
duced electromagnetic noise is worse when a line termi-
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FIGURE 3. Internal Noise Sources
DISTORTION
The objective is the transmission and recovery of digital in-
telligence between subsystems, and to this end, the charac-
teristics of the data recovered must resemble the data trans-
mitted. In Figure 4 there is a difference in the pulse width of
the data and the timing signal transmitted, and the corre-
sponding signal received. In addition there is a further differ-
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