IDEAL TRANSMISSION-LINE FUNDAMENTALS - Advanced Signal Integrity for High-Speed Digital Design - page 65

3.6.2 Measuring inductance and capacitance of reactive structures 137

3.6.3 Understanding the TDR profile 140

References 140

Problems 141

Transmission lines are simple electrical structures that consist of an insulating
layer of dielectric material sandwiched between two layers of metal (usually, a
nonferrous metal such as copper). In high-speed digital design, transmission lines
are used for communicating between electrical components, such as a micropro-
cessor and a memory module, or chip set. In modern designs such as computer
systems, the data transfer rates are so high that the width (in time) of the digital
pulses are small compared to the time it takes to propagate a signal from one end
of a transmission line to another. As a result, it is quite common to have more
than one bit of information propagating on the transmission line at any given
instant in time, where the transmission line is essentially “storing” information
before the receiving circuitry can latch the data. Consequently, to preserve the
quality of the digital pulse stream to a degree where it can be captured with
no errors at the receiving agent, great attention must be given to the construc-
tion and design of the transmission lines so that the electrical characteristics are
controlled and predictable. Designing a successfull bus for high-data-rate infor-
mation transfer requires a thorough understanding of how the signals propagate
on transmission lines. In this chapter we introduce basic transmission-line struc-
tures typically used in digital systems and present fundamental transmission-line
theory for the ideal case.

3.1 TRANSMISSION-LINE STRUCTURES

Transmission lines come in many shapes and sizes. One of the most common
transmission lines is the coaxial cable, which is used with cable television. When
designing a high-speed digital system, transmission lines are usually manufac-
tured on a printed circuit board (PCB) or a multichip module (MCM), which
typically consists of conductive traces buried in or attached to a dielectric with
one or more reference power and ground planes. The metal typically used is
copper (although the copper is often electroplated with silver or nickel to prevent
corrosion) and the dielectric is often FR4, a fiberglass-resin composite discussed
in detail in Chapter 6. The two most common types of transmission lines used
in digital designs are microstrips and striplines. A microstrip is routed on an
outside layer of the PCB and has only one reference plane. There are two types
of microstrips, buried and nonburied. A buried (sometimes called embedded )
microstrip is simply a transmission line that is embedded into the dielectric but