High-Speed Board Design Techniques

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INTRODUCTION
The most important factor in the design of many systems today is speed. 66-MHz thru 200-MHz
processors are common; 233 and 266-MHz processors are becoming readily available. The demand
for high speed results from: a) the requirement that systems perform complex tasks in a time frame
considered comfortable by humans; and b) the ability of component manufacturers to produce
high-speed devices. An example of a) is the large amount of information that must be processed
to perform even the most rudimentary computer animation. Currently, Programmable Array Logic
(PAL
®
) devices are available with propagation delays of 4.5 ns, and complex PLDs such as MACH
®
have propagation delays of 5 ns. While this might seem fast, it is not the propagation delay that
creates the potential for problems, but rather the fast edge rates needed to obtain the fast
propagation delays. In the future, much faster devices will become available, with correspondingly
faster edge rates.
Designing high-speed systems requires not only fast components, but also intelligent and careful
design. The analog aspect of the devices is as important as the digital. In high-speed systems, noise
generation is a prime concern. The high frequencies can radiate and cause interference. The
corresponding fast edge rates can result in ringing, reflections, and crosstalk. If unchecked, this
noise can seriously degrade system performance.
This application note presents an overview of the design of high-speed systems using a PC-board
layout. It covers:
u
the power distribution system and its effect on boardinghouse generation,
u
transmission lines and their associated design rules,
u
crosstalk and its elimination, and
u
electromagnetic interference.
1. POWER DISTRIBUTION
The most important consideration in high-speed board design is the power distribution network.
For a noise-free board, it is necessary to have a noise-free power distribution network. Note that
it is just as important to develop a clean V
CC
as it is to get a clean ground. For AC purposes, which
is what this application note mainly discusses, V
CC
is
ground.
The power distribution network also must provide a return path for all signals generated or received
on the board. This is often overlooked because the effect of the return path is less apparent at lower
frequencies. Many designs work even when the nature of the return path is ignored.
1.1 Power Distribution Network as a Power Source
1.1.1 The Effect of Impedance
Consider a 5" x 5" board with digital ICs and a power supply of +5.0 V. The goal is to deliver exactly
+5.0 V to the power pins of every device on the board, regardless of its position relative to the
power source. Furthermore, the voltage at the pins should be free of line noise.
【文件名】:061021@52RD_HighSpeedBoardDesign.pdf
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