10-9
PHYSICAL DESIGN AND SYSTEM DEBUGGING
Figure 10-6. Decoupling Leaded Capacitors
10.3 HIGH-FREQUENCY DESIGN CONSIDERATIONS
The overwhelming concern in dealing with high speed technologies is the management of trans-
mission lines. As the edge rates of the signal increase, the physical interconnections between de-
vices behave like transmission lines. Although transmission line theory is straightforward, the
difference between ordinary interconnection and transmission line is fairly complex. Transmis-
sion lines have distributed elements which are hard to define and designers tend to over-compen-
sate for the effects of these elements.
Efficient Intel486 CPU designs require the identification of the transmission lines over backplane
wiring, printed circuit board traces, etc. Once this task is accomplished, the designer’s next con-
cern should be to deal with three major problems which are associated with electromagnetic prop-
agation: impedance control, propagation delay, and coupling (electromagnetic interference).
The following sections discuss the negative effects of a transmission line that occur when oper-
ating at higher frequencies.
10.3.1 Transmission Line Effects
As a general rule, any interconnection is considered a transmission line when the time required
for the signal to travel the length of the interconnection is greater than one-eighth of the signal
rise time. The rise time can be either rise time or fall time, whichever is smaller, and it corre-
Intel486™
Processor
= 0.1
µF
= 1.0
µF
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