Intel Embedded Intel486, Hardware Reference Manual

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EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
10-12
Figure 10-8. Strip Lines
The propagation delay is given by the following formula:
t
pd
= 1.017 ns/ft
For G-10 fiberglass epoxy boards (e
r
= 5.0), the propagation delay of the strip lines is 2.26 ns/ft.
Typical values of the characteristic impedance and propagation delay of these types of lines are
as follows:
Z
0
= 50 ohms
t
pd
= 2 ns/ft (or 6"/ns)
The three major effects of transmission line phenomenon are impedance mismatch, coupling and
skew. The following section discusses them briefly and provide solutions to minimize their ef-
fects. For more information on high-frequency design, refer to High-Speed Digital Design, A
Handbook of Black Magic by Howard W. Johnson and Martin Graham (Publisher: Prentice-Hall
Inc.).
10.3.2 Impedance Mismatch
As mentioned earlier, the impedance of a transmission line is a function of the geometry of the
line, its distance from the ground plane, and the loads along the line. Any discontinuity in the im-
pedance causes reflections.
Impedance mismatch occurs between the transmission line characteristic impedance and the in-
put or output impedances of the devices that are connected to the line. The result is that the signals
are reflected back and forth on the line. These reflections can attenuate or reinforce the signal de-
pending upon the phase relationships. The results of these reflections include overshoot, under-
shoot, ringing and other undesirable effects.
t
Ground
Planes
Strip Line
w
h
b Dielectric
Insulating
e
r