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PHYSICAL DESIGN AND DEBUGGING
11.4.1.1.2 Strip Lines
A strip line
is
a strip conductor centered in a dielectric medium between two voltage
planes. The characteristic impedance
is
given theoretically
by
the equation below:
ZO
=
(601
ve;
In (5.98bhr (0.8 W + t)) ohms
where b = distance between the planes for controlled impedance
as
shown in
Figure
11-8.
The propagation. delay
.is
given
by
the following formula
tpd = 1.017 \l'Et.ns/ft
For G-I0 fiberglass epoxy boards (e
r
= 5.0), the propagation delay of the strip lines
is
2.27 ns/ft.
Typical values of the characteristic impedance and propagation delay of these types of
lines are
as
follows:
ZO
= 50 ohms
tpd
= 2 ns/ft (or
6"
Ins )
The three major effects of transmission line phenomenon are impedance mismatch, cou-
pling and skew.
The
follQwing
section will discuss them briefly and provide solutions to minimize their
effects.
Ground
Planes
Strip Line
~---------
+ Dielectric
Figure 11-8. Strip Lines
11-11
231732ill-8