Intel Embedded Intel486 Hardware Reference Manual page 295

PHYSICAL DESIGN AND SYSTEM DEBUGGING
where e is the relative dielectric constant of the board material and h, w, and t are the dimensions
r
of the strip. Knowing the line width, the thickness of Cu and the height of dielectric, the charac-
teristic impedance can be easily calculated.
The propagation delay (t ) associated with the trace is a function of the dielectric only. This is
pd
calculated as follows:
( )
t = 1.017 0.475e + 0.67 ns/ft
pd
r
For G-10 fiberglass epoxy boards (e = 5.0), the propagation delay of micro-strip is calculated to
r
be 1.77 ns/ft.
w
Micro-strip
t
h
Dielectric
Ground
Plane
Figure 10-7. Micro-Strip Lines
10.3.1.3 Strip Lines
A strip line is a flat conductor centered in a dielectric medium between two voltage planes. The
characteristic impedance is given theoretically by the equation below:
Z = [60/ e ] ln (5.98b/π (0.8w + t)) ohms,
0
r
where b = distance between the planes for controlled impedance as shown in Figure 10-8
10-11