Chapter 7
In all cases, the reflections happen quickly, during and just after an output
switches. But on a long line, the reflections are more likely to continue for
enough time to cause the receiver to misread logic levels. On short lines, the
reflections occur sooner and have no effect on received logic levels."
A pair of wires has several sources of impedance (Figure 7-4A). All of these
sources together determine a line’s characteristic impedance:- Series resistance varies with the wire’s diameter, length, and temperature.
- Series inductance varies with the wire’s diameter and distance from a ground
plane. - Parallel capacitance is a measure of the electric field between the wires.
- Parallel leakage resistance is a measure of the effectiveness of the wires’ insu-
lation. The leakage resistance is typically a very high value often expressed as
conductance (1/R).
One way to calculate characteristic impedance is to think of a pair of wires as a
series of identical short segments with each segment having the impedance
sources listed above (Figure 7-4B). To find the overall impedance of a long line,
find the impedance of a short segment and use this value to calculate the
impedance of an infinite series of identical segments strung together. For each
added segment, the existing line is in parallel with the new segment’s parallel
impedance, and this combined impedance is in series with the new segment’s
series impedance.
As you increase the line’s length, each segment added has less and less effect on
the total impedance, which approaches a fixed value. This value is the imped-
Figure 7-4: (A) A pair of wires has several impedance sources. (B) One way to find the
characteristic impedance of a line is to think of the line as a series of short segments.