Digital Signal Processing Reference
In-Depth Information
By examining Figure 11.4(a), we see that when the pulse (
V
p
) is narrow enough
to return to zero before reflected pulse arrives, the total near-end voltage (
V
t
) is the
sum of the incident and reflected pulses.
However, as shown in Figure 11.4(b), if the pulse is wide enough to still be
present when the reflection arrives, the incident and reflected pulses add during the
period when the pulses overlap, combining to change the shape of the waveform.
Although in Figure 11.4 the incident and reflected waves are positive going, they
can be negative going.
11.5
What Is the Behavior When There Are Multiple Refl ections?
Commonly in digital systems an impedance mismatch is present at both ends of the
transmission line. This causes reflections to rebound from both ends, which can
significantly alter the wave shape and signal amplitude at both ends of the line. A
demonstration of this setup appears in Figure 11.5.
V
p
(
i
V + V
)
r
V
t
2td
V
p
(
i
V + V
)
r
V
t
Voltage
Time
Figure 11.4
The total voltage (
V
t
) (a) when the launched (
V
p
) pulse is narrow and (b) when it is
wide enough to combine with the refl ection.
