Digital Signal Processing Reference
In-Depth Information
appears as a positive reflection that exponentially returns to the baseline as its im-
pedance falls. These behaviors are evident in Figure 11.10(a, d).
The charging behavior of the series capacitor configuration is apparent in Fig-
ure 11.10(b), and the shunt inductor in Figure 11.10(c). Initially the capacitor is
a low impedance and its impedance increases as it is charged by the launched cur-
rent. The inductor is a high impedance, but as the inductor charges, its impedance
decreases, shunting increasingly more of the signal energy to ground.
11.7 Main Points
The energy transmitted down a transmission line consists of voltage and cur-
rent waves.
•
Conservation of energy requires that a reflection wave be created from the
incident wave when a change in impedance is encountered.
•
The reflection coefficient (
) indicates how much of the incident wave is re-
flected from a discontinuity.
ρ
•
An open circuit creates a positive voltage reflection and a negative current
reflection.
•
1.00V
1.00V
0.75V
0.75V
Series capacitor
Shunt capacitor
0.50V
0.50V
0.25V
0.25V
0.00V
0.00V
0.0s
1.0 ns
0.0s
1.0 ns
(a)
(b)
1.00V
1.00V
0.75V
0.75V
Shunt inductor
Series inductor
0.50V
0.50V
0.25V
0.25V
0.00V
0.00V
0.0s
1.0 ns
(c)
0.0s
1.0 ns
(d)
Figure 11.10
(a-d) Waveforms created by discontinuities as measured at
V
ne
for a step voltage.
