Digital Signal Processing Reference
In-Depth Information
50
Ω
V
ne
V
fe
R
g
=10
Ω
2 ns
2V
200 ps
Figure 11.5
Circuit setup demonstrating refl ections when the generator and load resistances do
not match the transmission line impedance.
, 2-ns-long lossless
transmission line. The far end of the line is open-circuited, representing the input
of a high-impedance receiver. The driver launches a single pulse that has a width
of 200 ps, which is shorter than the electrical length of the line, and very fast rise
and fall times. A circuit simulation showing the behavior of this circuit appears in
Figure 11.6.
Figure 11.6(a) shows the load voltage,
V
fe
. The voltage's near end,
V
ne
, is
shown in Figure 11.6(b). Multiple reflections are present and it is apparent that the
voltage at the near and far ends alternate between a (+) going and (
Figure 11.5 shows a 2-V, 10
Ω
driver connected to a 60
Ω
) going wave.
The voltage at the load caused by the first reflection actually exceeds the driver's
2-V supply voltage.
Hand calculations can recreate the simulation results, but because there are
multiple reflections, keeping track of the various sums becomes cumbersome. A
−
4
3.34V
2
1.5V
0
-0.9V
−
2
−
2.
2
4
V
(a)
4
1.67V
2
0.55V
0.25V
0
−
0.37V
−
2
0.0s
2.0 ns
4.0 ns
6.0 ns
8.0 ns
10.0 ns 12.0 ns 14.0 ns
(b)
Figure 11.6
Simulation results for Figure 11.5. Voltages (a) at the load (
V
fe
) and (b) at the genera-
tor (
V
ne
).
