Digital Signal Processing Reference
In-Depth Information
3V
2V
1V
0V
(a)
3V
2V
1V
0V
(b)
3V
2V
1V
0V
0s
5 ns
10 ns
15 ns
20 ns
25 ns
(c)
Figure 12.17
(a) Load capacitor
C
L
1
voltage when
R
s
= 25
,
R
term
not present and
T
1 -
T
5 = 50
.
Ω
Ω
(b)
R
s
removed,
R
term
= 50
,
V
term
= 1.65V. (c)
R
term
= 50
,
V
term
= 1.65V,
T
1 = 25
.
Ω
Ω
Ω
the parallel combination of transmission lines at a branch is not as significant as it
would be if the loads were not terminated. This means that because diode clamps
clip the reflections and do not prevent them from being created, they usually are
not a useful substitute in this situation for one of the other parallel terminations.
12.9 Main Points
Transmission lines may be terminated at the source (source series termina-
tion) or at the load (parallel termination).
•
Source series termination raises the impedance of the driver and can prevent
reflected energy from rereflecting from the generator.
•
Parallel termination impedance matches a high-impedance load to a lower-
impedance transmission line. There are several variants, and circuit simula-
tion should be used to determine which is best suited in a given application.
•
Capacitive loads connected to a transmission line will either appear as a
sharp discontinuity (if the signal rise time is very fast) or will combine with
the transmission line capacitance to lower it.
•
Connections made to a transmission line are either stubs (when short) or
branches (when they are long). Their effects are very different.
•
