Digital Signal Processing Reference
In-Depth Information
6.5.1 What Voltage Gets Launched Down a Transmission Line?
Provided the line has low losses (which means that
R
and
G
in Figure 6.3 can be
ignored), the transmission line impedance appears as a simple resistance. The volt-
age and current waves at the input of an infinite length of line obey Ohm's law, and
will have a ratio equal to
Z
o
:
V
Z
=
i
(6.3)
o
I
i
where the launched voltage is called
V
i
, the incident voltage, and the launched cur-
rent is called
I
i
, the incident current.
Equation (6.3) cannot be used when the impedance changes along the line or
at its end, which gives rise to signal reflections. Signal reflections are waves that
travel along the line independently of the launched wave. These waves combine in
complex ways, but for lines with no reflections, (6.3) shows that the characteristic
impedance determines the relationship between the voltage and current along the
line.
Because the impedance of a lossless transmission line acts like a simple resis-
tance, the resistance voltage divider principle can be used to find the launched volt-
age if the power supply voltage (
V
cc
) is known. This is illustrated in Figure 6.5 and
in (6.4), where the impedance of the driver (such as an ASIC I/O driver) is
Z
g
, and
the transmission line characteristic impedance is
Z
o
.
Z
VV
ZZ
=
o
(6.4)
i
cc
+
o
g
This model shows how the power supply voltage, line impedance, and I/O
driver impedance interact:
For a given driver impedance and
V
cc
value, a higher voltage will be launched
if the transmission line characteristic impedance increases. This can be used
to improve the received noise margins by increasing the voltage at the re-
ceiver, but care must be taken to prevent excessive reflections and overshoots.
•
Z
o
×
V
= 1.65V
V
i
=
cc
ZZ
+
og
Z
50
Z
50
=Ω
=Ω
g
o
V
= 3.3V
cc
Figure 6.5
Voltage divider between the generator and transmission line impedances determines
the launched voltage (
V
i
). The line is long enough that the load is not a factor.
