Digital Signal Processing Reference
In-Depth Information
6”
(1.05 ns)
6”
(1.05 ns)
R
S
3
.3V
TX
TT1
T2
65
Ω
Z =65
Ω
Z =65
Ω
V
ne
C
L
2
C
L
1
3.5pF
3.5 pF
Figure 12.13
Capacitive loads
C
L
1
and
C
L
2
placed along and at the end of a series terminated trans-
mission line. Signal rise time determines
T
1's apparent impedance.
3.5V
3.0V
2.5V
1.65V
2.0V
1.5V
1.57V
1.0V
0.5V
0.0V
0s
2 ns
4 ns
6 ns
8 ns
10 ns
Figure 12.14
Voltage measured at
V
ne
when the signal has a very sharp rise time (dotted curve) and
when it is longer (solid curve). The two voltages show that the impedance is not the same.
driver voltage this confirms the transmission line impedance is 65
Ω
. The negative
going pulse caused by
C
L
is clearly visible.
The solid curve shows the response when the signal rise time is extended to 2.5
ns. The plateau is still present, but it has been shifted down to 1.57V. As we first
saw in Chapter 6, the voltage divider principle can be used to determine that the
transmission line impedance has fallen from 65
. The negative-going pulse
caused by load capacitor
C
L
1
that is so evident with the sharp-edged pulse is no
longer visible.
Lengthening the signal rise time causes these changes because now the line's
distributed capacitors and inductors become indistinct and blend together, appear-
ing as a single inductor and capacitor. Capacitor
C
L
adds to the transmission line
capacitance, increasing it, but it does not change the transmission inductance.
The transmission line inductance (
L
) and capacitance (
C
) affects the transmis-
sion line impedance (
Z
o
) as is shown in (12.5) (which was first presented in Chap-
ter 6).
Ω
to 59
Ω
L
(12.5)
Z
=
o
C
