Chapter 5
Signal Generation Fundamentals
NI PXIe-5450 User Manual
5-8
ni.com
The following figure shows what happens when, as in this example, a
coaxial cable of the wrong characteristic impedance (50
Ω
) is used with
75
Ω
source and load impedances.
The pulse encounters impedance mismatches at each end of the cable,
causing the pulse to be partially reflected. The reflected pulse traverses the
cable back and forth numerous times, diminishing at each end by the
reflection coefficient,
Γ.
.
where
v
r
= reflected voltage
v
i
= incident voltage
z
t
= terminating impedance
z
0
= characteristic impedance
The resulting voltage waveform is distorted by the asymptotic decay of the
reflected pulse as shown, exaggerated for visual effect. Impedance
discontinuities of smaller magnitude and/or duration have correspondingly
smaller effects. Also displayed is the waveform that results when a cable of
matched impedance (75
Ω
) is used.
Time
0
t
3t
5t
7t
9t
V
S
ign
a
l
Gener
a
tor
Volt
a
ge
With M
a
tched C
ab
le
0
