Chapter 5
Signal Generation Fundamentals
©
National Instruments Corporation
5-7
NI PXIe-5450 User Manual
Impedance Matching
When broadband signals are carried on transmission lines of any significant
length, care must be taken that the transmission line is matched to its
terminations. The source and load impedances should equal the
characteristic impedance of the transmission line, as this minimizes signal
reflections. The presence of impedance discontinuities or mismatches
degrade the amplitude and phase accuracy, as well as the temporal fidelity,
of waveforms generated with a signal generator.
One of the most common mismatch errors encountered in such
measurements is shown in the following figure.
In this example, selectable source impedances are provided at the signal
generator outputs to accommodate the most popular coaxial cable
characteristic impedances: 50
Ω
and 75
Ω
.
75
Ω
V
75
Ω
Ch
a
r
a
cteri
s
tic Imped
a
nce = 50
Ω
Del
a
y = t
S
o
u
rce
Co
a
xi
a
l
C
ab
le
Inp
u
t (Lo
a
d)
To DAC
