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Source leveling
When amplifiers operate in their nonlinear region,
the measured response may differ from the true
response of the amplifier. If the amplifier is operating
in its nonlinear region, variations in the stimulus
signal might not be duplicated at the output of the
amplifier. In contrast, the response of an amplifier
tested in its linear range is not affected by this
level variation. By ratioing the response to the
stimulus, the network analyzer removes the effects
of the stimulus variation and displays the true
performance of the amplifier.
However, if the amplifier is operating in its nonlinear
range, at or near saturation, the amplifier does not
produce an output signal with a variation propor-
tional to the variation of the signal present at the
input. The ratioing process in the network analyzer
then creates an erroneous display (Figure 2).
To optimize measurements in the nonlinear region, a
leveling scheme must be used to produce a stimulus
signal with a frequency response that is as flat as
possible.
Figure 2. When a device operates linearly, its true
response can be measured. However, a device that oper-
ates nonlinearly is affected by nonlevel input signals.
Source leveling using power-meter calibration
Using power-meter calibration in continuous sampling
mode is one way to level the source (Figure 3). The
power level at each frequency point in the sweep is
measured with a power meter. The network analyzer,
connected to the power meter via GPIB, adjusts its
source power until the power meter measures the
desired power level. Then the measurement is
made. Since the accuracy of the power meter is
very high (uncertainty in the tenths of a dB range),
you can have confidence that the power level is
accurate as well as flat.
8753E with Option 011
RF out
R
A
Power
splitter
Figure 3. Power-meter calibration levels the source
signal and removes the nonlinear ripple.
Power meter
GPIB
B
DUT
Power sensor
15
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