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Refer to Figure 5.2 for connection details. Note that the Mode! FG3B Sweep/Function Genera-
tor connects to both the amplifier input and
the
Channel
1 input of
the
oscilloscope.
The amplifier
output
is terminated with a Joad resistor (typically an
8
ohm, 100
watt
non-inductive unit
for audio
power
amplifiers) and the output drives the Channel
2 oscilloscope input.
Note that the power amplifier
input is terminated
with a SOQ resistor to prevent
high frequency
effects caused by the connecting
cable from affecting
the test results.
High frequency capacitive
loading
of the cable appears
as
ringing
and other
distortions in the
signal.
In operation, the Mode! FG3B Sweep/Function Generator
is set for
a 20Hz to 20kHz linear-type
sweep. The sine wave output
is selected, and
the output leve! is
set for
half the rated
amplifier's
power
output. This method
is safe for the
amplifier and gives
optimum
results
.
Or
if desired,
future sweep
testing may be done at one-fourth rated
power, or full
power.
Use caution when
testing at full power to
avoid overheating and damaging the
amplifier.
The oscilloscope is
set
up to properly display both
signals.
The Channel 1 and 2 Volts/Div
controls are set for
about 3
divisions
peak-to-peak
of the
input and
the
output signals. Use
the vertical
centering controls
to
position the
input signal
on the top half of the
display,
and the output
signal
on the
bottom half of the
display.
Then,
observe the variations
in
the
output signal
during the
sweep
cycle. The
input signal may be repositioned
over
the
output signal for a
better
comparison, if
desired.
If the oscilloscope hasan ADD mode and INVERT
switch, signal comparison
becomes
easier.
Only the difference between the
input and output
levels may be
displayed. Sim
ply press ADD and
INVERT,
then adjust the
vertical centering controls
to
center
the trace
on
the display.
18
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