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Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
4. Use the knob to place the marker on the AM modulated signal and press
MARKER--+ AUTO FFT again. The spectrum analyzer will perform the
following steps:
a. Save the present instrument state in state register 8.
b. Reduce the span to zoom in on the signal.
c. Set the detector mode to sample.
d. Set the scale to linear.
e. Change the span to zero span.
f. Start the continuous FFT function.
g. Set the FFT stop frequency to 10
kHz.
5. Press FFT STOP FREQ , 250 (Hz). This sets the spectrum analyzer to
include the fourth harmonic of the 60 Hz modulation signal on the screen.
6. To conErm that the resolution bandwidth and video bandwidth are correct
for measuring the modulation amplitude, use the following procedure:
a. Press
INIKR)
and use the knob to move the marker to the desired
modulation signal. In this example, place the marker on the 60 Hz
fundamental modulation signal.
b. Press
lew)
and decrease the resolution bandwidth using the
(JJ
key,
until measured signal amplitude drops. Then press
lj’jj
to increase the
bandwidth until the signal amplitude stops increasing and stays the
same, or until the maximum resolution bandwidth is reached. Use
the narrowest bandwidth that does not cause a change in the signal
amplitude.
NOTE
As the resolution bandwidth is stepped down, the modulated signal must be recentered on the
spectrum analyzer display If this is not done, the signal amplitude can decrease due to off tuning of
the spectrum analyzer and not because of the resolution bandwidth chosen,
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