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Making Measurements Using Spectrum
Analyzer
Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
c.
Press
IBW],
VID BW AUTO MAN (MAN) and use the step keys to
decrease the video bandwidth until the amplitude of the measured
signal drops. Then step the bandwidth up until the signal amplitude
stops increasing, or until the maximum video bandwidth is reached.
Use the narrowest video bandwidth that does not cause a change in
the signal amplitude.
For the best amplitude accuracy, the video bandwidth should be about
10 times greater than the highest modulation frequency of interest.
For the 60 Hz fundamental, a 1
kHz
video bandwidth works well.
il.
Press
(vj
and
%
AM ON OFF so that ON is underlined. The
spectrum analyzer reads out the percent AM of the largest modulation
frequency. An arrow indicates the signal being measured. See Figure 5-1.
This measurement does not include all of the harmonics of the
modulating signal.
Note that when the FFT measurement is active, pressing the
ljMEAS/USER]
key will cycle between the
MEAS/USER
and FFT menus.
NOTE
The percent AM function will not run if the SIGNAL CLIPPED error message is being displayed.
Increase the reference level until the error message goes away
5-8