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Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
For the best amplitude accuracy, the resolution bandwidth should
be about 10 times greater than the highest modulation frequency of
interest.
For the 60 Hz fundamental, a 1
kHz
resolution bandwidth works well.
(If harmonics are not a concern, a 100
kHz
resolution bandwidth can be
used and it will provide a faster update rate.)
c.
Press
(Bw),
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
7.
Press
(MEAS/USER)
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-l.
This measurement does not include all of the harmonics of the modulating
signal.
NOTE
l 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
l To return to the spectrum analyzer state prior to running the
FFT
function, press the FFT OFF
softkey
This turns off the
FFT
function. Press (RECALL], INTERNAL
+STATE
, and
8
to
recall the state from state register
8.
l When the
FFT
measurement is active, pressing the
c-1
key will cycle between the
MEASNSER
and
FFT
menus.
5-11