Filter
Top Products
I
-
Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
9. Press
C-1,
FFT Menu , and CONTINUS FFT The spectrum
analyzer will now be taking
FF’l3
continuously, updating the
measurement at the end of every sweep.
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.
10. To conErm that the resolution bandwidth and video bandwidth are
correct for measuring the modulation amplitude, use the following
procedure:
a. Press
(MKR)
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
Isw]
and decrease the resolution bandwidth using the
@J
key,
until measured signal amplitude drops. Then press
@)
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 in the filter
bandwidth. This is a zero span display To center the signal, select [FREQUENCY) and adjust the
center frequency to maximize the amplitude of the trace. 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.
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.)
5-7