User Manual
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FFT is used to compute the fast Fourier transform using analog input channels or reference waveform. FFT takes the digitized
time record of the specified source and transforms it to the frequency domain. When the FFT function is selected, the FFT
spectrum is plotted on the oscilloscope display as magnitude in dBV versus frequency. The readout for the horizontal axis
changes from time to frequency (Hertz) and the vertical readout changes from volts to dB. The mathematical calculation
function can convert the 2048 center points of the time domain waveform into an FFT spectrum. The final FFT spectrum
contains 1024 points from DC (0 Hz) to the Nyquist frequency. Normally, the display screen compresses the FFT spectrum
horizontally to 250 points, but you can use "FFT zoom" to expand the FFT spectrum to more clearly see the frequency
components of each of the 1024 data points in the FFT spectrum. FFT operation can facilitate the following works:
Measure harmonic components and distortion in the system
Measure the characteristics of the noise in DC power
Analyze vibration
To display FFT waveform:
1. Press the [Math] button on the front panel to open the MATH function menu.
2. Press the Operation softkey and then turn the Multifunctional Knob to select FFT. The resulting math waveform is
displayed on the screen and labeled with “M”.
3. Press the Source softkey, and then turn the Multifunctional Knob to select the source to do FFT operation. Analog
channels (CH1~CH2) can be used as the source.
4. Press Center softkey and then turn the Multifunctional Knob to adjust the frequency of the frequency domain waveform
corresponding to the horizontal center of the screen.
5. Press Span softkey and then turn the Multifunctional Knob to adjust the the horizontal scale of the frequency domain
waveform.
6. Press the Vertical Units softkey to select the unit of vertical axis. The units of the vertical axis can be dB or Vrms which
use a logarithmic scale or a linear scale to display vertical amplitude respectively.
7. Press the Scale softkey to select the vertical scale.
8. Press the Window softkey, and then turn the Multifunctional Knob to select an appropriate window.
Spectral leakage can be considerably decreased when a window function is used. The scope provides six kinds of FFT
window functions which have different characteristics and are applicable to measure different waveform. You need to select
the window function according to different waveform and their characteristics. Please read the table below carefully to make
an appropriate option according to the input signal.
Pulse or Transient Waveform
Special-purpose window applicable to discontinuous waveform.
This is actually the same as no windows.
Better frequency, poorer amplitude accuracy than Flattop
A litter bit better frequency resolution than Hanning.
Single frequency signal, search
for higher order harmonics.
The best amplitude resolution; the poorest frequency resolution
Narrow band signal with
stronger
Better frequency resolution.
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