2. Displaying the FFT Spectrum
Press the MATH button to display the Math Menu. Use the options to select the
Source channel, Window algorithm, and FFT Zoom factor. You can display only one
FFT spectrum at a time. You can select Full screen or Split in Display option to
display FFT waveform on full screen or display channel waveform and its FFT
waveform on half screen at a time.
FFT window
Set dBVrms to be the vertical Scale unit
Time base
Sampling rate
3. Select FFT window
Windows reduce spectral leakage in the FFT spectrum. The FFT assumes that
the YT waveform repeats forever. With an integral number of cycles, the YT
waveform starts and ends at the me amplitude and there are no discontinuities in
the signal shape A non-integral number of cycles in the YT waveform causes the
signal start and end points to be at different amplitudes. The transitions between
the start and end points cause discontinuities in the signal that introduce
high-frequency transients.
Table 2-10 FFT window instruction
Function
Setting
Introduction
Rectangular
Best frequency resolution,
worst magnitude resolution.
This is essentially the same
as no window.
Symmetric transients or bursts.
Equal-amplitude sine waves with
fixed frequencies. Broadband
random noise with a relatively
slowly varying spectrum.
Hanning
Hamming
Better frequency, poorer
magnitude accuracy than
Rectangular. Hamming has
slightly better frequency
resolution than Hanning.
Blackman
Best magnitude, worst
frequency resolution.
Single frequency waveforms, to
find higher order harmonics.
Sine, periodic, and narrow-band
random noise. Asymmetic
transients or bursts.
4. Magnifying and Positioning an FFT Spectrum
You can magnify and use cursors to take measurements on the FFT spectrum.
The oscilloscope includes an FFT Zoom option to magnify horizontally, press this
option button to select 1X, 2X, 5X or 10X. Moreover, you also can turn the
Universal knob to magnify FFT waveform horizontally in a 1-2-5 step. To magnify
vertically; you can turn the Volts/div knob.
20
Digital
S
torage
O
scilloscope
Digital Storage Oscilloscope
21
According to the tested options and source speciality, make sure the window you
need to use.
