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UTD2000 Series User Manual
22
2.8 FFT spectral analysis
Enter FFT (fast fourier transform) operation, the time domain (YT) signal can be converted
into frequency domain signal. At FFT operation, it is convenient to observe signals of the
following types:
● Measure harmonic content and distortion in the system
● Display noise characteristic in DC power supply
● Analyze vibration
Table 2-4：FFT Menu Description
Function Menu Setting Description
Type FFT FFT mathematical operation
Source
CH1
You can select signal source as
CH1 or CH2
CH2
Window
Hanning
Set to select Hanning, Hamming,
Blackman, or Rectangle window
function
Hamming
Blackman
Rectangle
Vertical unit
Vrms
Set vertical units as Vrms or
dBVrms
dBVrms
Select FFT window
Assuming that YT waveform is continuously repeated, DSO shall conduct FFT conversion
for time record with finite length. In this case, when the cycle is an integer, YT waveforms
are of the same amplitude at the starting and ending positions, without interrupting.
However, if the cycle of YT waveform is not an integer, the waveform amplitudes shall be
different at starting and ending positions. Therefore, high-frequency transient interruption
shall occur at the joints. In frequency domain, this effect is called leakage. Therefore, in
order to avoid the generation of leakage, by multiplying original waveform by a window
function, the value at forced starting and ending positions is 0. See the table below for
window function application:
Table 2-5：FFT window function
FFT Window Features The most suitable measurement content
Rectangle
The best frequency resolution and the worst
amplitude resolution. It is basically similar to
the condition without window.
Signal levels, before and after transient state or
short pulse, are basically equal. Constant
amplitude sinusoid with very similar frequency
possesses bandwidth random noise with slow
spectrum change.
Hanning
Compared with rectangular window, it has
better frequency resolution and worse
amplitude resolution.
Random noise of sine, cycle and narrow-band.
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