UTD2000 Series User Manual
22
Set to select Hanning, Hamming,
Blackman, or Rectangle window
function
Set vertical units as Vrms or
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
The most suitable measurement content
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.
Compared with rectangular window, it has
better frequency resolution and worse
amplitude resolution.
Random noise of sine, cycle and narrow-band.
The frequency resolution of Hamming window
is slightly superior to Hanning window.
Signal levels, before and after transient state or
short pulse, are of large difference.
The best amplitude resolution and the worst
frequency resolution.
Mainly used for single frequency signal seeking
for higher subharmonic.
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