Selection Of Fft Window - UNI-T UTD2025C Operating Manual

Operating Manual for UTD2000/3000

Selection of FFT window

Mathematical storage oscillograph will carry out FFT conversion for time record with limited length when
assuming continuous repetition of YT waveform. YT waveform will have the same amplitude in beginning
and ending locations for no interrupation. However, high-frequency transient interruption will be caused in
connector due to different waveform amplitudes for non-integral cycle of YT waveform. Such effect shall be
called as leakage in frequency domain. Therefore, value in forced beginning and ending locations is 0 by
multiplying original waveform by a window function to avoid leakage. Please refer to following form for
application of window function.
Form 2-4
FFT window
Characteristics
The best frequency and the poorest
Rectangle
amplitude resolution to be basically
similar to circumstances without window;
With excellent frequency resolution and
poor amplitude resolution by comparing
Hanning
with rectangular window;
Frequency resolution of Hamming window
Hamming
is better than that of Hanning window slightly.
The best amplitude resolution and the
Blackman
poorest frequency resolution
The most appropriate measurement content
Transient or short impulse with basically equivalent
signal level; Sine with equivalent amplitude with
similar frequency; Wide band random noise of wave
spectrum with slow variation;
Sine, cycle and narrow band random noise
Transient or short impulse with large difference
for signal level;
Mainly used for single-frequency signal to search
for higher order of harmonic wave.
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