UTD2000 Series User Manual
22
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 2-4：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.
Hamming
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.
Blackman
The best amplitude resolution and the worst
frequency resolution.
Mainly used for single frequency signal seeking
for higher subharmonic.
Explanation of nouns
FFT resolution: defined as the quotient of sampling and operation point. When the
number of operation point are fixed, the lower the sampling rate is, the better the FFT
resolution will be.
Nyquist frequency: for waveform with maximum frequency of f, at least 2f sampling rate
is required to reconstruct original waveform. It is also called Nyquist rule, where f refers to
Nyquist frequency, while 2f refers to Nyquist sampling rate.
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