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UTD4000 Four-channel User Manual
Table 2-4
FFT Window
Feature
Most Suitable Measurement Item
Rectangle
The best frequency recognition rate
the worst amplitude recognition rate
Basically similar to a status without
adding window
,
.
.
Temporary or fast pulse. Signal level is generally
the same before and after. Equal sine wave of very
similar frequency. There is broad-band random
noise with slow moving wave spectrum.
Hanning
Frequency recognition rate is better
than the rectangle window, but
amplitude recognition rate is poorer.
Sine, cyclical and narrow band random noise
-
.
Hamming
Frequency recognition rate is
marginally better than Hanning
window.
Temporary or fast pulse. Signal level varies greatly
before and after.
Blackman
The best amplitude recognition rate
and the poorest frequency
recognition rate.
Mainly for single-frequency signals to search for
higher-order harmonic wave.
Definition :
FFT recognition rate
Nyquist frequency :
.
,
,
means the quotient of the sampling and math points
When
math point value is
fixed
the sampling rate should be as low as possible relative to the FFT recognition rate.
To rebuild the original waveform, at least 2f sampling rate should be used for waveform
with a maximum frequency of f. This is known as Nyquist stability criterion
where f is the Nyquist
frequency and 2f is the Nyquist sampling rate.