Select Fft Window; Fft Window Function; Fft Operation Skills - Tenma 72-2650 Operating Manual

72-2650/72-10510/72-8225A /72-8705A/72-8710A user manual

FFT operation skills

Signal with DC component or deflection will lead to
error or deviation of FFT waveform component. AC
coupling mode can be selected to reduce DC
component. In order to reduce random noise of
repeated or single pulse events and aliasing frequency
components, the acquiring mode of DSO is set to be
average acquiring.

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 2-4 FFT window function
FFT
Features
Window
The best frequency
resolution and the
worst amplitude
Rectangle
resolution. It is
basically similar to
the condition
without window.
Compared with rectangular
window, it has better
Hanning
frequency resolution and
worse amplitude resolution.
The most suitable
measurement content
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.
Random noise of sine,
cycle and narrow-band.
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