FFT
CH2
Set the source to CH2
Window
Hanning
Set to Hanning window
Hamming
Set to Hamming window
Blackman
Set to Blackman window
Rectangle
Set to Rectangle window
Vertical
Unit
Linear
To set vertical unit to linear or dBV
dBV
FFT Spectrum Analysis
FFT (Fast Fourier Transform algorithm) mode can convert time domain (YT) into Frequency domain for
signal waveform graph, which helps to easily observe signals of following kinds:
●
Harmonic component and distortion in the measurement system.
●
Characterize noises in DC source
●
Analyze the virbration
FFT Operation Tips
Signals with DC components or bias may cause error or deviation to FFT waveform components. In order to
reduce DC component, you can set coupling to AC mode. And you can set ACQUIRE type to AVG to lower
down random noise or mixed overlapped frequency component in repetitive or single pulse event.
Select FFT Window
Suppose YT waveforms repeats constantly and the Oscilloscope then performs FFT on limited length record.
When the period numers are integer, YT waveform at the start and finish points share same amplitude
values and will not cause any interruption to the waveform. If it is non-integral, the start and finish
amplitude are different, and high-frequency transient interruption will happen at joint points, which is
called leakage in frequency domain. To avoid the leakage, FFT mode will multiply original waveform with
one window function to force the start and finish values all go to 0. Refer to following windows for details:
Table 6-21
FFT
Window
Features
Best Applications
Hanning
Compared with rectangle type, it has
better amplitude accuracy and less
so for frequency resolution.
Used to measure sine, periodic and
narrowband random noises.
Hamming
Frequency resolution for Hamming is
a little better than that of Hanning.
Used to measure transient or short
pulse; and for signals whose signal
levels before and after the event are
greatly different.
Blackman
Offers best amplitude accuracy and
worst for the frequency resolution.
Mainly for measuring single
frequency signals and looking for
higher order harmonics
Rectangle
Offer best frequency resolution, but
worst for the amplitude accuracy. No
big change before and after adding
window function.
Applicable for measuring transient or
short pulse, the signal levels before
and after the event are approximately
equal. And also for measuring
equal-amplitude sinewave with very
close frequencies, broadband random
noise with relatively slow changing
spectrum.
Term Explanation
FFT Resolution:
defined as Quotient between sample rate and operation points. When operation points are
fixed, the lower sample rate is, the better FFT resolution will be.
NYQUIST Frequency:
For waveforms with highest frequency f, it at least needs to apply 2f sample rate in
order to recreate original waveform. It is called NYQUIST principle, f is NYQUIST frequency and 2f is
NYQUIST sample rate.
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UTD1000L User Manual
