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4.3.1 Math FFT
This chapter elaborates on the Math FFT (Fast Fourier Transform) functionality. The Math FFT mode
may be used to convert a normal time-domain (YT) signal to its frequency components (spectrum), and
to observe the following:
Analyze harmonics in power cords;
Measure harmonic content and distortion in systems;
Characterize noise in DC power supplies;
Test impulse response of filters and systems;
Analyze vibration.
To use the Math FFT mode, perform the following tasks:
Set the source (time-domain) waveform;
Display the FFT spectrum;
Choose a type of FFT window;
Adjust the sample rate to display the fundamental frequency and harmonics without aliasing;
Use zoom controls to magnify the spectrum;
Use cursors to measure the spectrum.
4.3.1.1 Setting Time-domain Waveform
It is necessary to set the normal time-domain (YT) waveform before using the FFT mode. Follow the
steps below:
1. Push the AUTO button to display a standard YT waveform.
2. Click the VOLTS Key to ensure the entire waveform is visible on the screen. If the waveform is
invisible, the oscilloscope may display erroneous FFT results by adding high-frequency
components.
3. Click the Vertical Position key to vertically move the YT waveform to the center (zero division) so as
to ensure the FFT will display a true DC value.
4. Click the Horizontal Position key to position the part of the YT waveform to be analyzed in the
center eight divisions of the screen. The oscilloscope uses the 2048 center points of the
time-domain waveform to calculate the FFT spectrum.
5. Click the TIME/DIV key to provide the resolution needed in the FFT spectrum.
If possible, set the oscilloscope to display multiple signal cycles. If the TIME/DIV key is clicked to
select a faster setting (fewer cycles), the FFT spectrum will display a larger frequency range and
reduce the possibility of FFT aliasing.
