Basic Operation
5.3.1Math FFT
This chapter elaborates how to use the Math FFT (Fast Fourier Transform). You can use the Math
FFT mode to convert a time-domain (YT) signal into its frequency components (spectrum), and to
observe the following types of signals:
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.
5.3.1.1
Setting Time-domain Waveform
It is necessary to set the time-domain (YT) waveform before using the FFT mode. Follow the
steps below.
1. Push the AUTOSET button to display a YT waveform.
2. Turn the VERTICAL POSITION knob to vertically move the YT waveform to the center (zero
division) so as to ensure the FFT will display a true DC value.
3.
Turn the HORIZONTAL POSITION knob 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.
4. Turn the VOLTS/DIV knob to ensure the entire waveform remains on the screen. If the entire
waveform is invisible, the oscilloscope may display wrong FFT results by adding high-
frequency components.
5. Turn the SEC/DIV knob to provide the resolution you need in the FFT spectrum.
6.
If possible, set the oscilloscope to display multiple signal cycles.
If you turn the SEC/DIV knob to select a faster setting (fewer cycles), the FFT spectrum will
display a larger frequency range and reduce the possibility of FFT aliasing.
DSO5000 Series Digital Storage Oscilloscope User Manual 24
