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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.
To set the FFT display, follow the steps below.
1.
Push the MATH MENU button;
2.
Set the Operation option to FFT;
3.
Select the Math FFT Source channel.
In many situations, the oscilloscope can also generate a useful FFT spectrum despite the YT waveform not being
triggered. This is especially true if the signal is periodic or random (such as noise).
Note:
You should trigger and position transient or burst waveforms as close as possible to the screen center.
Nyquist Frequency
The highest frequency that any real-time digital oscilloscope can measure without errors is half of the sample rate,
which is called the Nyquist frequency. Frequency information beyond the Nyquist frequency is undersampled which
brings about the FFT aliasing. The math function can convert the center 2048 points of the time-domain waveform to
an FFT spectrum. The resulting FFT spectrum contains 1024 points from DC (0Hz) to the Nyquist frequency. Usually,
the screen compresses the FFT spectrum horizontally to 250 points, but you can use the FFT Zoom function to
expand the FFT spectrum so that you can clearly view the frequency components at each of the 1024 data points in
the FFT spectrum.
Note:
