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Basic Operation
DSO5000 Series Digital Storage Oscilloscope User Manual 22
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: The oscilloscope’s vertical response is a little bit larger than its bandwidth (60MHz,
100MHz or 200MHz, depending on the model; or 20MHz when the Bandwidth Limit option is
set to Limited). Therefore, the FFT spectrum can display valid frequency information above
the oscilloscope bandwidth. However, the amplitude information near or above the
bandwidth will not be accurate.
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