Basic Operation
DSO4000 Series Digital Storage Oscilloscope 18
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 (70MHz,
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.
2.7.2.2 Displaying FFT Spectrum
Push the MATH MENU button to display the Math menu. Use the options to select the Source
channel, the Window algorithm and the FFT Zoom factor. Only one FFT spectrum can be
displayed at a time.
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