Basic Operation
DSO5000P Series Digital Storage Oscilloscope User Manual 25
5.3.1.4 FFT Aliasing
Problems occur when the time-domain waveform acquired by the oscilloscope contains frequency
components higher than the Nyquist frequency. The frequency components above the Nyquist
frequency will be undersampled and displayed as lower frequency components that ‘fold back’
from the Nyquist frequency. These erroneous components are called aliases.
5.3.1.5 Eliminating Aliases
To eliminate aliases, use the following methods.
Turn the SEC/DIV knob to set a faster sample rate. Because the Nyquist frequency increases
as you increase the sample rate, the aliased frequency components will be displayed correct.
If too many frequency components appear on the screen, you may use the FFT Zoom option
to magnify the FFT spectrum.
If there is no need to observe the frequency components above 20MHz, set the Bandwidth
Limit option to Limited.
Filter the signal input from outside and limit the bandwidth of the source waveform to lower
than the Nyquist frequency.
Identify and ignore the aliased frequencies.
Use zoom controls and cursors to magnify and measure the FFT spectrum.
5.3.1.6 Magnifying and Positioning FFT Spectrum
You may scale the FFT spectrum and use cursors to measure it through the FFT Zoom option
which enables the horizontal magnification. To vertically magnify the spectrum, use the vertical
controls.
Horizontal Zoom and Position
You can use the FFT Zoom option to magnify the FFT spectrum horizontally without changing the
sample rate. The available zoom factors are X1(default), X2, X5 and X10. When the zoom factor is
set to X1 and the waveform is located at the center graticule, the left graticule line is at 0Hz and
the right is at the Nyquist frequency.
You magnifies the FFT spectrum to the center graticule line when you change the zoom factor.
That is, the axis for horizontal magnification is the center graticule line. Turn the Horizontal
Position knob clockwise to move the FFT spectrum to the right. Push the SET TO ZERO button to
position the center spectrum at the center of the graticule.
Vertical Zoom and Position
When the FFT spectrum is being displayed, the channel vertical knobs become the zoom and
position controls corresponding to their respective channels. The VOLTS/DIV knob provides the
following zoom factors: X1(default), X2, X5 and X10. The FFT spectrum is magnified vertically to
the marker M (math waveform reference point on the left edge of the screen). Turn the VERTICAL
POSITION knob clockwise to move up the spectrum.
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