FFT
Eliminating Aliases
To eliminate aliases, try the following remedies:
Turn the
Horizontal Scale
(seconds/division) knob to set the sample rate to
a faster setting. Since you increase the Nyquist frequency as you increase
the sample rate, the aliased frequency components appear at their proper
frequency. If too many frequency components are shown on the screen, you
can use the FFT Zoom option to magnify the FFT spectrum.
If you do not need to view frequency components above 20 MHz, set the
Bandwidth Limit option to On.
Put an external
fi
lter on the source signal to bandwidth limit the source
waveform to frequencies below that of the Nyquist frequency.
Recognize and ignore the aliased frequencies.
Use zoom controls and the cursors to magnify and measure the FFT spectrum.
Magnifying and Positioning an FFT Spectrum
You can magnify and use cursors to take measurements on the FFT spectrum. The
oscilloscope includes an FFT Zoom option to magnify horizontally. To magnify
vertically, you can use the vertical controls.
Horizontal Zoom and
Position
The FFT Zoom option lets you horizontally magnify the FFT spectrum without
changing the sample rate. Zoom factors are X1 (default), X2, X5, and X10. At
zoom factor X1, and with the waveform centered in the graticule, the left graticule
line is at 0 Hz and the right graticule line is at the Nyquist frequency.
When you change the zoom factor, the FFT spectrum is magni
fi
ed about the
center graticule line. In other words, the axis of horizontal magni
fi
cation is the
center graticule line.
Turn the
Horizontal Position
knob clockwise to move the FFT spectrum to the
right. Push the knob to position the center of the spectrum at the center of the
graticule.
Vertical Zoom and Position
The channel vertical knobs become vertical zoom and position controls for their
respective channels when displaying the FFT spectrum.
Turn the
Vertical Position
knob clockwise to move the spectrum up for the
source channel.
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