Digital Storage Oscilloscope 19
2.7.2.5 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.
2.7.2.6 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
