Math Waveforms 4
Keysight InfiniiVision 3000 X-Series Oscilloscopes User's Guide 83
FFT Measurement
FFT is used to compute the fast Fourier transform using analog input channels or
an arithmetic operation g(t). FFT takes the digitized time record of the specified
source and transforms it to the frequency domain. When the FFT function is
selected, the FFT spectrum is plotted on the oscilloscope display as magnitude in
dBV versus frequency. The readout for the horizontal axis changes from time to
frequency (Hertz) and the vertical readout changes from volts to dB.
Use the FFT function to find crosstalk problems, to find distortion problems in
analog waveforms caused by amplifier non-linearity, or for adjusting analog filters.
To display a FFT waveform:
1 Press the [Math] key, press the Function softkey and select f(t), press the Operator
softkey and select FFT.
• Source 1 — selects the source for the FFT. (See "To perform transforms or
filters on an arithmetic operation" on page 75 for information about using
g(t) as the source.)
• Span — sets the overall width of the FFT spectrum that you see on the
display (left to right). Divide span by 10 to calculate the number of Hertz per
division. It is possible to set Span above the maximum available frequency, in
which case the displayed spectrum will not take up the whole screen. Press
the Span softkey, then turn the Entry knob to set the desired frequency span
of the display.
• Center — sets the FFT spectrum frequency represented at the center vertical
grid line of the display. It is possible to set the Center to values below half
the span or above the maximum available frequency, in which case the
displayed spectrum will not take up the whole screen. Press the Center
softkey, then turn the Entry knob to set the desired center frequency of the
display.
• Scale — lets you set your own vertical scale factors for FFT expressed in
dB/div (decibels/division). See "To adjust the math waveform scale and
offset" on page 75.
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