Math Waveforms 4
Agilent InfiniiVision 3000 X-Series Oscilloscopes User's Guide 83
• "Units for Math Waveforms" on page 76
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.
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