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FFT Commands
The :MEASure:FFT commands control the FFT measurements that are
accessible through the Measure subsystem.
Measurement Setup
To make a measurement, the portion of the waveform required for that
measurement must be displayed on the oscilloscope.
• For a period or frequency measurement, at least one and a half
complete cycles must be displayed.
• For a pulse width measurement, the entire pulse must be displayed.
• For a rise time measurement, the leading (positive-going) edge of the
waveform must be displayed.
• For a fall time measurement, the trailing (negative-going) edge of the
waveform must be displayed.
In jitter mode with jitter statistics enabled, measurements are made on
all data regardless of what is on screen.
User-Defined Thresholds
If you choose to set user-defined thresholds, they must be set before
actually sending the measurement command or query.
Measurement Error
If a measurement cannot be made because of a lack of data, because the
source waveform is not displayed, the requested measurement is not
possible (for example, a period measurement on an FFT waveform), or
for some other reason, the following results are returned:
• 9.99999E+37 is returned as the measurement result.
• If SENDvalid is ON, the error code is also returned.
Making Measurements
If more than one period, edge, or pulse is displayed, time measurements
are made on the first, left-most portion of the displayed waveform.
When any of the defined measurements are requested, the oscilloscope
first determines the top (100%) and base (0%) voltages of the waveform.
From this information, the oscilloscope determines the other important
voltage values (10%, 90%, and 50% voltage values) for making
measurements.
The 10% and 90% voltage values are used in the rise time and fall time
measurements when standard thresholds are selected. The 50% voltage
value is used for measuring frequency, period, pulse width, and duty
cycle with standard thresholds selected.
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