Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 145
Measurement Definition Chapter 4
Table 30 - FFT
Parameter Values Comment
Enable TWF Data
Storage
Enable (checked) / Disabled (not
checked)
Select the checkbox to save the time waveform (TWF). The module saves the TWF to any defined Trend buffers,
and makes the most recent sample available for external access.
Tip: External access to “Live” TWF and FFT data, as defined on this page, requires that the TWF and/or FFT be
enabled here.
Clear the checkbox so the waveform does not save.
FFT processing requires that the module measure a TWF using the TWF attributes defined on this properties
page); however, saving the FFT does not require saving the TWF. If you do not choose to save the TWF, the module
discards it after the calculating the FFT.
Number of
Samples
Select from:
•256
•512
• 1024
• 2048
• 4096
• 8192
Select the number of samples to be captured in the TWF. While this TWF and the TWF that the module uses to
calculate the FFT begin with the same sample, it is not necessary that they have the same number of samples.
Therefore the Number of Spectrum Lines (for the FFT) is not related to this Number of Samples (for the TWF).
Enable FFT Data
Storage
Enable (checked) / Disabled (not
checked)
Select the checkbox to make the module process and save the FFT so it is available to be read externally from the
module. The module also saves the FFT in the Trend buffers.
Tip: External access to “Live” TWF and FFT data, as defined on this page, requires that the TWF and/or FFT be
enabled here, AND that Dynamic Data be enabled on the TREND page.
Clear the checkbox so the FFT does not process in the module.
Number of
Spectrum Lines
Select the number of lines for the FFT. The available number of FFT Lines that are displayed depends on the signal source and if the measurement is
decimated (the FMAX is lower than the ADC out FMAX).
Signal Detection Select from:
•Peak
•Peak to Peak
•RMS
Select the scaling (detection) method for the FFT line (bin) values.
An FFT is calculated from a set of samples that contain many signal cycles of the various signals that are present.
While the shape of the waveform reflects the interaction between the various signal components, it isn’t possible
to discern, from an FFT, the “true” deflection of a signal. The amplitude results from the FFT are representative
RMS values that are calculated from the whole sample set. One result is calculated that can be presented as RMS,
or (scaled) peak, or (scaled) peak to peak for each discrete frequency line or bin.
Decimation
None Alternate
Path
(1)
(1) Or if the Measurement Type (See Hardware
Configuration Page) is either of the
aeroderivative selections.
Primary
Path
1800 1000 600
900 500 300
450 250 150
225 125 75
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