Allen-Bradley 1444-TSCX02-02RB User Manual page 296

Chapter 10 Operate the Module
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Table 72 - High-level Signal Processing Considerations (continued)
High-level Signal Processing Considerations
Page Parameter
Bands FFT Bands are used to measure component signals in complex measurements, typically
from measurements where there are many different frequency signals present. In these
cases, the high-level signal processing that is applied, and the configuration of the FFT,
used in the bands measurement, are critical to correct measurements – and changes
based on the characteristics of the signal of interest.
The accuracy of FFT Band measurements can only be verified when the frequencies of
included signals are stable. When signal frequencies change (or "slew") within a band,
or move between bands, the FFT function effectively distributes the signal energy
within multiple FFT bins. This action can result in band measurements that are lower
than actual.
Some signal-processing techniques can be applied to minimize these effects, for
example:
• Configure higher frequency measurements to increase the bin-width (Hz per bin)
which reduces the amount of leakage that is associated with minor changes in
signal frequency.
• Sample synchronously to move the FFT bins effectively as speed changes.
• Define bands in the order domain to keep the bands centered on order frequencies.
While these and other techniques can reduce the effects of frequencies changing
within the measurement, how effective they are depends on the following:
• How rapidly the signal frequencies are changing.
• The presence of other non-synchronous signals.
• The type of FFT Window that is applied, and more.
While it is possible to improve the accuracy of measuring synchronous signals as speed
changes, doing so conversely reduces the accuracy of measuring any non-synchronous
signals that are present.
Detection
FFT Window
Number of Averages
Measurement Mode
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
Considerations
See previously mentioned "detection" in this table.
Set the window to Flat top
The Flat top FFT Window provides the best possible
magnitude accuracy for band measurements.
It is best to specify some amount of averaging if signal
noise is expected. However, if the frequency of the signal
of interest is not stable, then the averaging smears the
signal unless sampling synchronously. But in that case, it
smears any non-synchronous signals that are present.
The FFT Band function provides two options for
magnitude measurement:
• Band Overall
Use Band Overall if the measured signal includes only
one frequency. This frequency verifies the best
accuracy when the frequency of the signal is not
centered within an FFT bin, or if its frequency changes
within the band. However, accuracy can still be
reduced with the frequency of the signal is at the edges
of the specified Band limits.
• Use Max Peak in Band
Use when multiple frequencies can be present within
the FFT Band limits.