Allen-Bradley 1444-TSCX02-02RB User Manual page 421

Table 151 - Sampling Control (continued)
Category
Disabling a LP filter
Decimation of asynchronous samples
FFT Data Filter (SR_FILTER)
Higher Frequency modes
Description
You can disable the LP filter to use more of the available bandwidth for the overall (0)
measurement. Minimizing (unnecessary) filtering is also beneficial for reducing module
processing load and generally retaining the fidelity of the signal.
While within a particular channel application type you cannot explicitly choose to disable
an LP filter, it can be achieved by choosing to set the filter cutoff frequency at the
maximum allowed: SRD / 2.048 (note that for calculation purposes, '40 kHz mode' has an
SRD of '1' .
On receiving the configuration, if the configuration setting is above or within 5 Hz of, the
calculated maximum then the module automatically disables that LP filter. Main path and
alternate path (asynchronous) filters are considered separately as appropriate to the
configuration.
A decimation of n further reduces sample rate by retaining only the nth sample.
Decimation is commonly used for the following purposes to:
• Provide the user with an FFT whose Fmax is lower than what is implied or attainable
by the SRD
• Permit internal sample transfer for CM data purposes (such transfers cannot support
the 40 kHz bandwidth)
• Reduce the sample rate into an HP filter, where the difference between the filter -3 dB
and the sample rate is a large ratio.
The latter is not considered likely to be necessary unless the ratio of sample rate to HP
filter cutoff frequency significantly exceeds 3000. On a 4 kHz measurement bandwidth
(SRD 9) that equates to an HP filter cutoff, lower than 3 Hz.
In all cases decimation requires prior LP filtering of the samples, to avoid aliasing.
Dependent on the application/path, filtering can be by -24 or -48 dB/octave filters. To
avoid the possibility of aliasing, it is recommended that these maximum filter cutoff
settings are imposed:
• -24 dB (LP followed by an HP): 0.25 x the decimated sample rate
• -48 dB (LP alternate path only): 0.36 x the decimated sample rate
• -60 dB (Aeroderivativemode only):0.385 x the decimated sample rate
Synchronous resampling also requires anti-alias protection and this protection is provided
by a -48 dB LP filter. The difference between this and asynchronous decimation by a -48
dB LP filter is that in the synchronous case the filter -3 dB point is continually and
automatically adjusted according to the machine speed.
When FFT data is requested that relies on a signal that has been decimated or resampled
synchronously, as described previously, one of the internal LP filters must be used to
provide anti- aliasing protection for the resampled stream. Due to the relatively slow roll-
off of these filters, they have to be positioned well within the expected FFT FMAX. This
positioning means that a proportion of the FFT lines reflect frequencies at which the
signal amplitudes are significantly attenuated. If the FFT Data Filter capability is enabled
(appropriate bit set in the FFT request), then if the data to be returned is affected by this
then only 50% of the Normal lines are returned. This return allows for the worst case of
the -24 dB/octave filter. With this control, you have the choice at the point of request
whether to receive the full or the reduced (filtered) Data-Set.
• If the bit is set and the data is not decimated or synchronously resampled, then 100%
of the available FFT lines are returned.
• A bit in ucDataSelect of the FFT header indicates if the data has been filtered.
• The actual filter setting is not checked, simply whether this filter is decimated or
synchronously resampled data
• Where data filtering results in different number of lines being returned, the data
requests are appropriately grouped, for like data lengths
• This data filter can be requested on any CM data object that supports the return of FFT
spectral data
In the 'normal' (20 kHz max) case:
SRD settings are in the range 2 to 32 and are set appropriately for the channel application.
The default decimation is 1.
In the gSE/40 kHz case:
The SRD is fixed at 2 and internally the A/D set is set at double mode (that combination is
equivalent to an SRD of 1).
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
CIP Objects Appendix A
421