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Filters
Page 13
3. Filters
The data filter is a digital FIR filter and the unit automatically sets the filter to suit the selected data output
rate. So that the 803 can be used for turbulent flow applications where users wish to recreate the flow
characteristics, the filters are set so that the cut-off frequency is half the data rate. Attenuation [dB and Signal
ratio] are given in both graphical and tabular form in Appendix 2.
A characteristic of the filters are that they have a fixed time delay for the output signal [i.e. phase delay is
independent of signal frequency]. If data is being synchronised with other sources of data, then this delay
needs to be taken into account in data processing – note that the delay times are exact multiples of the time
between data points.
The filter characteristics can be summarised as follows:
Data Rate
[Hz]
-3dB point
[Hz]
Cut off
[Hz]
Data delay
[secs]
Data delay
[samples]
16
3.56
8
0.3125
5
8
3.01
4
1.875
15
4
1.17
2
1.75
7
2
0.61
1
4
8
1
0.30
0.5
8
8
