Spider DSA User’s Manual
310
Real Time Digital Filter
Real Time Digital Filters can be used to filter a measured signal in real time. Filter
characteristics can be defined by the user to meet the requirements of a specific
application. Real-time digital filters are applied in the data conditioning phase.
The filters are designed with a graphic design tool and then uploaded to the front-
end for real-time calculation. The graphic design tool defines the filter
performance vertical axis with a dB scale. The horizontal axis is defined as relative
frequency.
For example, a user might want to look at the energy distribution for a specific
band of frequencies over time instead of for the entire frequency spectrum. This
can be done by creating a band-pass filter and then applying an RMS estimator to
the output of the filter.
The figure below shows a graphical representation of the process used to define a
real-time filter in the EDM software. The icon on the left, CH1 represents the
native measured time stream. It is connected to an IIR Filter which computes a
signal named iir filter (ch1) which is connected to an RMS estimator. The output
of the RMS estimator is a signal named rms(iirfilter(ch1)).
Figure 175. Example of real time digital filter application.
Another example: Perhaps a user wishes to look at the energy in bands from 100
Hz to 200 Hz and from 1000 Hz to 2000 Hz. This can be done by deriving two
output streams from the native channel 1 and then applying the band-pass filter
to each path as shown in Figure 176.
Figure 176. A Digital Real Time Filter with two output streams.
In another example, you might want to sample a signal at a high rate to capture
high frequency events but may also wish to sample the same channel at a lower
rate to see lower frequency events. This can be done by applying a decimation
filter to the native time stream. The native channel time stream is split into two
streams so the signal from the same channel is recorded at both high and lower
sampling rates.
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