Page 66 - Prosig Catalogue 2005

SOFTWARE PRODUCTS
THE BASICS OF DIGITAL FILTERING

Training & Support

Figure 10: Swept sinewave with low pass filter applied Figure 12 shows the full swept sine wave after we have applied the band
Figure 13: Swept sinewave with band pass filter applied
Condition Monitoring but as it passes the 500Hz cut off more of the signal passes through the frequency of the swept sinewave passes through the 250Hz to 750Hz
Figure 11 shows the full swept sine wave after we have applied the high
stop filter. Clearly we can see how the filter attenuates the signal as the
pass filter. Here we see how the signal is attenuated at lower frequencies,
region.
filter.
Figure 13 shows the full swept sine wave after we have applied the band
pass filter. Here, we see the opposite effect, where the filter only passes
frequencies lying between the two cut offs.
Several properties of a filter can affect the precise form of the output.
There are, for instance, many different types of filter (Butterworth,
Chebyshev etc.). Also, we should consider the number of passes. This
is simply the number of times we apply the filter algorithm to the signal.
The more times it is applied the sharper the roll off rate. However, as well
as changing the amplitude, passing data through a filter causes phase
changes or delays in the output signal. The real change is frequency
sensitive and depends on the number of passes, the cut off frequency and
the filter type. To find out more about this and how you can use phaseless
techniques to filter data, see the article “Removing Phase Delay Using
Software Figure 11: Swept sinewave with high pass filter applied Phaseless Filters” on the Prosig Noise & Vibration Measurement
Blog at http://blog.prosig.com/2001/06/06/removing-phase-delay-using-
phaseless-filtering/

Hardware Figure 12: Swept sinewave with band stop filter applied

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