Page 17 - Shock and Vibration Overview

Data Acquisition (DAQ) System Selection

Figure 7: A 60 Hz sine wave when sampled at 100 Hz can erroneously make the vibration profile
look like a 40 Hz signal.

Now the question remains as to what type of filter should you use? An ideal filter would
uniformly pass all frequencies below a specified limit and eliminate all above that limit.
This ideal filter would have a perfectly linear phase response to the same upper
frequency limit. But ideal filters don’t exist; there is some compromise that needs to be
made on a filter’s amplitude and phase response. There are four main different types of
filters:

1) Butterworth
A Butterworth filter is known for its maximally flat amplitude response and a
reasonably linear phase response. The Butterworth filter is the most popular for
vibration testing.

2) Bessel
The Bessel filter has nearly perfect phase linearity so it is best suited for transient
events like shock testing. It has a fairly good amplitude response but its amplitude
roll-off is slower than the Butterworth or Chebyshev filter.

3) Chebyshev
The Chebyshev has a faster roll-off in the amplitude response which is achieved by
introducing a ripple before the roll-off. They have a relatively nonlinear phase
response.

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