Polytec PDV-100, User Manual

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4-3
4 Making Measurements
4.2.3 Setting the High Pass Filter
To suppress low-frequency background vibrations, the PDV has got a
switchable high pass filter of a 3rd order Butterworth with a cutoff frequency of
100 Hz ( − 3 dB). The high pass filter is switched on (Y) and off (N) on the
display at HP .
For technical reasons this filter has been realized in conventional analog
technnology and therefore only has an effect on the analog voltage output.
The high pass filter can always be used beneficially for analog signal
acquisition if you are working with high-level background interference e.g.
caused by machine vibrations when a relatively small signal is to be
measured at high frequencies. Under these conditions please take care of the
internal saturation of the selected measurement range (refer also to
SECTION
4.2.3). Attention has to be paid to the following properties for the high
pass filter:
•
The changeover from the pass band to the stop band occurs with a more
gradual frequency roll-off ( − 60 dB / dec) than with the low pass filter.
•
Near the cutoff frequency a significant amplitude error occurs (refer to
FIGURE
4.1).
•
For frequencies > 140 Hz, the amplitude error is < − 0,5 dB ( − 5%).
Figure 4.1: Amplitude and phase frequency response of a 3 rd order Butterworth high pass
filter
The lower graph in
FIGURE
4.1 represents the phase frequency response.
The 360° phase jump can only be seen on the diagram for presentation
reasons.