Polytec PSV 300, Hardware Manual

Page: 43 / 105

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                                     
      In the PSV 300-F and -U low pass filters with 3rd order Bessel characteristics In the PSV 300-F and -U low pass filters with 3rd order Bessel characteristics
and cutoff frequenci
and cutoff frequencies of 5 es of 5 kHz, 20 kHz, 20kHz or 100 kHz or 100 kHz are used. Characteri kHz are used. Characteristic for stic for
Bessel filters is the phase linearity from the Bessel filters is the phase linearity from the frequency zero up to frequency zero up to the cutoff the cutoff
frequency, i.e. the phase shift increases in proportion to the frequency. The frequency, i.e. the phase shift increases in proportion to the frequency. The
filter however
filter however, causes amplitude errors in the , causes amplitude errors in the pass band which can be pass band which can be roughly roughly
estimated: estimated:
 
Up to 20% of the cutoff frequency, the maximum amplitude error is Up to 20% of the cutoff frequency, the maximum amplitude error is
±±
1%. 1%.
 
At the cutoff frequency, the amplitude error is At the cutoff frequency, the amplitude error is −− 3dB (approx. 3dB (approx. −− 30%). 30%).
The phase shift increases in proportion to The phase shift increases in proportion to the frequency from Zero degree a the frequency from Zero degree att
the frequency Zero the frequency Zero to approximately to approximately
−−
100 degrees at the cutoff frequency 100 degrees at the cutoff frequency
(refer to (refer to fi fi gu gu re re 4. 4. 6 6 ). Due to ). Due to this linear phase frequency response, the filter this linear phase frequency response, the filter
shows optimal transmissio shows optimal transmission behavior for pulses, a n behavior for pulses, as all frequencies of a s all frequencies of a
complex wave are subjected to the same time delay. Thus the shape of the complex wave are subjected to the same time delay. Thus the shape of the
pulse is not falsified but it is merely delayed.
pulse is not falsified but it is merely delayed.
The complete amplitude frequency response of a 3 The complete amplitude frequency response of a 3rd order Bessel low pass rd order Bessel low pass
filter is shown in filter is shown in fi fi gu gu re re 4. 4. 4 4 . The frequency is normalized to the cutoff . The frequency is normalized to the cutoff
frequency f frequency f
c c
..
Figure 4.4: Amplitude frequency response of a 3rd order Bessel low pass filter Figure 4.4: Amplitude frequency response of a 3rd order Bessel low pass filter
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