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The controller is equipped with an adjustable low pass filter which adapts the
bandwidth of the measurement signal to the respective application. When dis-
playing a signal in the time domain, the signal-to-noise ratio can be improved
by limiting the bandwidth to the necessary extent. When analyzing in the fre-
quency domain with an external FFT analyzer, filters only play a subordinate
role. Here they can prevent the FFT analyzer from overranging due to noise
spikes.
In the OFV-3001 controller, low pass filters with 3rd order Bessel characteris-
tics are used. Characteristic of this type of filter is the phase linearity from the
frequency zero up to the cutoff frequency i.e. the phase shift increases pro-
portionally to the frequency. These filters however cause amplitude errors in
the passband which can be roughly estimated:
Up to 40% of the cutoff frequency, the amplitude error is less than -5%.
This range can be considered to be exact for amplitude measurement.
Up to 70% of the cutoff frequency, the amplitude error increases to about
-15%.
The upper 30% of the passband should only be used for orientation mea-
surements. At the cutoff frequency of the filter, the amplitude error is -3dB
(approximately -30%).
The phase shift increases proportionally to the frequency from close to zero
degree at a few Hertz to approximately -100 degrees at the cutoff frequency
(refer to figure 4.5). Due to this linear phase frequency response, the filter
shows optimal transmission behavior for pulses as all frequencies of a com-
plex wave are subjected to the same time delay. Thus the shape of the pulse
is not falsified but it is merely delayed.
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