Polytec OFV-3001, User Manual

Page: 36 / 101

4-8
4 Making Measurements
Low pass
filter The controller is equipped with an adjustable low pass filter which adapts the
bandwidth of the measurement signal to the respective application. When
displaying 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 frequency 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
characteristics 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 proportionally 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
measurements. At the cutoff frequency of the filter, the amplitude error is
-3 dB (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
complex wave are subjected to the same time delay. Thus the shape of the
pulse is not falsified but it is merely delayed.
The complete amplitude frequency response of a 3rd order Bessel low pass
filter is shown in
figure 4.3 . The frequency is normalized to the cutoff
frequency f
c
.
Figure 4.3: Amplitude frequency response of a 3rd order Bessel low pass filter
- 6 0
- 5 0
- 4 0
- 3 0
- 2 0
- 1 0
0
0 . 1 0 . 2 0 . 5 1 2 5 1 0 2 0
A m p l i t u d e / d B
f
f
C
1 0