4-4
4 Making Measurements
For the tracking filter settings, you can use the following rules of thumb:
•
Use the tracking filter to improve the signal noise if the optical signal is
weak. If you have a good optical signal, it is not possible for the tracking
filter to improve the signal-to-noise ratio for physical reasons. In this case
it should be switched off.
•
Select the Slow, Medium, or Fast mode depending on the peak
acceleration.
•
The Medium and Slow settings are not suitable for high accelerations.
When the maximum acceleration is exceeded, the optical sensitivity is
reduced. However, an exceeding within the range of the crosshatched
area is harmless (refer to the following figure). If the maximum
acceleration of Slow is exceeded, the Medium setting takes better effects.
If the maximum acceleration of Medium is exceeded, the Fast setting
takes better effects. So, if necessary, you will have to try out the most
favorable setting of the tracking filter. To do so, select sample points with
a high vibration amplitude.
When you perform particularly precise measurements at high optical signal
level you should switch off the tracking filter as frequency response errors,
linearity errors, and harmonic distortions can also be generated by the Fast
setting. The additional frequency response error can be up to 0.25dB and
occurs at frequencies above 75% of the bandwidth set.
Figure 4.1: Selection of suitable tracking filter settings depending on velocity and
frequency
Frequency in % of the selected bandwidth
Velocity in % of the selected
measurement range
Acceleration limit
Acceleration limit
Acceleration limit
100
1
10
100
10
1
SLOW
MEDIUM
FAST
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