An additional time delay is caused by the velocity decoder. It depends on theAn additional time delay is caused by the velocity decoder. It depends on the
measurement range and is approximately a few microseconds. The resultingmeasurement range and is approximately a few microseconds. The resulting
overall phase shiftoverall phase shift
∆Φ∆Φ
can be can be estimated using the following simple equation:estimated using the following simple equation:
Equation 4.1Equation 4.1
The tracking filter is used to The tracking filter is used to improve the signal-to-noise ratio of theimprove the signal-to-noise ratio of the
interferometeinterferometer signal. This r signal. This is particularly good for is particularly good for bridging short dropoutsbridging short dropouts
which occur due to which occur due to the speckled natured of the light the speckled natured of the light scattered back. Thescattered back. The
bridging capability is generally better with a high time constant SLOW,bridging capability is generally better with a high time constant SLOW,
however, it may not be possible to follow highly dynamic signals any more. Inhowever, it may not be possible to follow highly dynamic signals any more. In
this case, FAST or OFF have to be selected. The best setting therefore has tothis case, FAST or OFF have to be selected. The best setting therefore has to
be determined from case to case or be estimated based on the range diagrambe determined from case to case or be estimated based on the range diagram
ininfi
figugurere 4.4.77. The range . The range diagram shows the dynamic limits for both settings ofdiagram shows the dynamic limits for both settings of
the tracking filter, plotted versus the frequency.the tracking filter, plotted versus the frequency.
A constant vA constant velocity limit of approxielocity limit of approximately 3mately 3mm// s is characteristis is characteristic for the lowerc for the lower
frequency range. If the velocity exceeds this value, it frequency range. If the velocity exceeds this value, it means that the trackingmeans that the tracking
filter generally has to be switched OFF. In the medium frequency range, thefilter generally has to be switched OFF. In the medium frequency range, the
velocity limit changes over to become an velocity limit changes over to become an acceleration limit, i.e. the velocityacceleration limit, i.e. the velocity
limit decreases in inverse proportion to the frequency. In the upper frequencylimit decreases in inverse proportion to the frequency. In the upper frequency
range a constant velocity limit becomes effective again.range a constant velocity limit becomes effective again.
ΦΦ
LPLP
...... phase shifphase shift of the lot of the low pass fw pass filterilter, refer to, refer tofi
figugurere 4.4.33andandfifigugurere 4.4.66
pp
ss
...... specific phspecific phase roll-ofase roll-off, refer to f, refer to specificatspecifications inions inse
sectctionion 7.17.1.3.3
ff ...... ffrreeqquueennccy y iin n kkHHzz
∆Φ ∆Φ ΦΦ
LPLP
pp
ss
ff⋅⋅++==
Figure 4.7: Operating ranges of the tracking filterFigure 4.7: Operating ranges of the tracking filter
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