An additional time delay is caused by the velocity decoder. It depends on the
An additional time delay is caused by the velocity decoder. It depends on the
measurement range and is approximately a few microseconds. The resulting
measurement range and is approximately a few microseconds. The resulting
overall phase shift
overall phase shift
∆Φ
∆Φ
can be
can be estimated using the following simple equation:
estimated using the following simple equation:
Equation 4.1
Equation 4.1
The tracking filter is used to
The tracking filter is used to improve the signal-to-noise ratio of the
improve the signal-to-noise ratio of the
interferomete
interferometer signal. This
r signal. This is particularly good for
is particularly good for bridging short dropouts
bridging short dropouts
which occur due to
which occur due to the speckled natured of the light
the speckled natured of the light scattered back. The
scattered 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. In
however, 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 to
this 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 diagram
be determined from case to case or be estimated based on the range diagram
in
in fi
fi
gu
gu
re
re
4.
4.
7
7
. The range
. The range diagram shows the dynamic limits for both settings of
diagram shows the dynamic limits for both settings of
the tracking filter, plotted versus the frequency.
the tracking filter, plotted versus the frequency.
A constant v
A constant velocity limit of approxi
elocity limit of approximately 3
mately 3 m
m // s is characteristi
s is characteristic for the lower
c for the lower
frequency range. If the velocity exceeds this value, it
frequency range. If the velocity exceeds this value, it means that the tracking
means that the tracking
filter generally has to be switched OFF. In the medium frequency range, the
filter 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 velocity
acceleration limit, i.e. the velocity
limit decreases in inverse proportion to the frequency. In the upper frequency
limit 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.
Φ
Φ
LP
LP
...
... phase shif
phase shift of the lo
t of the low pass f
w pass filter
ilter, refer to
, refer to fi
fi
gu
gu
re
re
4.
4.
3
3
and
and fi
fi
gu
gu
re
re
4.
4.
6
6
p
p
s
s
...
... specific ph
specific phase roll-of
ase roll-off, refer to
f, refer to specificat
specifications in
ions in se
se
ct
ct
ion
ion
7.1
7.1
.3
.3
ff
...... ffrre
eq
qu
ue
en
nc
cy
y iin
n k
kH
Hz
z
∆Φ
∆Φ
Φ
Φ
LP
LP
p
p
s
s
ff
⋅⋅
+
+
=
=
Figure 4.7: Operating ranges of the tracking filter
Figure 4.7: Operating ranges of the tracking filter
Содержание PSV 300
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