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The signal is composed of low frequency
background noise, the Doppler frequency and high frequency components
in the order of the frequency of the laser beam.
The low frequency noise can be the result of some other light
source, the vibrations of the pump, or purely electronic noise.
Generally it increases with decreasing frequency and it is particularly
high for frequencies below the 2 kHz.
The Doppler signal has been discussed briefly in the theory.
The addition of waves of equal intensity and differing frequency
yields a light wave with an irradiance that is modulated with
the so-called beat frequency. There is however not continuously
a scattering particle present. The Doppler signal increases in
intensity when a particle enters the measurement volume up to
the center and decreases again upon leaving the measurement volume.
This is the result of the Gaussian distribution of the intensity
of the laser beams, that reflects on the intensity distribution
of the measurement volume. For a single particle this results
in an additional modulation with a 'frequency' that corresponds
to:
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(4.1) |
Here the 'wavelenght' is equal to the width
of the measurement volume in the direction of the flow. The Doppler
signal itself is always accompanied by this 'carrier wave' ,
also called the pedestal wave. The result is a so-called signal
burst that is composed of the pedestal wave, with the Doppler
wave superposed on it. A scetch of these signals is presented
in fig. 4.1. The signal burst manifests itself as a highly
recognizable burst, both for its frequency as for its intensity.
This is a very useful property when one is adjusting the data
processing instrumentarium through adjustment of the diaphragm,
the bias of the amplifier and the amplification and it is therefore
advisable to perform the optimazition of these while keeping
an eye on the signal in the time domain. This should be done
for each measurement performed, since the intensity of the signal
may vary heavily for changing circumstances. This implies that
the signal intensity may be so low that nothing is detected or
that the photodiode may suffer from an overload. In the first
case there will hardly by any signal in the frequency dimension,
but in the second case there will be additional frequency components
present. The occasional overload may be interpreted as an additional
square wave. Since this type of waves is built up of a series
of high frequency components the square wave will, under a Fourier
transform, result in this decomposition.
Apart from the low frequency noise and the signal burst another
frequency component is present. It is in the order of the frequency
of the laser light and will therefore remain undetected since
the photodiode can measure frequencies up to only 50 kHz. In
fig. 4.2 an actual example of a measurement is shown, comprising
all the effects mentioned. |
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| a
: Pedestal wave |
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| b
: Doppler wave |
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Figure 4.1
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Scetches of the components of the measured signal as shown in
fig. 4.2.
The pedestal wave (a) results from the intensity increase and
decrease of the scattered light when a particle crosses the measurement
volume.
The Doppler signal (b) results from the interference of the scattered
light originating from the two beams.
Finally the signal burst (c) is the resulting signal, i.e. the
Doppler wave superposed on the pedestal wave. |
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| c
: Signal burst |
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Figure 4.2 : An example of an actual measured signal. The higher
frequency component corresponds to the Doppler frequency, whereas
the low frequency component is in the order of the Pedestal wave. |
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After a clearly established measurement of
the signal burst on a regular basis one can start the spectral
measurements. These are to be averaged over a temporal period
that is substantially larger than the period of the occurence
of a signal burst, since it is highly dependent of the velocity
of passing particles (see complications) and the regularity of
the fringe pattern (see complications).
Once a series of spectra is obtained, one may estimate the center
frequency and the width of the spectral peaks. The next step
is an error analysis, including the propagation errors that result
from the calculation of velocity. This should include all relevant
parameters like the distance between the beams passing lens L1,
the focal length of lens L1, the position of the measurement
volume etc.
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