D-2
D Functional Description of the Controller
The velocity decoder is followed by a low pass filter which suppresses
spurious RF components and limits the bandwidth of the output signal to
reduce background noise. Via the system control, various cutoff frequencies
can be set for the low pass filter.
The last block in each signal path is an amplifier which scales the output
signals and can optimally drive subsequent signal processing units.
The system control manages the communication of the controller with the
environment via the front panel and PC interfaces and also the internal setting
of all parameters for the individual functional blocks.
Not included in the block diagram is the power supply unit which generates all
supply voltages for the sensor head and the controller.
D.2 Signal Conditioning
The RF signal from the sensor head first of all has to be pre-processed to
optimally drive the various demodulators. The signal conditioning includes the
following functions:
•
Measurement of the input signal level
•
Stabilization of the signal amplitude
•
Limitation of the bandwidth
•
Drop-out reduction via the tracking filter
•
Down-mixing of the frequency
Measurement
of the input
signal level
The measurement of the input signal level is required to provide the user with
information of the back scattering properties of the object and as a help to
optimally focus the laser beam. The level is converted to a logarithmically
scaled DC voltage. This signal is visualized on the sensor head and on the
controller as a bar display and is available at the BNC jack SIGNAL for
external usage.
Stabilization
of the signal
amplitude
Stabilization of the signal amplitude is necessary for the following signal
processing steps as the input signal level can fluctuate by several orders of
magnitude due to the extremely different back scattering properties of the
objects.
Limitation of
the bandwidth
Limitation of the bandwidth at the input of the signal processing electronics is
required because, for low velocities, only a narrow section of the system
bandwidth is occupied by the FM signal. In the remaining bandwidth only
noise is recorded. For this reason, at the input section of the controller, a
switchable filter is installed which limits the noise bandwidth depending on the
velocity measurement range set. As this is carried out right at the input,
however, the bandwidth limitation also affects the displacement decoding and
under certain circumstances has to be taken into consideration (refer also to
, Optimizing the RF bandwidth).
Summary of Contents for OFV-3001
Page 28: ...3 16 3 First Steps...
Page 54: ...5 12 5 Operating the Vibrometer...
Page 94: ...D 10 D Functional Description of the Controller...
