D-6
D Functional Description of the Controller
D.4.2 Operating Principle
The velocity decoder determines the essential measurement properties of the
vibrometer. Velocity decoding is in principal an FM demodulation process
which converts the velocity dependent Doppler frequency of the
interferometer signal into an AC voltage. The linearity and bandwidth of the
demodulator determine the accuracy of the vibrometer. In contrast to FM radio
which works with the same modulation procedure, considerably higher
frequency deviations occur in the vibrometer which make significantly higher
demands on the demodulators:
In the OFV-3001 controller, different respectively optimally adapted
modulators are switched on in the individual velocity measurement ranges. A
maximum of two velocity decoders can be installed which in measurement
ranges which are in part the same, satisfy differing requirements with regards
to the maximum frequency or linearity. The decoders and measurement
ranges are selected via the system control and the internal bus. At the same
time, the corresponding settings on the subassemblies oscillator and signal
conditioning are carried out internally.
D.4.3 Low Pass Filter
The signal generated by the FM demodulator always contains spurious RF
components and its noise bandwidth corresponds with the maximum
frequency of the respective measurement range. A subsequent low pass filter
suppresses the RF components and limits the noise bandwidth according to
its cutoff frequency. This makes a rough adaptation of the measurement
bandwidth to the application possible and makes the signal evaluation in the
time domain significantly easier due to the improved signal-to-noise ratio. The
filters are adjusted via the system control.
In the OFV-3001 controller low pass filters with 3rd order Bessel
characteristics are used. Bessel filters have the advantage of a linear phase
response and thus optimal transmission behavior for pulses. Bessel filters
were selected, as the signal processing before the low pass also shows a
linear phase response and thus this advantage is retained even when the
filter is switched on. Due to the characteristics of the Bessel filter, however
signal amplitudes are attenuated even at relative low frequencies. This has to
be taken into consideration for accurate measurements. The filter
characteristics as well as rules of thumb for using them are provided in
FM radio:
max. frequency deviation 75 kHz,
max. modulation frequency 53 kHz (stereo)
Vibrometer:
max. frequency deviation 32 MHz,
max. modulation frequency 1.5 MHz
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...
