Polytec PSV 300, Руководство по оборудованию

Страница: 91 / 105

   
                                                       
     
The velocity decoder determines the essential measurement properties of The velocity decoder determines the essential measurement properties of the the
vibrometer. Velocity decoding is in principal an FM demodulation process vibrometer. Velocity decoding is in principal an FM demodulation process
which converts the velocity dependent Doppler frequency of the
which converts the velocity dependent Doppler frequency of the
interferomete
interferometer signal into r signal into an AC voltage. The linearity and an AC voltage. The linearity and bandwidth of the bandwidth of the
demodulat demodulator determine the accurac or determine the accuracy of the vibrometer. In contrast to FM y of the vibrometer. In contrast to FM radio radio
which works with the same modulation procedure, considerably higher which works with the same modulation procedure, considerably higher
frequency deviations occur in frequency deviations occur in the vibrometer which make the vibrometer which make significan significantly higher tly higher
demands on the demands on the demodulators: demodulators:
In the OFV-3001S controller, different respectively optimally adapted In the OFV-3001S controller, different respectively optimally adapted
demodulators are switched on in
demodulators are switched on in the individual velocity measurement ranges. the individual velocity measurement ranges.
The decoders and measurement ranges are The decoders and measurement ranges are selected via the system control selected via the system control
and the internal bus. and the internal bus. At the same time, the corresponding settings on the At the same time, the corresponding settings on the
subassemblies oscillator and signal conditioning are carried out internally. subassemblies oscillator and signal conditioning are carried out internally.
       
The signal generated by the The signal generated by the FM demodulator always contains spurious RF FM demodulator always contains spurious RF
components and its noise bandwidth corresponds with the maximum components and its noise bandwidth corresponds with the maximum
frequency of the respective measurement range. A subsequent low pass filter
frequency of the respective measurement range. A subsequent low pass filter
suppresses the RF components and limits the noise bandwidth according to
suppresses the RF components and limits the noise bandwidth according to
its cutoff frequency. This makes a rough adaptation of the measurement
its cutoff frequency. This makes a rough adaptation of the measurement
bandwidth to the application possible which makes the signal evaluation
bandwidth to the application possible which makes the signal evaluation in in
the time domain significantly easier due to the time domain significantly easier due to the improved signal-to-noise ratio. the improved signal-to-noise ratio.
The filters are adjusted via the The filters are adjusted via the system control. In position OFF the low pass system control. In position OFF the low pass
filter has no effect. With the filter switched on, its influence on both amplitude filter has no effect. With the filter switched on, its influence on both amplitude
and phase of the measurement signal has to be
and phase of the measurement signal has to be taken into consideration. The taken into consideration. The
filter characterist filter characteristics as well as rules of thumb for using them are ics as well as rules of thumb for using them are provided in provided in
sec sec tio tio n n 4.2 4.2 .2 .2 ..
F FM r M ra ad dio io:: m ma ax x. f . fre requ que enc ncy d y dev evia iattio ion 7 n 75kH 5kHz z,,
max. modulation frequency 53 max. modulation frequency 53kHz (Ste kHz (Stereo) reo)
Vi Vibro brome meter ter:: ma max. x. fre freque quency ncy dev deviat iation ion 32MHz, 32MHz,
max. modulation frequenc max. modulation frequency 1.5 y 1.5 MHz MHz