Polytec PSV 300

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    

The velocity decoder determines the essential measurement properties of The velocity decoder determines the essential measurement properties of thethe

vibrometer. Velocity decoding is in principal an FM demodulation processvibrometer. Velocity decoding is in principal an FM demodulation process

which converts the velocity dependent Doppler frequency of thewhich converts the velocity dependent Doppler frequency of the

interferometeinterferometer signal into r signal into an AC voltage. The linearity and an AC voltage. The linearity and bandwidth of thebandwidth of the

demodulatdemodulator determine the accuracor determine the accuracy of the vibrometer. In contrast to FM y of the vibrometer. In contrast to FM radioradio

which works with the same modulation procedure, considerably higherwhich works with the same modulation procedure, considerably higher

frequency deviations occur in frequency deviations occur in the vibrometer which make the vibrometer which make significansignificantly highertly higher

demands on the demands on the demodulators:demodulators:

In the OFV-3001S controller, different respectively optimally adaptedIn 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 controlselected via the system control

and the internal bus. and the internal bus. At the same time, the corresponding settings on theAt 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 RFFM demodulator always contains spurious RF

components and its noise bandwidth corresponds with the maximumcomponents and its noise bandwidth corresponds with the maximum

frequency of the respective measurement range. A subsequent low pass filterfrequency of the respective measurement range. A subsequent low pass filter

suppresses the RF components and limits the noise bandwidth according tosuppresses the RF components and limits the noise bandwidth according to

its cutoff frequency. This makes a rough adaptation of the measurementits 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 inin

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 passsystem control. In position OFF the low pass

filter has no effect. With the filter switched on, its influence on both amplitudefilter 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. Thetaken into consideration. The

filter characteristfilter characteristics as well as rules of thumb for using them are ics as well as rules of thumb for using them are provided inprovided in

sec