'
')XQFWLRQDO'HVFULSWLRQRIWKH&RQWUROOHU
' 2SHUDWLQJ3ULQFLSOH
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 interferome-
ter 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 devia-
tions occur in the vibrometer which make significantly higher demands on the
demodulators:
In the OFV-3001 controller, different respectively optimally adapted modula-
tors are switched on in the individual velocity measurement ranges. A maxi-
mum 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 condition-
ing are carried out internally.
' /RZ3DVV)LOWHU
The signal generated by the FM demodulator always contains spurious RF
components and its noise bandwidth corresponds with the maximum fre-
quency 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 characteris-
tics 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 section 4.2.2.
FM radio: max. frequency deviation 75kHz,
max. modulation frequency 53kHz (stereo)
Vibrometer: max. frequency deviation 32MHz,
max. modulation frequency 1.5MHz
To Next Page
Loading...
Need help?
General
