Campbell Scientific AVW200-series Vibrating Wire Interfaces - Manual (Page 17)

AVW200-series 2-Channel Vibrating Wire Spectrum Analyzer Modules
TABLE 2-2. AVW200 Measurement Outputs
Output Units Description
Resonant Frequency Hz Frequency of the peak response
Response Amplitude mV rms Amplitude of the peak response 1
Signal-to-Noise Ratio unitless Response amplitude divided by
amplitude of largest noise candidate 1
Noise Frequency Hz Frequency of largest noise candidate 1
Decay Ratio unitless Ending time-series amplitude divided
by the beginning time-series
amplitude 1
Thermistor Resistance
(see Section 2.2)
Ohms On-gage thermistor resistance 2
Notes:
1. Use for measurement diagnostics.
2. Optional output, not measured if Therm50_60Hz is set to 0.
When using firmware version Std.04 (or higher) and the Response Amplitude
diagnostic is measured as less than 0.01 mV rms (10 microvolts), the Resonant
Frequency reading will be modified to warn the user about the occurrence of
low signal strength amplitudes. If SDI-12 is used to communicate with the
AVW200-series device, the frequency will be given as -9,999,999 under those
conditions. For all other communication methods, the frequency will be given
as NAN (not-a-number) when experiencing this low signal strength condition.
If the user desires the frequency to be returned as NAN for a higher (i.e., more
pessimistic) threshold than 0.01 mV, this can be done by using an optional
parameter in the AVW200 CRBasic Instruction. See Section 6.1, AVW200
Instruction, for details about how this can be done.
The Resonant Frequency reading is also used to warn the user when there is an
invalid voltage supply in the hardware of the device (firmware Std.04 and
higher). If an internal calibration factor is outside of the expected range, then
the value of -555,555 is returned for the frequency measurement. This
indicates to the user that there is a hardware issue on the device which requires
a factory examination and/or repair. Contact Campbell Scientific for
instructions when this value is given as the Resonant Frequency reading.
FIGURE 2-2 (a representative output from the Device Configurator’s
Troubleshooter) illustrates the AVW200 measurements. The bottom graph
shows the raw time series data recorded from a vibrating sensor after the sensor
has been excited with the frequency-swept voltage signal, and the top graph
shows the spectrum after the AVW200 has applied the fast Fourier transform
(FFT). In addition to the wire’s resonant frequency, the spectrum shows the
response amplitude, noise amplitude, and noise frequency. The AVW200
computes the signal-to-noise ratio diagnostic by dividing the response
amplitude by the noise amplitude. The AVW200 computes the decay ratio
diagnostic from the time series ending amplitude divided by the beginning
amplitude shown on the bottom graph in FIGURE 2-2.
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