Campbell Scientific CR6 CR6 Measurement and Control System - Page 352
Response, Amplitude, Noise Amplitude, Response Amplitude, Resonant Frequency, VibratingWire
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Section 8. Operation Signal-to-Noise Ratio Diagnostic The spectrum data shown in the figure Unconditioned Spectrum-Analysis Data (p. 351) are derived from the time-series data through an FFT (p. 495). These data emphasize the amplitude of the natural-resonant frequency, labeled Response Amplitude, and the amplitude of noise frequencies, labeled Noise Amplitude. The signal-to-noise ratio is calculated as follows: Signal-to-Noise Ratio = Response Amplitude / Noise Amplitude Low-Signal-Strength Amplitude Warning When the response amplitude is measured as less than 0.01 mV RMS, the Resonant Frequency value reports NAN indicating that low signal-strength amplitudes have occurred. The 0.01 mV threshold can be modified in the VibratingWire() instruction. Low-Amplitude Warning Setting If application requires the frequency be returned as NAN for a higher (in other words, more pessimistic) threshold than 0.01 mV, use an optional parameter in the AVW200 CRBasic Instruction. See Section 6.1 for details about how this can be done. Invalid Voltage-Supply Warning A Resonant Frequency value of -555555 is an error code indicating an invalid voltage supply in the hardware of the CR6, which is an internal problem requiring factory repair. Vibrating-Wire Quality The following may improve measurement quality: • Match frequency ranges to expected frequencies • Reject noise • Minimize resonant decay • Prevent spectral leakage Match Measurement Ranges to Expected Frequencies Measurements are best when the frequency ranges of the swept excitation and of the response analysis match the range of resonant frequencies expected from the sensor. The swept and analysis ranges for specific sensors are determined using the Device Configuration Utility (p. 116) software CR6 vibrating-wire analyzer utility. Once determined, the ranges are then programmed into the CRBasic program by adjusting the BeginFreq and EndFreq parameters in the VibratingWire() instruction. Reject Noise The figures Narrow Sweep, Low Noise (p. 353) and Wide Sweep, Low Noise (p. 354) show measurement results from a sensor subjected to narrow- and wide-swept ranges. The narrow measurement was taken with a swept frequency between 200 and 2200 Hz and the wide measurement with a swept frequency between 200 and 352
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