Simulation of the two-point interrogation strategy—each pulse is generated as a rectangular 30 μs long window—probing a resonator fitted with a Butterworth-van Dyke model (C 0 = 1.7 pF, C 1 = 0.56 F, L 1 = 241 μH, R 1 = 73 Ω).
Experimental setup: A music tuning fork is fitted with a quartz strain gauge resonator interrogated through a wireless link. The interrogation algorithm is implemented in the flexible digital interrogation unit.
Top right: Time domain records of the DAC output of the wireless acoustic sensor reader probing a quartz resonator strain gauge bound to a music tuning fork: the voice coil driving voltage at 442 Hz (maximum response amplitude) is increased from 0 to 1.8 V pp . As described in the text, the voltage-to-frequency conversion factor is 4.2 × 10−6 V/Hz, so that the 20 mV amplitude indicates a frequency shift amplitude of 362 Hz or a stress variation at the quartz strain gauge bound at the base of one of the prongs of about 650 kPa. Left: Fourier transform of the interrogation unit DAC voltage, sampled by a digital oscilloscope at 25 ksamples/s. The strain gauge signal is visible at 442 Hz—the driving voltage at resonance of the tuning fork (set at 441.737 Hz)—with a magnitude dependent on the driving signal amplitude, and vanishing when no excitation signal is applied (0 V). The interrogation unit sampling rate is visible at f s /2 = 2412 Hz, with a power independent on the voice coil driving voltage.
Top: Two-minute measurement of the frequency output of the resonator interrogation unit running the two-point algorithm, here with a digital communication of the measurement through an asynchronous serial link, yielding a rather slow measurement rate of 135 Hz. Bottom: Allan deviation of the resulting dataset, exhibiting a sub-kHz standard deviation (two-point standard deviaton) and sub-100 Hz Allan deviation at 1 s (135 sample averages) integration time.
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