Time reversal: a flexible approach for identifying and measuring surface acoustic wave delay lines acting as wireless, passive sensors 1
T. R´etornaz1,2 , N. Chr´etien1,2 , J.-M Friedt1 , G. Martin2 , S. Ballandras1,2 FEMTO-ST, UMR CNRS 6174, Univ. of Franche Comt´e, Besan¸con, France 2 SENSEOR SAS, Besan¸con, France Email:
[email protected]
Context: surface acoustic wave (SAW) devices used as passive sensors interrogated through a wireless link. Objective: develop interrogation strategies compatible with identification and physical quantity measurement + anti-collision. Impulse response of two CTR SAW device 10
Cross-correlation Oscilloscope
Waveform generator
Acoustic sensor esnopseR
CTR17 CTR44+5
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Experimental setup: arbitrary waveform generator used to emit the time-reversed inverse Fourier transform of the spectra (measured or simulated). Demonstration at 2450 and 868 MHz Left: experimental setup. Right: impulse response of two 2450 MHz delay lines used for this demonstration.
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Experimental data
Time-reversal for SAW delay line identification Acoustic delay lines act as correlators: if incoming signal matches transfer function, a compressed cross-correlation pulse is returned Assuming orthogonal transfer functions, a single delay line should return a pulse to a probe function. Interrogation de la puce CTR17
Interrogation de la puce CTR44
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time (s)
Simulation results
0.0003 Echo 17 Echo 44
Echo 44 Echo 17
0.0003 0.0002
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Time-reversal for measurement (application to temperature) Physical quantity variation → velocity variation → time stretching Sweep sampling clock of arbitrary waveform generator → track sampling frequency for which cross-correlation is maximized, “hardware” (acoustic) cross correlation ⇒ fast measurement (no digital signal processing)
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Sum of abs(cross correlation) (a.u.)
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88°C
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70°C -0.0002
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Time-dependent returned signal when probing a delay line with the timereversed transfer function (max) and that of another device (1 bit difference).
Issue: how to select orthogonal codes ? Simulation tools appropriate to model all possible combinations of open and shorted mirrors ⇒ case of a 8-mirror delay line cout1=max(xcorr)
cout2=sidelobes
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Sampling frequency (Hz)
Cross correlation at different temperatures, and probing different chips Optimal sampling rate (Hz)
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Heating (oven) 50
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P1
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P2
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Cost function: 1. maximize returned power (maximum number of mirrors) 2. maximize auto-correlation function time (a.u.) 3. minimize cross-correlation with other chips Tracking sampling rate maximizing cross-correlation as sensor is heated. 4. minimize sidelobes Conclusion: time-reversal is used for identifying a single device within a population (identification & anti-collision) and measure the physical quantity changing the acoustic velocity (temperature) Perspectives: improve orthogonality & conversion of the laboratory experiment to an embedded interrogation electronics
