Time reversal: a flexible approach for identifying and measuring surface acoustic wave delay lines acting as wireless, passive sensors
T. R´etornaz1,2, N. Chr´etien1,2, J.-M Friedt1, G. Martin2, S. Ballandras1,2
1FEMTO-ST, UMR CNRS 6174, Univ. of Franche Comt´e, Besan¸con, France
2SENSEOR SAS, Besan¸con, France Email: [email protected]
Context: surface acoustic wave (SAW) devices used aspassive sensorsinterrogated through a wireless link.
Objective:develop interrogation strategies compatible withidentificationand physical quantitymeasurement+anti-collision.
Oscilloscope
Waveform
Cross-correlation
Acoustic
Response generator
sensor
Experimental data Simulation results
Experimental setup: arbitrary wave- form generator used to emit the time-reversed inverse Fourier trans- form 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 demonstra-
tion. -4
-2 0 2 4 6 8 10
1e-06 1.5e-06 2e-06 2.5e-06 3e-06
Magnitude (U.A.)
time (s) Impulse response of two CTR SAW device
CTR17 CTR44+5
Time-reversal for SAW delay line identification
Acoustic delay lines act ascorrelators: 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.
-0.0003 -0.0002 -0.0001 0 0.0001 0.0002 0.0003 0.0004
0 20000 40000 60000 80000 100000
Interrogation de la puce CTR17
Echo 17 Echo 44
-0.0003 -0.0002 -0.0001 0 0.0001 0.0002 0.0003
0 20000 40000 60000 80000 100000
Interrogation de la puce CTR44
Echo 44 Echo 17
Time-dependent returned signal when probing a delay line with the time- reversed transfer function (max) and that of another device (1 bit difference).
Issue: how to select orthogonal codes ?
Simulation tools appropriate to modelall possible combinations of open and shorted mirrors⇒case of a 8-mirror delay line
cout1=max(xcorr)
50 100 150 200 250
50 100 150 200 250
cout2=sidelobes
50 100 150 200 250
50 100 150 200 250
P1
50 100 150 200 250
50 100 150 200 250
P2
50 100 150 200 250
50 100 150 200 250
Cost function:
1. maximize returned power (maximum number of mirrors) 2. maximize auto-correlation function
3. minimize cross-correlation with other chips 4. minimize sidelobes
Time-reversal for measurement (application to temper- ature)
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 measure- ment (no digital signal processing)
70°C 88°C
Sum of abs(cross correlation) (a.u.)
Sampling frequency (Hz)
Cross correlation at different temperatures, and probing different chips
time (a.u.) Heating (oven)
Optimal sampling rate (Hz)
Tracking sampling rate maximizing cross-correlation as sensor is heated.
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
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