_applied_  
_sciences_  
_Article_  
Development of Wireless and Passive SAW Temperature Sensor with Very High Accuracy  
[PERSON] 1,2, [PERSON] 1, [PERSON] 1,*, [PERSON] 1, [PERSON] 1, [PERSON] 1,2,*, [PERSON] 1,2,  
[PERSON] 3 and [PERSON] 3 4  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
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Footnote 7:  
the transceiver and transponder [13]. The electromagnetic (EM) signal with characteristic frequency was emitted from the transceiver, received by the SAW transponder through the antenna, and transferred into SAW propagating along the piezoelectric substrate by the IDTs. The reflected SAW from the reflectors was retransmitted into the EM signal received by the transceiver. The changes in temperature modulate the SAW velocity owing to the thermal expansion effect of the piezoelectric substrate and corresponding shift in sensing device frequency (\(\Delta\)f) or phase (\(\Delta\)\(\Phi\)) is extracted as the sensing signal [14; 15; 16]. The pattern for the temperature sensor chip usually employs a one-port resonator or reflective delay line [17], as shown in Figure 1. The one-port resonator (Figure 1a) is composed of interdigital transducers (IDTs) and two reflectors positioned on each side of the IDTs. The reflective delay line pattern consists of an IDT and several reflectors positioned along the SAW propagation path. The temperature sensor using the one-port resonator pattern had been successfully applied to the health monitoring of power equipment. However, it is not easy to further improve the accuracy because of the limitation of the frequency sweep in the transceiver. For example, the sensitivity of a resonant pattern sensor is 1 KHz/\({}^{\circ}\)C. To achieve an accuracy of 0.1 \({}^{\circ}\)C, the frequency sweep of the receiver should be below 100 Hz, but it is difficult to achieve. The reflective delay line pattern (Figure 1b) provides a way to improve the sensing accuracy by decoupling the phase signal, attracting more interest for high-accuracy temperature measurement. Corresponding temperature sensors have been frequently reported in recent literature. [PERSON] et al. proposed a well calibration towards the SAW sensor, which allows high accuracy better than 0.5 \({}^{\circ}\)C [18]. [PERSON] et al. conducted optimization towards the SAW sensing device with a multi-reflector delay line pattern and satisfactory accuracy of \(\sim\)0.8 \({}^{\circ}\)C was achieved [19]. In addition, [PERSON] et al. proposed a multi-iteration-enhanced two-point simple moving average (MI-2P-SMA) method to reduce the measurement error. The results showed the relative error of the original temperature measurement data in the testing range of 40\(\sim\)60 \({}^{\circ}\)C is only \(\pm\)0.15%, while the unprocessed data are \(-\)3.4% to 1.87% [20]. [PERSON] et al. performed the median filtering algorithm to eliminate the noise of measurement data, and the temperature measurement error is reduced to \(\pm\)0.6 \({}^{\circ}\)C [21]. Figure 1: (**a**) One-port resonator pattern, (**b**) Reflective delay line pattern, (**c**) COM model for IDT.