A novel scheme called dual-frequency interrogation and hierarchical evaluation (DFI-HE) is proposed to resolve integer ambiguity of phase difference and obtain precise time delay between reflectors.  With DFI-HE, only two reflectors are needed as the sensing elements for SAW reflective delay-line sensors.  Additionally, DFI-HE has the advantage of fast ambiguity resolution to enable both a wide range and high accuracy sensing.  The basic principle of sensing with DFI-HE is elaborated, and simulations and temperature sensing experiments are carried out to verify the proposed scheme, based on a SAW reflective delay-line temperature sensor with two reflectors fabricated on 128° YX-LiNbO3 substrate. 
DFI-HE
Dual-Frequency Interrogation and Hierarchical Evaluation Scheme for SAW Reflective Delay-Line Sensors.The resolution of phase difference ambiguity is critically important for simultaneously achieving wide range and high accuracy sensing with surface acoustic wave (SAW) reflective delay-line sensors.  With DFI-HE, only two reflectors are needed as the sensing elements for SAW reflective delay-line sensors. 
SAW Reflective Delay-Line Sensors
A novel scheme called dual-frequency interrogation and hierarchical evaluation (DFI-HE) is proposed to resolve integer ambiguity of phase difference and obtain precise time delay between reflectors.  With DFI-HE, only two reflectors are needed as the sensing elements for SAW reflective delay-line sensors.  Additionally, DFI-HE has the advantage of fast ambiguity resolution to enable both a wide range and high accuracy sensing.  The basic principle of sensing with DFI-HE is elaborated, and simulations and temperature sensing experiments are carried out to verify the proposed scheme, based on a SAW reflective delay-line temperature sensor with two reflectors fabricated on 128° YX-LiNbO3 substrate. 
DFI-HE
A novel scheme called dual-frequency interrogation and hierarchical evaluation (DFI-HE) is proposed to resolve integer ambiguity of phase difference and obtain precise time delay between reflectors.  With DFI-HE, only two reflectors are needed as the sensing elements for SAW reflective delay-line sensors.  Additionally, DFI-HE has the advantage of fast ambiguity resolution to enable both a wide range and high accuracy sensing.  The basic principle of sensing with DFI-HE is elaborated, and simulations and temperature sensing experiments are carried out to verify the proposed scheme, based on a SAW reflective delay-line temperature sensor with two reflectors fabricated on 128° YX-LiNbO3 substrate. 
DFI-HE
The basic principle of sensing with DFI-HE is elaborated, and simulations and temperature sensing experiments are carried out to verify the proposed scheme, based on a SAW reflective delay-line temperature sensor with two reflectors fabricated on 128° YX-LiNbO3 substrate. 
128° YX-LiNbO3 substrate