Simultaneous and wireless measurement of CO2 and NO2 using a saw reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) based micro-sensor was developed on a 41deg YX LiNbO
₃
piezoelectric substrate for simultaneous measurement of CO
₂
and NO
₂
. The developed sensor consists of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, CO
₂
sensitive film (Teflon AF 2400), and NO
₂
sensitive film (Indium Tin Oxide). The coupling of mode (COM) modeling was conducted to determine optimal device parameters prior to fabrication. According to simulation results, the device was fabricated and then wirelessly measured using the network analyzer. The measured reflective coefficient S
₁₁
in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. From the experimental results, high selectivity and sensitivity were observed in each concentration of gases. The obtained sensitivity was 2.12deg/ppm for CO
₂
and 55.4deg/ppm for NO
₂
. Temperature compensation was performed during sensitivity evaluation process.