Digital phase detection approach and its application for AlN dual-mode differential surface acoustic wave sensing

Performance of a conventional phase detector implemented using analog circuitry is affected by, among other factors, the parameter value drifting of the analog components with environment (e.g., temperature) and time (i.e., aging). It is also adversely impacted by electromagnetic interferences likely existing in the analog circuits when the detector operates in a high frequency range. More importantly, such a detector is inflexible to compensate various nonlinear characteristics of sensors and is very difficult to realize sophisticated signal processing strategies desired for practical applications. To address issues like these, we have developed a digital system approach that uses the zero-crossing algorithm for phase detection. The phase detection limit and dynamic phase response of the system were assessed using surface acoustic wave (SAW) sensors. Our preliminary evaluation involved aluminum nitride (AlN) dual-mode differential sensors, which were fabricated on an AlN(0 0 0 1)/Al2O3(1 1 2̄ 0) thin film structure, with the SAW and shear horizontal SAW (SH-SAW) operating at approximately 243 MHz and 256 MHz, respectively. Due to the hardware constraints, the digital phase processing was carried out in an off-line fashion. Our baseline experiments without sensor indicate that the system can achieve a lower level of phase noise with the standard deviation being about 0.005◦. Experiments with the differential sensors running in the SAW and SH-SAW modes exhibit that the system can reach a phase detection limit of 0.02◦. The differential system showed small temperature coefficient at ppm level measured by varying the sensor temperature using a thermal control oven. Finally, our use of the system in measuring the response of the SH-SAW sensor to sodium chloride (NaCl) solution conductivity shows that the system is capable of performing microanalysis of liquid properties. We conclude that a real-time fully digital phase detection system can be practically achieved. © 2008 Elsevier B.V. All rights reserved.