Comparative analysis of the experience obtained from the use of SAW and BAW wireless resonator temperature sensors for surgery

Two separate approaches for temperature sensor systems with sensitive elements utilizing acoustic waves were compared. The first approach uses a pair of surface acoustic waves (SAW) resonators for the frequency range around 915 MHz. The quartz cut is 70°Y and the orientations are 70°Y-90°X for the BGS wave and 70°Y-80°X for the Rayleigh wave (the TCF difference is close to 142 ppm/°C). The second approach uses a single 5.28 MHz bulk acoustic wave (BAW) resonator made with Y-cut (the TCF is greater than 80 ppm/°C). Both types of sensors are hermetically sealed with a proprietary flip-chip technique using metallized frames at the die edges. Miniature antennas with different designs were incorporated into the packages and tested. Variants of the interrogators with a single common antenna and with separate antennas for the transmitter and for the receiver were studied. In order to increase the working distance interrogating signals were designed to maximize the reradiated energy. Interrogators use the signal processing approach based on the optimum solving rule for discrete signal classification. Extensive testing of both systems brought a lot of valuable practical information on their performance. Experiments on sheep have shown the possibility of reading information from the sensors with the implantation depth up to 18 cm. The specifications of the system stated that the working distance from the sensor location to the interrogator antenna may reach up to 60 cm. This distance to the implanted sensor required the interrogator average and impulse power levels of 10 mW and 2 W respectively. Because of the lower level of the electromagnetic wave propagation loss at lower frequencies inside living objects and of the higher resonator Q-factor the system based on BAW resonators has shown better performance in this range of distances.