A direct digital synthesis system for surface acoustic wave sensors

Current designs for surface acoustic wave sensor system electronics are typically based on SAW oscillators, phase detectors, or phase-locked loops to measure changes in SAW velocity. The advantage of SAW oscillators is a high resolution frequency output as compared to phase systems which are more stable and can more easily provide amplitude information. Phase-locked loops offer advantages of both the SAW oscillator and phase detection systems but have the disadvantages of a fixed frequency range and the need for frequency counting circuitry. The objectives of this work were to study the performance of a direct digital synthesis based phase-locked loop system with the advantages of a programmable center frequency, elimination of the need for frequency counting circuitry, and tolerance to large SAW sensor insertion loss. The DDS system had a resolution of 4 Hz and a range of 80 to 120 MHz with 100 MHz SAW sensors in humidity and temperature sensing applications. This indicates that the DDS based PLL is a practical electronic system for SAW sensors.