A digital PLL circuit for resonator sensors

Resonant mechanical sensors are powerful tools for measuring, e.g., physical properties of fluids. Suitable systems for real-time monitoring of a sensor's resonance behavior are, e.g., oscillator circuits and phase-locked loop circuits. In this contribution, we present a novel specific digital phase-locked loop circuit with the ability to determine damping, as well as resonance frequency, and providing automatic compensation of spurious parallel capacitances of the sensor. Instead of a phase-frequency detector, the system measures phase and amplitude of the sensor signal with a 16-bit ADC and uses this information to control a numerically-controlled oscillator. Due to the used 4-channel digital synthesizer, the sensor can be excited with up to three frequencies at the same time facilitating enhanced performance in terms of simultaneous measurement of higher order resonances and compensation of spurious capacitances.