Frequency-modulated enhancement of microwave resonator sensing

We use the Pound-Drever-Hall (PDH) technique to characterize the frequency stability of a microwave-frequency surface acoustic wave (SAW) resonator-based sensor. The multi-mode acoustic resonator is integrated in a notch geometry with a transmission line, all fabricated on Y-cut lithium niobate. We measure the amplitude and phase of the resonator transfer function and the PDH signal across the resonator full spectral range. We use these measurements to emphasize the differences between the PDH measurement and a standard Phase-Locked Loop (PLL) technique. As compared to a PLL, we demonstrate that PDH is insensitive to phase error and exhibits a reduced Allan deviation of the center frequency measurement, in each case by up to an order of magnitude. The method rejects spurious effects and background frequency drift, demonstrating the enhancements possible with PDH-based measurements, which can be realized in a wide range of microwave-frequency resonator-based sensors and devices.