Sub-10 μHz/ √ Hz measurement instrumentation for 140-dB DR frequency-modulated MEMS sensors

The field of frequency modulated (FM) microelectromechanical system (MEMS) sensors is emerging as an alternative to conventional amplitude modulated (AM) architectures, especially because of new applications requirements on extended full-scale and low noise, which turn into a wide dynamic range (DR). As a consequence, a need for characterization tools dedicated to this novel class of transducers is arising. This work presents the development of the key building blocks of an instrument for the characterization of FM sensors, including an integrated circuit and external board-level components. In the instrument design, the focus is set on versatility in terms of input range frequencies and on guaranteeing a dynamic range in the order of 140 dB. The instrument core is based on a frequency to digital converter, implemented through a type-II phase-locked-loop with a differentiator implementing a period meter. Low quantization noise is obtained using a high frequency (100 MHz) time reference, while versatility is guaranteed at the same time by having programmable parameters within the integrated circuit. The instrument is tested across the 27 kHz to 260 kHz input range and demonstrates sub-10 μHz/ √Hz frequency noise density in the range from 10 Hz to 100 Hz offset from the carrier and up to 100-Hz full scale. The instrument is available for testing and use to whoever is interested in its features.