Maezawa Koichi, Mori Masayuki, Andoh Hiroya
Faculty of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan.
Department of Information and Computer Engineering, National Institute of Technology, Toyota College, 2-1 Eiseicho, Toyota 471-8525, Japan.
Sensors (Basel). 2021 May 16;21(10):3470. doi: 10.3390/s21103470.
Frequency delta-sigma modulator (FDSM) employing a variable frequency oscillator is a novel replacement of the classical delta-sigma modulators. This is advantageous for application to sensors, because an ADC can be intrinsically integrated with the sensors. We have already proposed to use this technique to various sensors. However, the signal-to-noise ratio was significantly degraded by noise floor, in the previous papers. In this paper, we have investigated the origin of the noise floor in the FDSM microphone sensors as a promising example. It was demonstrated that improving the phase noise of the oscillator can drastically reduce the noise floor. For this reduction we improved the Q-factor of the cavity resonator, and the design of the oscillator circuit. With these improvements, the phase noise, and, hence, the noise floor, were improved by approximately 40 dB. In addition, we obtained an SNR of 57 dB for 114 dBSPL sound input with 96 kHz bandwidth, which corresponds to the dynamic range of 87 dB for maximum 140 dBSPL. A much larger dynamic range of around 120 dB is expected by increasing the sampling rate and decreasing the Al diaphragm thickness. These results also indicate the promise of the FDSM to varieties of physical sensors.
采用可变频率振荡器的频率增量 - 累加调制器（FDSM）是经典增量 - 累加调制器的一种新型替代方案。这对于传感器应用具有优势，因为模数转换器（ADC）可以与传感器进行本质集成。我们已经提议将该技术应用于各种传感器。然而，在之前的论文中，信噪比因本底噪声而显著降低。在本文中，我们以FDSM麦克风传感器为例，研究了本底噪声的来源。结果表明，改善振荡器的相位噪声可以大幅降低本底噪声。为了降低本底噪声，我们提高了腔谐振器的品质因数以及振荡器电路的设计。通过这些改进，相位噪声以及本底噪声提高了约40 dB。此外，对于96 kHz带宽、114 dBSPL的声音输入，我们获得了57 dB的信噪比，这对应于最大140 dBSPL时87 dB的动态范围。通过提高采样率和减小铝膜厚度，预计可实现约120 dB的更大动态范围。这些结果也表明了FDSM在各种物理传感器方面的应用前景。
