Noise Floor Reduction in Frequency Delta-Sigma Modulation Microphone Sensors

Abstract

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.

A Novel Displacement Sensor Based on a Frequency Delta-Sigma Modulator and its Application to a Stylus Surface Profiler

“…A significant implication in this figure is that this sensor can measure the displacement with very wide dynamic range. An extremely low total noise power of -143 dBFS was demonstrated with a sampling rate of approximately 12.6 GHz [14]. This indicates that this sensor can measure the frequency shift as small as 22 Hz.…”

“…Figure 1 illustrates the FDSM microphone we demonstrated in the previous paper [14] together with the block diagram of the FDSM. The variable frequency oscillator consists of a cylindrical cavity resonator and a gain block.…”

“…Figure 5 shows the photographs of the fabricated cavity resonator oscillator. The cavity resonator was the same one used in [14]. It was fabricated from oxygen-free, highconductivity copper to achieve a high Q-factor, which is essential for noise floor reduction.…”

“…Moreover, it is robust against external noises, and eliminates noises added during signal amplification. We have recently proposed some sensors based on the FDSM concept [10]- [14]. Among these sensors, the microphone sensor based on a microwave oscillator using a cavity resonator demonstrated a very low noise power of -143 dBFS (decibel fullscale) for 96 kHz band width [11], [14].…”

“…We have recently proposed some sensors based on the FDSM concept [10]- [14]. Among these sensors, the microphone sensor based on a microwave oscillator using a cavity resonator demonstrated a very low noise power of -143 dBFS (decibel fullscale) for 96 kHz band width [11], [14]. This sensor uses a cylindrical cavity resonator for the oscillator, where one end of the resonator is replaced by a thin metal diaphragm which moves in response to sound pressure.…”