Quadrature oscillator with pre-distorted waveforms for application in MEMS-based mechanical spectrum analyser

An AC-operated capacitive accelerometer with electrostatic force feedback is employed for direct mechanical spectrum analysis. A suitable electrostatic AC field is used to make the accelerometer selectively sensitive to only the coherent mechanical frequency component. By sweeping the frequency of the drive voltage over a selected range, the mechanical (vibration) spectrum is analysed in the mechanical domain. Operation requires the simultaneous generation of predistorted sine- and cosine-like waveforms. A DDFS with DAC, based on a specially scaled resistive string, is designed, fabricated in CMOS and tested. Spurious free dynamic range is 22 dB over a frequency range of DC up to 1 kHz. The spectral performance is comparable to FFT-based systems for spectral analysis on a time series supplied by a conventional accelerometer, while the overall system features a reduced complexity, simple spectral in-zooming and potential for low power consumption.