A micromechanical bandpass filter with adjustable bandwidth and bidirectional control of centre frequency

This paper introduces a tunable, narrow-band, micromechanical filter whose bandwidth and centre frequency can be adjusted independently. The filter is made of two micro-resonators that are electrostatically coupled using a middle electrode. A low coupling strength results in nearly constant bandwidth while one tunes the centre frequency bi-directionally by applying a DC voltage to the coupling electrode. On the other hand, the bandwidth of the filter is independently modified by applying axial stress to one of the resonators, without affecting the signal attenuation through the filter. Analytic and numerical models for the behaviour of the filter are also presented. Test devices with a centre frequency of about 300 kHz were fabricated in a standard micromachining process. Experimental results support the design principle and validity of the proposed models.