This paper reports audio frequency electrothermal filter design using micromachined thermally-isolated diaphragm structures. Generally, large capacitance is needed for low frequency filters. In this paper, micromachined electrothermal structures have been used to obtain equivalent large capacitance suitable for audio frequency range filter design. Low frequency filter operation has been achieved by a thermal time constant ($\cong 10msec$) in the electrothermal structures. Design parameters with respect to filter characteristics have been analyzed and design guidelines have been identified. Electrothermal filters using electrothermal structures with driving circuitry are realized. Measured responses show that characteristic frequency can be electrically tuned ranging from 30Hz to 300Hz. Canceling scheme of the thermal conductance, is proposed and is adapted to improve electrothermal filter characteristics. According to the proposed design guidelines, electrothermal filters ranging from 25Hz to 2.5kHz have been designed and simulated.
