Design and analysis of micro-electro-mechanical variable capacitors for high-frequency filter applications

This dissertation investigates the design of polysilicon surface micro-machined varactors and their applications to RF and VHF filter design. A previously proposed multi-finger differential topology is adopted to micro-machined variable capacitors and optimized to increase the achievable quality factor.; Approximate analytical expressions for capacitor quality factors is derived and used to maximize the achievable quality factor. It is shown that this topology can be used to design high quality factor capacitors with only one available low-resistively layer. A simple analytical model of the zipper-action variable capacitor tuning is proposed.; Experimental parallel-plate and zipper-action varactors based on this topology have been designed, fabricated in the commercial MUMPs polysilicon surface micro-machining process, and measured. The parallel-plate variable capacitors tune from 3.5 pF to 6.5 pF and have the quality factor of 14 at 1 GHz. The zipper-action variable capacitors tune from 3.4 pF to 4.9 pF and have the quality factor of 36 at 1 GHz. The fractal topology has been proposed to increase the achievable quality factor and a zipper-action fractal variable capacitor has been fabricated and characterized.; An active LC second-order band-pass filter tunable with micro-machined varactor has been designed. The filter core was fabricated in 0.35 μm CMOS process. Bonding wires were used to realize high-Q inductors and to connect the varactor that was fabricated on a separate die to the CMOS circuit. The filter center frequency is tunable from 914 MHz to 1089 MHz. The quality factor is better than 15.8 across the tuning range. The filter core and buffer consume 4.9 mA and 16.6 mA, respectively, from a 3.3V supply and has spurious-free dynamic range of 53dB.; A transconductance-C biquadratic filter has also been fabricated in 0.35 μm CMOS process. This filter realizes the second-order Butterworth low-pass response which is tuned with two micro-machined variable capacitors. The cut-off frequency of this filter tunes from 92.8 MHz to 102.8 MHz. This filter consumes 19.5 mA from a 3.0 V supply and has the spurious-free dynamic range of 49 dB.