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Tunable Inductor and Thin Film Resonator Filter with Adjustable Bandwidth

IP.com Disclosure Number: IPCOM000010290D
Original Publication Date: 2002-Nov-18
Included in the Prior Art Database: 2002-Nov-18
Document File: 3 page(s) / 101K

Publishing Venue

Motorola

Related People

Jeffrey Petsinger: AUTHOR [+3]

Abstract

A multi-turn piezoelectric bimorph tunable inductor, in which part or parts of the inductor are suspended and can be manipulated by the application of DC voltages to vary their vertical position and hence turn spacing, hence coupling between turns, hence inductance. The tunable inductor can be monolithically integrated with a piezoelectric thin film bulk acoustic wave (BAW) resonator, since both are made with similar materials, for example aluminum electrodes and aluminum nitride piezoelectric layers. This electronic device that will be useful for many wireless communication devices requiring variable inductance, but especially in software definable radio (SDR) applications requiring reconfigurable electronics.

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Tunable Inductor and

Thin Film Resonator Filter with Adjustable Bandwidth

Jeffrey Petsinger, Gregory Dunn, and William Ziemer

Abstract

A multi-turn piezoelectric bimorph tunable inductor, in which part or parts of the inductor are suspended and can be manipulated by the application of DC voltages to vary their vertical position and hence turn spacing, hence coupling between turns, hence inductance.  The tunable inductor can be monolithically integrated with a piezoelectric thin film bulk acoustic wave (BAW) resonator, since both are made with similar materials, for example aluminum electrodes and aluminum nitride piezoelectric layers.  This electronic device that will be useful for many wireless communication devices requiring variable inductance, but especially in software definable radio (SDR) applications requiring reconfigurable electronics.

Problem

BAW filters offer several advantages over surface acoustic wave (SAW) devices, including a higher power handling capability and compatibility with frequencies above 2 GHz. 

Reconfigurable radio concepts would be greatly enhanced by the ability to vary filter bandwidth on command, for example, to temporarily increase bandwidth to accommodate transmission at higher data rates, while also optimizing narrow band voice transmission for highest quality and minimal interference.  It is known that placing inductors in series with or in parallel with filter elements will increase filter bandwidth. 

Tunable inductors are known in the art, but a search of the patent and technical literature reveals only concepts for tunability that are cumbersome and not amenable to monolithic integration.  WO0161848 proposes that an inductor be divided into segments that can be short-circuited by a matrix of electrostatic MEMS switches to provide a selectable inductance.  This concept would produce an overly complex and large structure, which is not continuously tunable, may degrade the quality factor (Q) of the inductor, and moreover is not easily integrated with fabrication of a piezoelectric thin film resonator.  U.S. Patent 6,184,755 teaches a MEMS inductor fabricated with residual stresses such that the inductor curls away from the substrate upon release; the inductor is tuned by varying its temperature.  The residual stress is likely to be very difficult to control with precision in manufacturing, and thermal actuation requires significant power, which is impractical given the very low power requirements for portable el...