# US 8,786,384 B2 · Self-matched band reject filter > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,786,384 B2 - Document ID: US8786384B2 - Status: Granted - Kind: B2 - Application number: 13/125,362 - Filed: 2009-11-02 - Published: 2011-09-29 - Granted: 2014-07-22 - Earliest claimed: 2008-10-31 - Assignees: Apple Inc. - Applicants: Chun-Yun Jian - Inventors: Chun-Yun Jian - Web page: https://patentgrep.com/patent/US8786384B2 ## CPC - H04B 1/50: using different frequencies for the two directions of communication - H03H 9/542: {including passive elements (H03H9/545 takes precedence)} - H03H 9/547: {Notch filters, e.g. notch BAW or thin film resonator filters} - H03H 9/6409: {SAW notch filters} - H04B 1/48: in circuits for connecting transmitter and receiver to a common transmission path, e.g. by energy of transmitter {(H04B1/46 takes precedence)} - H04L 25/0278: {Arrangements for impedance matching} - Y10T 29/4902: Electromagnet, transformer or inductor - H04B: TRANSMISSION - H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P) - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) - Y10T: TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION ## Abstract The present application describes a radio frequency band reject filter including an input port, an output port, a plurality of acoustic resonators and an inductor for matching the impedance of the plurality of acoustic resonators. The inductor is positioned within the band reject filter in respect of the plurality of acoustic resonators such that a static capacitance between the input port and the inductor is substantially equivalent to a static capacitance between the output port and the inductor. The plurality of acoustic resonators may be a plurality of parallel resonators, a plurality of series resonators or a combination of series and parallel resonators. The radio frequency band reject filter is fabricated using any of surface acoustic wave (SAW) technology, thin film bulk acoustic resonator (FBAR) technology, and bulk acoustic wave (BAW) technology. ## Claims ### Claim 1 A radio frequency band reject filter comprising: an input port; an output port; a plurality of acoustic resonators coupled between the input port and the output port, wherein the plurality of acoustic resonators comprise a first plurality of acoustic resonators and a second plurality of acoustic resonators; and an inductor positioned within the band reject filter, and comprising: a first terminal coupled to ground; and a second terminal coupled to a common node between the first plurality of acoustic resonators and the second plurality of acoustic resonators, wherein a static capacitance between the input port and the common node is substantially equivalent in value to a static capacitance between the output port and the common node; wherein a combined impedance of the inductor and the plurality of acoustic resonators is equivalent in value to at least one of: an impedance of the radio frequency band reject filter from the input port to ground; or an impedance of the radio frequency band reject filter from the output port to ground. ### Claim 2 (depends on 1) The radio frequency band reject filter of claim 1 wherein the plurality of acoustic resonators are one of: a plurality of parallel acoustic resonators; a plurality of series acoustic resonators; and a combination of series acoustic resonators and parallel acoustic resonators. ### Claim 3 (depends on 2) The radio frequency band reject filter of claim 2, wherein the combination of series acoustic resonators and parallel acoustic resonators comprises: N, where N>2, series acoustic resonators; and M, where M≧2, parallel acoustic resonators. ### Claim 4 (depends on 3) The radio frequency band reject filter of claim 3, wherein N is an even number and there are N/2 series acoustic resonators between the input port and the common node and N/2 series acoustic resonators between the output port and the common node. ### Claim 5 (depends on 3, 4) The radio frequency band reject filter of claim 3 or claim 4, wherein M is an even number and there are M/2 parallel acoustic resonators between the input port and the common node and M/2 parallel acoustic resonators between the output port and the common node. ### Claim 6 (depends on 1) The radio frequency band reject filter of claim 1 wherein the radio frequency band reject filter is fabricated using one of: surface acoustic wave (SAW) technology; thin film bulk acoustic resonator (FBAR) technology; and bulk acoustic wave (BAW) technology. ### Claim 7 (depends on 1) The radio frequency band reject filter of claim 1 wherein the inductor is a short stub on-chip inductor. ### Claim 8 (depends on 7) The radio frequency band reject filter of claim 7 wherein the inductor has an inductance that is equal to or less than 0.1 nH. ### Claim 9 (depends on 1) The radio frequency band reject filter of claim 1 cascaded with one or more other radio frequency band reject filters. ### Claim 10 (depends on 9) The radio frequency band reject filter of claim 9, wherein at least one of the one or more other radio frequency band reject filters has an inductor for matching impedance of the respective at least one other radio frequency band reject filters. ### Claim 11 (depends on 1) The radio frequency band reject filter of claim 1, wherein the plurality of acoustic resonators comprise a combination of series acoustic resonators and parallel acoustic resonators, and wherein each series acoustic resonator of the series acoustic resonators and each parallel resonator of the parallel acoustic resonators is formed by a set of interdigital electrodes extending from a pair of parallel conductive elements. ### Claim 12 (depends on 11) The radio frequency band reject filter of claim 11, wherein for the parallel acoustic resonators, at least one of the pair of parallel conductive elements is coupled to ground. ### Claim 13 (depends on 11) The radio frequency band reject filter of claim 11, wherein for at least one series acoustic resonator or at least one parallel acoustic resonator, or both, at least one of the pair of parallel conductive elements forming the respective acoustic resonator is coupled to the inductor. ### Claim 14 (depends on 13) The radio frequency band reject filter of claim 13, wherein the inductor is a short stub element located between the at least one of the pair of parallel conductive elements and ground. ### Claim 15 (depends on 1) The radio frequency band reject filter of claim 1 wherein the inductance of the inductor can be fabricated accurately enough that external matching circuits are not used with the filter. ### Claim 16 (depends on 1) A telecommunication base station comprising: at least one antenna; transmit circuitry configured for modulating one or more carrier signals having a desired transmit frequency or frequencies; receiving circuitry configured for receiving a radio frequency signal bearing information from one or more remote transmitters; a baseband processor configured for: processing a received signal received by the receiving circuitry; and encoding a signal for transmission by the transmit circuitry; at least one of the transmit circuitry or receiving circuitry comprising the radio frequency band reject filter of claim 1. ### Claim 17 (depends on 1) A telecommunication wireless terminal comprising: at least one antenna; transmit circuitry configured for modulating one or more carrier signals having a desired transmit frequency or frequencies; receiving circuitry configured for receiving a radio frequency signal bearing information from one or more remote transmitters; a baseband processor configured for: processing a received signal received by the receiving circuitry; and encoding a signal for transmission by the transmit circuitry; at least one of the transmit circuitry or receiving circuitry comprising the radio frequency band reject filter of claim 1. ### Claim 18 (depends on 1) A duplexer comprising the radio frequency band reject filter of claim 1. ### Claim 19 A method of impedance matching during the fabrication of a radio frequency band reject filter comprising: fabricating an input port; fabricating an output port; fabricating a plurality of acoustic resonators between the input port and the output port, comprising fabricating a first plurality of acoustic resonators and a second plurality of acoustic resonators; fabricating an inductor that comprises a first terminal configured to couple to ground, and further comprises a second terminal; wherein fabricating the inductor comprises coupling the second terminal of the inductor to a common node between the first plurality of acoustic resonators and the second plurality of acoustic resonators, wherein a static capacitance between the input port and the common node is substantially equivalent in value to a static capacitance between the output port and the common node; and wherein a combined impedance of the inductor and the plurality of acoustic resonators is equivalent in value to at least one of: an impedance of the radio frequency band reject filter from the input port to ground; or an impedance of the radio frequency band reject filter from the output port to ground. ### Claim 20 (depends on 19) The method of claim 19, wherein fabricating the inductor comprises fabricating a short stub on-chip element that has a desired inductance. ### Claim 21 (depends on 20) The method of claim 20 wherein fabricating a short stub on-chip element that has a desired inductance comprises controlling at least one of the length, width, and thickness of the short stub on-chip element.