Band reject filters

Abstract

Distributed band reject filters are disclosed. A band reject filter includes a first acoustic resonator and a second acoustic resonator, each of which has either shunt resonators adapted to resonate substantially at respective resonance frequencies defining a rejection frequency band or series resonators adapted to anti-resonate substantially at respective anti-resonance frequencies defining the rejection frequency band. These resonators are connected through a phase shifter which imparts an impedance phase shift of approximately 45° to 135°. Exemplary applications of the band reject filters disclosed herein include implementation as an inter-stage band reject filter for a base station power amplifier for a wireless communication system, as a radio frequency band reject filter in a duplexer for a wireless communication terminal, and in a low noise amplifier input stage.

1 . A network element for a wireless communication system, the network element comprising:
 an antenna system;   a transmitter coupled to the antenna system to transmit radio frequency signals in a transmit frequency band via the antenna; and   a receiver coupled to the antenna system to receive radio frequency signals in a receive frequency band different from the transmit frequency band via the antenna;   at least one of the transmitter and the receiver comprising a radio frequency band reject filter, the band reject filter comprising:   a coupler operable on an input signal applied to an input port to provide a respective output signal proportional to the input signal at each of a first output port and a second output port, the output signals having a phase shift between 45 degrees and 135 degrees with respect to the input signal; and   first, second and third resonators coupled respectively to the input port, the first output port and the second output port.   
     
     
         2 . The network element of  claim 1 , wherein the network element is a base station. 
     
     
         3 . The network element of  claim 1 , wherein:
 the transmitter comprises the band reject filter; and   the band reject filter is configured to attenuate the receive frequency band.   
     
     
         4 . The network element of  claim 1 , wherein:
 the receiver comprises the band reject filter; and   the band reject filter is configured to attenuate the transmit frequency band.   
     
     
         5 . The network element of  claim 1 , wherein:
 the transmitter comprises a first band reject filter configured to attenuate the receive frequency band; and   the receiver comprises a second band reject filter configured to attenuate the transmit frequency band.   
     
     
         6 . The network element of  claim 1 , wherein at least one of the acoustic resonators is a shunt resonator operable to resonate substantially at a resonance frequency defining a rejection frequency band. 
     
     
         7 . The network element of  claim 1 , wherein at least one of the acoustic resonators is a series resonator operable to anti-resonate substantially at an anti-resonance frequency defining a rejection frequency band. 
     
     
         8 . The network element of  claim 1 , wherein the band reject filter comprises multiple stages, each stage comprising a respective coupler and respective first, second and third resonators. 
     
     
         9 . The network element of  claim 1 , wherein the coupler comprises a Wilkinson combiner. 
     
     
         10 . The network element of  claim 1 , wherein the coupler comprises a quadrature coupler. 
     
     
         11 . The network element of  claim 10 , wherein:
 the quadrature coupler comprises a first input port and a second input port; and   the band reject filter comprises a fourth acoustic resonator coupled to the second input port.   
     
     
         12 . A method of operating a network element for a wireless communication system, the network element comprising an antenna system, a transmitter coupled to the antenna system to transmit radio frequency signals in a transmit frequency band via at least one antenna of the antenna system, and a receiver coupled to the antenna system to receive radio frequency signals in a receive frequency band different from the transmit frequency band via at least one antenna of the antenna system, the method comprising, at at least one of the transmitter and the receiver:
 receiving an input signal;   applying the input signal to a coupler operable to provide a respective output signal proportional to the input signal at each of a first output port and a second output port, the output signals having a phase shift between 45 degrees and 135 degrees with respect to the input signal, wherein first, second and third resonators are coupled respectively to the input port, the first output port and the second output port; and   the resonators in combination with the coupler attenuating the input signal in a predetermined frequency band.   
     
     
         13 . The method of  claim 12 , wherein the network element is a base station. 
     
     
         14 . The method of  claim 12 , wherein:
 the method is performed at the transmitter; and   the predetermined frequency band is the receive frequency band.   
     
     
         15 . The method of  claim 12 , wherein:
 the method is performed at the receiver; and   the predetermined frequency band is the transmit frequency band.   
     
     
         16 . The method of  claim 12 , wherein the method is performed at least one of:
 the transmitter to attenuate the receive frequency band; and   the receiver to attenuate the transmit frequency band.   
     
     
         17 . The method of  claim 12 , wherein at least one of the acoustic resonators is a shunt resonator operable to resonate substantially at a resonance frequency defining a rejection frequency band. 
     
     
         18 . The method of  claim 12 , wherein at least one of the acoustic resonators is a series resonator operable to anti-resonate substantially at an anti-resonance frequency defining a rejection frequency band. 
     
     
         19 . The method of  claim 12 , comprising coupling together multiple stages, each stage comprising a respective coupler and respective first, second and third resonators, the multiple stages being operable in combination to attenuate the input signal in the predetermined frequency band. 
     
     
         20 . The method of  claim 12 , wherein the coupler comprises a Wilkinson combiner. 
     
     
         21 . The method of  claim 12 , wherein the coupler comprises a quadrature coupler. 
     
     
         22 . The method of  claim 21 , wherein the quadrature coupler comprises a first input port and a second input port, further comprising coupling a fourth acoustic resonator to the second input port.