Tunable bridged-T filter

Abstract

A tunable filter ( 300 ) includes first ( 302 ) and second ( 304 ) “bridged-T” resonators. The first ( 302 ) and second ( 304 ) resonators are capacitively coupled together via bandwidth control/coupling section ( 306 ). A biasing section ( 314 ) provides the proper biasing and decoupling for the filter ( 300 ). Filter ( 300 ) provides for low-side injection mode operation, and provides for a very deep zero on the lower side of the passband resulting in excellent selectivity. Filter ( 300 ) provides for excellent selectivity while reducing the number of inductors and varactors needed in the filter circuit. A high side injection mode filter ( 500 ) similar to filter ( 300 ) is also described in the disclosure.

1 . A tunable filter, comprising: 
 first and second bridged-T resonator sections; and    a coupling section that couples the first bridged-T resonator section to the second bridged-T resonator section.    
     
     
         2 . A tunable filter as defined in  claim 1 , wherein the coupling section comprises only capacitors.  
     
     
         3 . A tunable filter as defined in  claim 1 , wherein the first bridged-T resonator section comprises: 
 an input port;    a varactor having first and second terminals;    a first capacitor coupled between the input port and the first terminal of the varactor;    an inductor;    a second capacitor; and    the inductor and second capacitor are coupled in parallel to the second terminal of the varactor.    
     
     
         4 . A tunable filter as defined in  claim 3 , wherein the second bridged-T resonator section comprises: 
 an output port;    a varactor having first and second terminals;    a first capacitor coupled between the output port and the first terminal of the varactor;    an inductor;    a second capacitor; and    the inductor and second capacitor are coupled in parallel to the second terminal of the varactor.    
     
     
         5 . A tunable filter as defined in  claim 1 , further comprising: 
 a biasing section coupled to the first and second bridged-T sections.    
     
     
         6 . A tunable filter as defined in  claim 5 , wherein the biasing section further includes a control port for receiving a control voltage used for tuning the tunable filter.  
     
     
         7 . A tunable filter as defined in  claim 6 , wherein the first and second bridged-T sections each includes at least one tunable element which is responsive to the control voltage.  
     
     
         8 . A tunable filter as defined in  claim 7 , wherein the at least one tunable element in the first and second bridged-T sections comprise varactors.  
     
     
         9 . A tunable filter as defined in  claim 1 , wherein the tunable filter operates in the low side injection mode.  
     
     
         10 . A receiver, comprising: 
 an antenna port for receiving an RF signal; and a    tunable filter coupled to the antenna port, including 
 first and second bridged-T resonator sections; and  
 a coupling section that couples the first bridged-T resonator section to the second bridged-T resonator section.  
   
     
     
         11 , A receiver as defined in  claim 10 , wherein the coupling section includes only capacitors.  
     
     
         12 . A receiver as defined in  claim 11 , further comprising: 
 a biasing section coupled to the first and second bridged-T sections.    
     
     
         13 . A tunable filter as defined in  claim 12 , wherein the biasing section further includes a control port for receiving a control voltage used for tuning the tunable filter.  
     
     
         14 . A tunable filter, comprising: 
 first and second resonator sections,    the first resonator section including: 
 an input port;  
 an inductor;  
 a varactor; and  
 a capacitor having first and second terminals, the first terminal coupled to the input port and the second terminal coupled to the varactor and inductor which are in parallel to each other;  
 the second resonator section including: 
 an output port;  
 an inductor;  
 a varactor; and  
 a capacitor having first and second terminals, the first terminal coupled to the output port and the second terminal coupled to the varactor and inductor which are in parallel to each other; and  
 a coupling section that couples the first resonator section to the second resonator section.  
 
   
     
     
         15 . A tunable filter as defined in  claim 14 , wherein the coupling section comprises only capacitors.  
     
     
         16 . A tunable filter as defined in  claim 14 , further comprising: 
 a biasing section coupled to the first and second bridged-T sections.    
     
     
         17 . A tunable filter as defined in  claim 16 , wherein the biasing section further includes a control port for receiving a control voltage used for tuning the tunable filter.