Surface acoustic wave filter

Abstract

A SAW filter includes a first SAW resonator having a pair of terminals and a predetermined resonance frequency (f rp ), the first SAW resonator being provided in a parallel arm of the SAW filter. A second SAW resonator has a pair of terminals and a predetermined resonance frequency (f rs ) approximately equal to a predetermined antiresonance frequency of the first SAW resonator (f ap ). The second SAW resonator is provided in a series arm of the SAW filter. An inductance element is connected in series to the first SAW resonator.

What is claimed is:  
     
       1. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising: 
       a pair of SAW filters having respective pass bands and comprising a plurality of one-port SAW resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals;  
       the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of SAW filters;  
       the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of SAW filters; and  
       an inductance element located between at least one of the SAW filters located at the first stage and the pair of band-pass filter input terminals and directly connected between the respective pair of input terminals of the at least one of the SAW filters and thereby in parallel to said at least one of the SAW filters.  
     
     
       2. A SAW filter comprising: 
       a plurality of first SAW resonators, each having a pair of terminals and a predetermined resonance frequency (f rp ), said first SAW resonators being connected in respective, parallel arms of the SAW filter;  
       a plurality of second SAW resonators, each having a pair of terminals and a predetermined resonance frequency (f rs ) approximately equal to an antiresonance frequency (f ap ) of each of the first SAW resonators, said second SAW resonators being provided in series arms of the SAW filter; and  
       inductance elements respectively connected in series with the first SAW resonators in the parallel arms and formed of wires.  
     
     
       3. The SAW filter as claimed in  claim 2 , further comprising: 
       a package accommodating the first and second resonators and the inductance elements; and  
       lead terminals extending from interiorly of the package to exteriorly thereof, said wires of the inductance elements being bonded to said lead terminals.  
     
     
       4. A band-pass filter having a predetermined pass-band characteristic and comprising: 
       a plurality of SAW resonators connected in a ladder formation, said plurality of resonators being connected in respective said serial arms and parallel arms; and  
       bonding inductance elements, said parallel arms of said ladder formation being connected to ground via respective said bonding inductance elements.  
     
     
       5. The band-pass filter as claimed in  claim 4 , wherein said bonding inductance elements comprise wires. 
     
     
       6. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising: 
       a pair of SAW filters having respective, different pass bands and each SAW filter having a pair of SAW filter input terminals and a pair of SAW filter output terminals and comprising a plurality of one-port SAW resonators connected in a latter structure between the input and output terminals and including at least a first stage having a series-arm SAW resonator connected to one of the pair of input terminals;  
       a pair of SAW filters having respective pass bands and comprising a plurality of one-port SAW resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals;  
       the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of SAW filters;  
       the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of SAW filters.  
     
     
       7. A band-pass filter having a predetermined pass-band characteristic and comprising: 
       a plurality of SAW resonators connected in a ladder configuration of respective serial arms and parallel arms, said plurality of SAW resonators being connected in respective said serial arms and parallel arms; and  
       bonding inductance elements respectively connecting said parallel arms of said ladder configuration to ground.  
     
     
       8. A SAW filter comprising: 
       
         a plurality of first SAW resonators, each having a pair of terminals and a predetermined resonance frequency, said first SAW resonators being connected in respective, parallel arms of said SAW filter;  
       
       
         inductance elements connected in series to respective ones of said first SAW resonators in the parallel arms of said SAW filter;  
       
       
         a plurality of second SAW resonators, each having a pair of terminals and a predetermined resonance frequency approximately equal to an antiresonance frequency of each of said first SAW resonators, said second SAW resonators being provided in series arms of said SAW filter;  
       
       
         a first product of an aperture length and a number of electrode finger pairs of each of said first SAW resonators being larger than a second product of an aperture length and a number of electrode finger pairs of each of said second SAW resonators. 
       
     
     
       9. An RF SAW filter having a relative bandwidth equal to or greater than  2 % , a suppression factor equal to or larger than  20  dB, and an insertion loss less than or equal to  5  dB comprising:    
       
         a plurality of first SAW resonators, each having a pair of terminals and a predetermined resonance frequency, said first SAW resonators being connected in respective, parallel arms of said SAW filter;  
       
       
         a plurality of second SAW resonators, each having a pair of terminals and a predetermined resonance frequency approximately equal to an antiresonance frequency of each of said first SAW resonators, said second SAW resonators being provided in series arms of said SAW filter; and  
       
       
         inductance elements respectively connected in series with said first SAW resonators in the parallel arms. 
       
     
     
       10. A SAW filter as claimed in  claim 9 , wherein an aperture length of each of said first SAW resonators is larger than an aperture length of each of said second SAW resonators. 
     
     
       11. A SAW filter as claimed in  claim 9 , wherein a number of electrode finger pairs of each of said first SAW resonators is larger than a number of electrode finger pairs of each of said second SAW resonators. 
     
     
       12. A SAW filter as claimed in  claim 2 ,  4 ,  7 ,  8  or  9 , wherein said inductance elements have different inductance values. 
     
     
       13. A SAW filter as claimed in  claim 2 ,  4 ,  7 ,  8  or  9 , wherein said SAW filter is mounted in a ceramic package. 
     
     
       14. A SAW filter as claimed in  claim 2 ,  4 ,  7 ,  8  or  9 , wherein said inductance elements are provided at a path which is between the parallel arms and ground level on a ceramic package in which said SAW filter is mounted. 
     
     
       15. A SAW filter as claimed in  claim 2 ,  4 ,  7 ,  8  or  9 , wherein each of said inductance elements includes a bonding wire connected in series to the corresponding one of said first SAW resonators in the parallel arms, the bonding wire being longer than bonding wires for input and output terminals of said SAW filter. 
     
     
       16. A SAW filter comprising: 
       
         a first SAW resonator in a parallel arm of said SAW filter and having a predetermined resonance frequency;  
       
       
         a second SAW resonator in a series arm of said SAW filter and having a predetermined resonance frequency approximately equal to an antiresonance frequency of said first SAW resonator; and  
       
       
         an inductance element connected in series with said first SAW resonator, said inductance element functioning to increase the admittance of the parallel arm and decrease the resonance frequency.