Surface-acoustic-wave filters with poles of attenuation created by impedance circuits

Abstract

A surface-acoustic-wave filter couples a two-port surface-acoustic-wave resonator filter circuit coupled in parallel or in series with a two-port impedance circuit. The two-port impedance circuit has an impedance that creates a pole of attenuation, by making the open-circuit impedance of the surface-acoustic-wave filter equal to the short-circuit impedance. High attenuation over a wide range of stopband frequencies is obtained in this way, combined with steep roll-off between the passband and stopband.

What is claimed is:  
     
       1. A surface-acoustic-wave filter with poles, comprising: 
       a two-port surface-acoustic-wave resonator filter circuit having a phase shifter, at least one series-arm surface-acoustic-wave resonator coupled in series with said phase shifter, and at least one shunt-arm surface-acoustic-wave resonator; and  
       a two-port impedance circuit coupled in parallel with said two-port surface-acoustic-wave resonator filter circuit, having an impedance that creates a pole of attenuation by making an open-circuit impedance of said surface-acoustic-wave filter equal to a short-circuit impedance of said surface-acoustic-wave filter.  
     
     
       2. The surface-acoustic-wave filter of  claim 1 , wherein said two-port surface-acoustic-wave resonator filter circuit has just two series-arm surface-acoustic-wave resonators. 
     
     
       3. The surface-acoustic-wave filter of  claim 1 , wherein said two-port impedance circuit comprises a first capacitor. 
     
     
       4. The surface-acoustic-wave filter of  claim 3 , wherein said two-port impedance circuit also comprises a first inductor. 
     
     
       5. The surface-acoustic-wave filter of  claim 4 , wherein said first inductor comprises a bonding wire having a certain inductive impedance. 
     
     
       6. The surface-acoustic-wave filter of  claim 4 , wherein said first capacitor and said first inductor are coupled in series. 
     
     
       7. The surface-acoustic-wave filter of  claim 6 , wherein said two-port impedance circuit further comprises a second capacitor coupled in parallel with said first capacitor and said first inductor. 
     
     
       8. The surface-acoustic-wave filter of  claim 4 , wherein said first capacitor and said first inductor are coupled in parallel. 
     
     
       9. The surface-acoustic-wave filter of  claim 8 , wherein said two-port impedance circuit further comprises a second inductor coupled in series with said first capacitor and said first indicator. 
     
     
       10. The surface-acoustic-wave filter of  claim 1 , wherein said surface-acoustic-wave filter is disposed in an antenna duplexer, and said phase shifter comprises an interconnecting line in said antenna duplexer. 
     
     
       11. The surface-acoustic-wave filter of  claim 1 , wherein said at least one series-arm surface-acoustic-wave resonator, said at least one shunt-arm surface-acoustic-wave resonator, and said phase shifter are formed on a piezoelectric substrate. 
     
     
       12. The surface-acoustic-wave filter of  claim 11 , wherein said phase shifter comprises a stripline formed on said piezoelectric substrate. 
     
     
       13. The surface-acoustic-wave filter of  claim 1 , wherein said at least one series-arm surface-acoustic-wave resonator, and said at least one shunt-arm surface-acoustic-wave resonator are formed on a piezoelectric substrate, said piezoelectric substrate is mounted on a main substrate, and said phase shifter is formed on said main substrate. 
     
     
       14. The surface-acoustic wave filter of  claim 1 , wherein said at least one series-arm surface-acoustic-wave resonator, and said at least one shunt-arm surface-acoustic-wave resonator are formed on a piezoelectric substrate, said piezoelectric substrate is mounted on a main substrate, and said phase shifter is formed partly on said piezoelectric substrate and partly on said main substrate. 
     
     
       15. A surface-acoustic-wave filter with poles, comprising: 
       a two-port surface-acoustic-wave resonator filter circuit having two series-arm surface-acoustic-wave resonators coupled in series with a phase shifter, and two shunt-arm surface-acoustic-wave resonators; and  
       a two-port impedance circuit coupled in series with said two-port surface-acoustic-wave resonator filter circuit, having an impedance that creates a pole of attenuation by making an open-circuit impedance of said surface-acoustic-wave filter equal to a short-circuit impedance of said surface-acoustic-wave filter.  
     
     
       16. The surface-acoustic-wave filter of  claim 15 , wherein said two-port impedance circuit comprises a first inductor. 
     
     
       17. The surface-acoustic-wave filter of  claim 16 , wherein said first inductor comprises a bonding wire having a certain inductive impedance. 
     
     
       18. The surface-acoustic-wave filter of  claim 16 , wherein said two-port impedance circuit also comprises a first capacitor. 
     
     
       19. The surface-acoustic-wave filter of  claim 18 , wherein said first inductor and said first capacitor are coupled in parallel. 
     
     
       20. The surface-acoustic-wave filter of  claim 19 , wherein said two-port impedance circuit further comprises a second inductor coupled in series with said first capacitor and said first inductor. 
     
     
       21. The surface-acoustic-wave filter of  claim 18 , wherein said first inductor and said first capacitor are coupled in series. 
     
     
       22. The surface-acoustic-wave filter of  claim 21 , wherein said two-port impedance circuit further comprises a second capacitor coupled in parallel with said first inductor and said first capacitor. 
     
     
       23. The surface-acoustic-wave filter of  claim 15 , wherein said two shunt-arm surface-acoustic-wave resonators are coupled back-to-back, sharing a common central electrode. 
     
     
       24. The surface-acoustic-wave filter of  claim 23 , wherein said two-port impedance circuit comprises a bonding wire attached to said common central electrode. 
     
     
       25. The surface-acoustic-wave filter of  claim 15 , wherein said two-port surface-acoustic-wave resonator filter circuit and said two-port impedance circuit form a first filter section, and said surface-acoustic-wave filter further comprises a second filter section cascaded with said first filter section. 
     
     
       26. The surface-acoustic-wave filter of  claim 25 , wherein said second filter section, when combined with said a two-port surface-acoustic-wave resonator filter circuit, forms a surface-acoustic-wave ladder filter. 
     
     
       27. The surface-acoustic-wave filter of  claim 25 , wherein said second filter section is similar in configuration to said first filter section, comprising a two-port surface-acoustic-wave resonator filter circuit having two series-arm SAW resonators and two shunt-arm SAW resonators, coupled in series with a two-port impedance circuit. 
     
     
       28. A surface-acoustic-wave filter comprising a plurality of surface-acoustic-wave filters as described in  claim 15 , coupled in cascade. 
     
     
       29. A surface- acoustic - wave filter with poles, comprising:    
         a two - port surface - acoustic - wave resonator filter circuit having two series - arm surface - acoustic - wave resonators and two shunt - arm surface - acoustic - wave resonators; and    
         a two - port impedance circuit coupled in series with said two - port surface - acoustic - wave resonator filter circuit, having an impedance that creates a pole of attenuation by making an open - circuit impedance of said surface - acoustic - wave filter equal to a short - circuit impedance of said surface - acoustic - wave filter.   
     
     
       30. The surface- acoustic - wave filter of    claim 29   , wherein said two - port impedance circuit comprises a first inductor.   
     
     
       31. The surface- acoustic - wave filter of    claim 30   , wherein said first inductor comprises a bonding wire having a certain inductive impedance.   
     
     
       32. The surface- acoustic - wave filter of    claim 30   , wherein said two - port impedance circuit further comprises a first capacitor.   
     
     
       33. The surface- acoustic - wave filter of    claim 32   , wherein said first inductor and said first capacitor are coupled in parallel.   
     
     
       34. The surface- acoustic - wave filter of    claim 33   , wherein said two - port impedance circuit further comprises a second inductor coupled in series with said first capacitor and said first inductor.   
     
     
       35. The surface- acoustic - wave filter of    claim 32   , wherein said first inductor and said first capacitor are coupled in series.   
     
     
       36. The surface- acoustic - wave filter of    claim 35   , wherein said two - port impedance circuit further comprises a second capacitor coupled in parallel with said first inductor and said first capacitor.   
     
     
       37. The surface- acoustic - wave filter of    claim 29   , wherein said two - shunt - arm surface - acoustic - wave resonators are coupled back - to - back, sharing a common central electrode.   
     
     
       38. The surface- acoustic - wave filter of    claim 37   , wherein said two - port impedance circuit comprises a bonding wire attached to said common central electrode.   
     
     
       39. The surface- acoustic - wave filter of    claim 29   , wherein said two - port surface - acoustic - wave resonator filter circuit and said two - port impedance circuit form a first filter section, and said surface - acoustic - wave filter further comprises a second filter section cascaded with said first filter section.   
     
     
       40. The surface- acoustic - wave filter of    claim 39   , wherein said second filter section is combined with said two - port - surface - acoustic - wave resonator filter circuit, forming a surface - acoustic - wave ladder filter.   
     
     
       41. The surface- acoustic - wave filter on    claim 39   , wherein said second filter section is similar in configuration to said first filter section, said second filter section comprising a two - port surface - acoustic - wave resonator filter circuit having two series - arm SAW resonators and two shunt - arm SAW resonators coupled in series with a two - port impedance circuit.   
     
     
       42. The surface- acoustic - wave filter of    claim 29   , further comprising a plurality of surface - acoustic - wave filters coupled in cascade.   
     
     
       43. A surface- acoustic - wave duplexer having a transmitting surface - acoustic - wave filter circuit and receiving surface - acoustic - wave filter circuit and a coupling circuit, the transmitting surface - acoustic - wave filter circuit comprising:    
         a two - port surface - acoustic - wave resonator filter circuit having two series - arm surface - acoustic - wave resonators and two shunt - arm surface - acoustic - wave resonators; and    
         a two - port impedance circuit coupled in series with said two - port surface - acoustic - wave resonator filter circuit, having an impedance that creates a pole of attenuation by making an open - circuit impedance of said transmitting surface - acoustic - wave filter equal to a short - circuit impedance of said transmitting surface - acoustic - wave filter.   
     
     
       44. The surface- acoustic - wave duplexer of    claim 43   , wherein said two - port surface - acoustic - wave resonator filter circuit forms a ladder surface - acoustic - wave resonator filter, and said two - port impedance circuit is coupled in series with said two shunt - arm surface - acoustic - wave resonators.   
     
     
       45. A surface- acoustic - wave filter with poles, comprising:    
         a first series - arm surface - acoustic - wave resonator coupled to an input terminal;    
         a second series - arm surface - acoustic - wave resonator coupled to said first series - arm surface - acoustic - wave resonator;    
         a third series - arm surface - acoustic - wave resonator coupled between said second series - arm surface - acoustic - wave resonator and an output terminal;    
         a first shunt - arm surface - acoustic - wave resonator coupled to said first and second series - arm surface - acoustic - wave resonators;    
         a second shunt - arm surface - acoustic - wave resonator coupled to said second and third series - arm surface - acoustic - wave resonators; and    
         an impedance circuit having first and second ends, wherein said first end is coupled only to said first and second shunt - arm surface - acoustic - wave resonators and said second end is coupled to a ground terminal, and wherein and impedance circuit has an impedance that creates a pole of attenuation by making an open - circuit impedance of said surface - acoustic - wave filter substantially equal to a short - circuit impedance of said surface - acoustic - wave filter.   
     
     
       46. The surface- acoustic - wave filter of    claim 45   , further comprising a phase shifter coupled between said input terminal and said first series - arm surface - acoustic - wave resonator.   
     
     
       47. The surface- acoustic - wave filter of    claim 45   , wherein said impedance circuit comprises a first inductor.   
     
     
       48. The surface- acoustic - wave filter of    claim 47   , wherein said first inductor comprises a bonding wire having a certain inductive impedance.   
     
     
       49. The surface- acoustic - wave filter of    claim 47   , wherein said impedance circuit further comprises a first capacitor.   
     
     
       50. The surface- acoustic - wave filter of    claim 49   , wherein said first inductor and said first capacitor are coupled in parallel.   
     
     
       51. The surface- acoustic - wave filter of    claim 50   , wherein said impedance circuit further comprises a second inductor coupled in series to said first capacitor and said first inductor.   
     
     
       52. The surface- acoustic - wave filter of    claim 49   , wherein said first inductor and said first capacitor are coupled in series.   
     
     
       53. The surface- acoustic - wave filter of    claim 52   , wherein said impedance circuit further comprises a second capacitor coupled in parallel to said first capacitor and said first inductor.   
     
     
       54. The surface- acoustic - wave filter of    claim 45   , wherein said first and second shunt - arm surface - acoustic - wave resonators are coupled back - to - back, sharing a common central electrode.   
     
     
       55. The surface- acoustic - wave filter of    claim 54   , wherein said impedance circuit comprises a bonding wire attached to said common central electrode.   
     
     
       56. A surface- acoustic - wave duplexer, comprising:    
         a transmitting surface - acoustic - wave filter circuit; and    
         a receiving surface - acoustic - wave filter circuit;    
         wherein said transmitting surface - acoustic - wave filter circuit comprises:    
         a first series - arm surface - acoustic - wave resonator coupled to an input terminal;    
         a second series - arm surface - acoustic - wave resonator coupled to said first series - arm surface - acoustic - wave resonator;    
         a third series - arm surface - acoustic - wave resonator coupled between said second series - arm surface - acoustic - wave resonator and an output terminal;    
         a first shunt - arm surface - acoustic - wave resonator coupled to said first and second series - arm surface - acoustic wave resonators;    
         a second shunt - arm surface - acoustic - wave resonator coupled to said second and third series - arm surface - acoustic - wave resonators; and    
         an impedance circuit having first and second ends, wherein said first end is coupled only to said first and second shunt - arm surface - acoustic - wave resonator and said second end is coupled to a ground terminal, and wherein said impedance circuit has an impedance that creates a pole of attenuation by making an open - circuit impedance of said transmitting surface - acoustic - wave filter substantially equal to a short - circuit impedance of said transmitting surface - acoustic - wave filter.   
     
     
       57. A surface- acoustic - wave filter with poles, comprising:    
         two series - arm surface - acoustic - wave resonators coupled in series between an input terminal and an output terminal;    
         two shunt - arm surface - acoustic - wave resonators coupled to said two series - arm surface - acoustic - wave resonators;    
         an impedance circuit having first and second ends, wherein said first end is coupled only to said two shunt - arm surface - acoustic - wave resonators and said second end is coupled to a ground terminal, and wherein said impedance circuit has an impedance that creates a pole of attenuation by making an open circuit impedance of said surface - acoustic - wave filter substantially equal to a short - circuit impedance of said surface - acoustic - wave filter.   
     
     
       58. The surface- acoustic - wave filter of    claim 57   , further comprising a phase shifter coupled between said input terminal and said first series - arm surface - acoustic - wave resonator.   
     
     
       59. The surface- acoustic - wave filter of    claim 57   , wherein said impedance circuit comprises a first inductor.   
     
     
       60. The surface- acoustic - wave filter of    claim 59   , wherein said first inductor comprises a bonding wire having a certain inductive impedance.   
     
     
       61. The surface- acoustic - wave filter of    claim 59   , wherein said impedance circuit further comprises a first capacitor.   
     
     
       62. The surface- acoustic - wave filter of    claim 61   , wherein said first inductor and said first capacitor are coupled in parallel.   
     
     
       63. The surface- acoustic - wave filter of    claim 62   , wherein said impedance circuit further comprises a second inductor coupled in series to said first capacitor and said first inductor.   
     
     
       64. The surface- acoustic - wave filter of    claim 61   , wherein said first inductor and said first capacitor are coupled in series.   
     
     
       65. The surface- acoustic - wave filter of    claim 64   , wherein said impedance circuit further comprises a second capacitor coupled in parallel to said first capacitor and said first inductor.   
     
     
       66. The surface- acoustic - wave filter of    claim 57   , wherein said two shunt - arm surface - acoustic - wave resonators are coupled back - to - back and share a common central electrode.   
     
     
       67. The surface- acoustic - wave filter of    claim 66   , wherein said impedance circuit comprises a bonding wire attached to said common central electrode.   
     
     
       68. The surface- acoustic - wave filter of    claim 57   , further comprising a filter section cascaded with said surface - acoustic - wave filter.   
     
     
       69. The surface- acoustic - wave filter of    claim 68   , wherein said filter section and said surface - acoustic - wave filter form a surface - acoustic - wave ladder filter.   
     
     
       70. The surface- acoustic - wave filter of    claim 68   , wherein said filter section in similar in configuration to said surface - acoustic - wave filter, and wherein said filter section comprises:    
         a second surface - acoustic - wave filter with poles having two series - arm surface - acoustic - wave resonators;    
         two shunt - arm surface - acoustic - wave resonators; and    
         an impedance circuit, said impedance circuit being coupled in series to said two shunt - arm surface - acoustic - wave resonators.   
     
     
       71. The surface- acoustic - wave filter of    claim 57   , further comprising a plurality of surface - acoustic - wave filters coupled in cascade.   
     
     
       72. A surface- acoustic - wave duplexer, comprising:    
         a receiving surface - acoustic - wave filter circuit; and    
         a transmitting surface - acoustic - wave filter circuit comprising:    
         two series - arm surface - acoustic - wave resonators coupled in series between an input terminal and an output terminal;    
         two shunt - arm surface - acoustic - wave resonators coupled to said two series - arm surface - acoustic - wave resonators; and    
         an impedance circuit having first and second ends, wherein said first end is coupled only to said two shunt - arm surface - acoustic - wave resonators and said second end is coupled to a ground terminal, and wherein said impedance circuit has an impedance that creates a pole of attenuation by making an open - circuit impedance of said transmitting surface - acoustic - wave filter substantially equal to a short - circuit impedance of said transmitting surface - acoustic - wave filter.   
     
     
       73. A surface- acoustic - wave filter with poles, comprising:    
         first and second series - arm surface - acoustic - wave resonators coupled in series between an input terminal and an output terminal;    
         first and second shunt - arm surface - acoustic - wave resonators coupled respectively to said first and second series - arm surface - acoustic - wave resonators; and    
         a first impedance circuit having first and second ends, wherein said first end is coupled only to said first and second shunt - arm surface - acoustic - wave resonators and said second end is coupled to a ground terminal, wherein said first impedance circuit has a first impedance that creates a pole of attenuation by making an open - circuit impedance of said surface - acoustic - wave filter substantially equal to a short - circuit impedance of said surface - acoustic wave filter.   
     
     
       74. The surface- acoustic - wave filter of    claim 73   , further comprising a phase shifter coupled between said input terminal and said first series - arm surface - acoustic - wave resonator.   
     
     
       75. The surface- acoustic waver filter of    claim 73   , wherein said impedance circuit comprises a first inductor.   
     
     
       76. The surface- acoustic - wave filter of    claim 75   , wherein said first inductor comprises a bonding wire having a certain inductive impedance.   
     
     
       77. The surface- acoustic - wave filter of    claim 75   , wherein said impedance circuit further comprises a first capacitor.   
     
     
       78. The surface- acoustic - wave filter of    claim 77   , wherein said first inductor and said first capacitor are coupled in parallel.   
     
     
       79. The surface- acoustic - wave filter of    claim 78   , wherein said impedance circuit further comprises a second inductor coupled in series to the first capacitor and the first inductor.   
     
     
       80. The surface- acoustic - wave filter of    claim 77   , wherein said first inductor and said first capacitor are coupled in series.   
     
     
       81. The surface- acoustic - wave filter of    claim 80   , wherein said impedance circuit further comprises a second capacitor coupled in parallel to said first capacitor and said first inductor.   
     
     
       82. The surface- acoustic - wave filter of    claim 73   , wherein said first and second shunt - arm surface - acoustic - wave resonators are coupled back - to - back, sharing a common central electrode.   
     
     
       83. The surface- acoustic - wave filter of    claim 82   , wherein said impedance circuit comprises a bonding wire attached to said common central electrode.   
     
     
       84. The surface- acoustic - wave filter of    claim 73   , further comprising a filter section cascaded with said surface - acoustic - wave filter.   
     
     
       85. The surface- wave filter of    claim 84   , wherein said filter section and said surface - acoustic - wave filter forms a surface - acoustic - wave ladder filter.   
     
     
       86. The surface- acoustic - wave filter of    claim 73   , further comprising a plurality of surface - acoustic - wave filters coupled in cascade.   
     
     
       87. The surface acoustic- wave filter of    claim 73   , further comprising another surface - acoustic - wave filter with poles, and another surface - acoustic - wave filter comprising:    
         third and fourth series - arm surface - acoustic - wave resonators coupled in series between said second series - arm surface - acoustic - wave resonator and said output terminal;    
         third and fourth shunt - arm surface - acoustic - wave resonators coupled respectively to said third and fourth series - arm surface - acoustic - wave resonators; and    
         a second impedance circuit having third and fourth ends, wherein said therein end is coupled only to said third and fourth shunt - arm surface - acoustic - wave resonators and said fourth end is coupled to a ground terminal, wherein said second impedance circuit has a second impedance that creates a pole of attenuation by making an open - circuit impedance of said another surface - acoustic - wave filter substantially equal to a short - circuit impedance of said another surface - acoustic - wave filter.   
     
     
       88. A surface- acoustic - wave duplexer, comprising:    
         a receiver surface - acoustic - wave filter circuit; and    
         a transmitting surface - acoustic - wave filter circuit comprising:    
         first and second series - arm surface - acoustic - wave resonators coupled in series between an input terminal and an output terminal;    
         first and second shunt - arm surface - acoustic - wave resonators coupled respectively to said first and second series - arm surface - acoustic - wave resonators; and    
         a first impedance circuit having first and second ends, wherein said first end is coupled only to said first and second shunt - arm surface - acoustic - wave resonators and said second end is coupled to a ground terminal, wherein said first impedance circuit has a first impedance that creates a pole of attenuation by making an open - circuit impedance of said transmitting surface - acoustic - wave filter substantially equal to a short - circuit impedance of said transmitting surface - acoustic - wave filter.   
     
     
       89. The surface- acoustic - wave duplexer of    claim 88   , further comprising a first surface - acoustic - wave filter including:    
         third and fourth series - arm surface - acoustic - wave resonators coupled in series between second series - arm surface - acoustic - wave resonator and said output terminal;    
         third and fourth shunt - arm surface - acoustic - wave resonators coupled respectively to said third and fourth series - arm surface - acoustic - wave resonators; and    
         a second impedance circuit having third and fourth ends, wherein said third end is coupled only to said third and fourth shunt - arm surface - acoustic - wave resonators and said fourth end is coupled to a ground terminal, wherein said second impedance circuit has a second impedance that creates a pole of attenuation by making an open - circuit impedance of said first surface - acoustic - wave filter substantially equal to a short - circuit impedance of said first surface - acoustic - wave filter.   
     
     
       90. A surface- acoustic - wave filter comprising:    
         a first series - arm surface - acoustic - wave resonator having a first series - arm terminal coupled to receive an input signal and a second series - arm terminal;    
         a second series - arm surface - acoustic - wave resonator having a third series - arm terminal connected to said second series - arm terminal of said first series arm surface - acoustic - wave resonator and a fourth series - arm terminal;    
         a third series - arm surface - acoustic - wave resonator having a fifth series - arm terminal connected to said fourth series - arm terminal of said second series - arm surface - acoustic - wave resonator and a sixth series - arm terminal coupled to output an output signal;    
         a first shunt - arm surface - acoustic - wave resonator having a first shunt - arm terminal connected to said second series - arm terminal of said first series - arm surface - acoustic - wave resonator and a second shunt - arm terminal;    
         a second shunt - arm surface - acoustic - wave resonator having a third shunt - arm terminal connected to said fourth series - arm terminal of said second series - arm surface - acoustic - wave resonator and a fourth shunt - arm terminal connected to said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator; and    
         a first impedance circuit having a first terminal connected to said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator and a second terminal connected to a ground terminal, wherein said first terminal of said first impedance circuit is connected only to said second and fourth shunt - arm terminals of said first and second shunt - arm surface - acoustic - wave resonators, and wherein said first impedance circuit has a first impedance that creates a pole of attenuation by making an open - circuit impedance of said surface - acoustic - wave filter substantially equal to a short - circuit impedance of said surface - acoustic - wave filter.   
     
     
       91. The surface- acoustic - wave filter according to    claim 90   , further comprising:    
       
         an input terminal receiving the input signal; and  
       
         a phase shifter connected between said input terminal and said first series - arm terminal of said first series - arm surface - acoustic - wave resonator.   
     
     
       92. The surface- acoustic - wave filter according to    claim 90   , wherein said first impedance circuit comprises a first inductor.   
     
     
       93. The surface- acoustic - wave filter according to    claim 92   , wherein said first inductor is a bonding wire connected between said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator, said bonding wire having a certain inductive impedance.   
     
     
       94. The surface- acoustic - wave filter according to    claim 92   , wherein said first impedance circuit further comprises a first capacitor.   
     
     
       95. The surface- acoustic - wave filter according to    claim 94   , wherein said first inductor and said first capacitor are connected in parallel.   
     
     
       96. The surface- acoustic - wave filter according to    claim 95   , wherein said first impedance circuit further comprises a second inductor connected in series to said first capacitor and said first inductor.   
     
     
       97. The surface- acoustic - wave filter according to    claim 94   , wherein said first inductor and said first capacitor are connected in series.   
     
     
       98. The surface- acoustic - wave filter according to    claim 97   , wherein said first impedance circuit further comprises a second capacitor coupled in parallel to said first capacitor and said first inductor.   
     
     
       99. The surface- acoustic - wave filter according to    claim 90   , wherein said first and second shunt - arm surface - acoustic - wave resonators have a common central electrode.   
     
     
       100. The surface- acoustic - wave filter according to    claim 99   , wherein said first impedance circuit is a bonding wire connected to said common central electrode.   
     
     
       101. The surface- acoustic - wave filter according to    claim 90   , further comprising:    
         a third shut - arm surface - acoustic - wave resonator having a fifth shunt - arm terminal connected to said sixth series - arm terminal of said third series - arm surface - acoustic - wave resonator and a sixth shunt - arm terminal; and    
         a second impedance circuit having a third terminal connected to said sixth shunt - arm terminal of said third shunt - arm surface - acoustic - wave resonator and a fourth terminal connected to said ground terminal.   
     
     
       102. A surface- acoustic - wave duplexer, comprising:    
         a receiving surface - acoustic - wave filter circuit; and    
         a transmitting surface - acoustic - wave filter circuit comprising:    
         a first series - arm surface - acoustic - wave resonator having a first series - arm terminal coupled to receive an input signal and a second series - arm terminal;    
         a second series - arm surface - acoustic - wave resonator having a third series - arm terminal connected to said second series - arm terminal of said first series - arm surface - acoustic - wave resonator and a fourth series - arm terminal;    
         a third series - arm surface - acoustic - wave resonator having a fifth series - arm terminal connected to said fourth series - arm terminal of said second series - arm surface - acoustic - wave resonator and a sixth series - arm terminal coupled to output an output signal;    
         a first shunt - arm surface - acoustic - wave resonator having a first shunt - arm terminal connected to said second series - arm terminal of said first series - arm surface - acoustic - wave resonator and a second shunt - arm terminal;    
         a second shunt - arm surface - acoustic - wave resonator having a third shunt - arm terminal connected to said fourth series - arm terminal of said second series - arm surface - acoustic - wave resonator and a fourth shunt - arm terminal connected to said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator; and    
         a first impedance circuit having a first terminal connected to said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator and a second terminal connected to a ground terminal, wherein said first terminal of said first impedance circuit is connected only to said second and fourth shunt - arm terminals of said first and second shunt - arm surface - acoustic - wave resonators, and wherein said first impedance circuit has a first impedance that creates a pole of attenuation by making an open circuit impedance of said transmitting surface - acoustic - wave filter substantially equal to a short - circuit impedance of said transmitting surface - acoustic - wave filter.   
     
     
       103. The surface- acoustic - wave duplexer according to    claim 102   , further comprising:    
       
         an input terminal receiving the input signal; and  
       
         a phase shifter connected between said input terminal and said first series - arm terminal of said first series - arm surface - acoustic - wave resonator.   
     
     
       104. The surface- acoustic - wave duplexer according to claim  102 , wherein said first impedance circuit comprises a first inductor.   
     
     
       105. The surface- acoustic - wave duplexer according to    claim 104   , wherein said first inductor is a bonding wire connected between said second shunt - arm terminal of said first shunt - arm surface - acoustic - wave resonator, said bonding wire having a certain inductive impedance.   
     
     
       106. The surface- acoustic - wave duplexer according to    claim 104   , wherein said first impedance circuit further comprises a first capacitor.   
     
     
       107. The surface- acoustic - wave duplexer according to    claim 106   , wherein said first inductor and said first capacitor are connected to parallel.   
     
     
       108. The surface- acoustic - wave duplexer according to    claim 107   , wherein said first impedance circuit further comprises a second inductor connected in series to said first capacitor and said first inductor.   
     
     
       109. The surface- acoustic - wave duplexer according to    claim 106   , wherein said first inductor and said first capacitor are connected in series.   
     
     
       110. The surface- acoustic - wave duplexer according to    claim 109   , wherein said first impedance circuit further comprises a second capacitor coupled in parallel to said first capacitor and said first inductor.   
     
     
       111. The surface- acoustic - wave duplexer according to    claim 102   , wherein said first and second shunt - arm surface - acoustic - wave resonators have a common central electrode.   
     
     
       112. The surface- acoustic - wave duplexer according to    claim 111   , wherein said impedance circuit is a bonding wire connected to said common central electrode.   
     
     
       113. The surface- acoustic - wave duplexer according to    claim 102   , further comprising:    
         a third shunt - arm surface - acoustic - wave resonator having a fifth shunt - arm terminal connected to said sixth series - arm terminal of said third series - arm surface - acoustic - wave resonator and a sixth shunt - arm terminal; and    
         a second impedance circuit having a third terminal connected to said sixth shunt - arm terminal of said third shunt - arm surface - acoustic - wave resonator and a fourth terminal connected to said ground terminal.