Patent Publication Number: US-2011068881-A1

Title: Acoustic wave filter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an acoustic wave filter. 
     2. Background Art 
     As shown in  FIG. 13 , conventional acoustic wave filter  130  includes unbalanced terminal  137 , first balanced terminal  138   a  for outputting a signal of an opposite phase to that of unbalanced terminal  137 , and second balanced terminal  138   b  for outputting a signal of the same phase as that of unbalanced terminal  137 . Furthermore, acoustic wave filter  130  includes first IDT (Inter Digital Transducer) electrode  131 , second IDT electrode  132 , third IDT electrode  133 , fourth IDT electrode  134  and fifth IDT electrode  135 . First IDT electrode  131  is electrically connected to unbalanced terminal  137 . Second IDT electrode  132  is electrically connected to first balanced terminal  138   a.  Third IDT electrode  133  is electrically connected to second balanced terminal  138   b.  Furthermore, acoustic wave filter  130  includes reflectors  136   a  and  136   b  formed so as to sandwich first to fifth IDT electrodes  131  to  135  from both sides in the acoustic wave propagation direction. 
     With such a configuration, based on an unbalanced signal input from unbalanced terminal  137 , a pair of balanced signals having opposite phases to each other are output from first balanced terminal  138   a  and second balanced terminal  138   b,  respectively. 
     Note here that an example of conventional art information related to the invention of this application includes Japanese Patent Application Unexamined Publication No. 2003-309452. 
     In conventional acoustic wave filter  130 , the degree of balance between a signal output from first balanced terminal  138   a  and a signal output from second balanced terminal  138   b  is not good. 
     SUMMARY OF THE INVENTION 
     An acoustic wave filter of the present invention includes an unbalanced terminal, a first balanced terminal, a second balanced terminal, a first IDT electrode, a second IDT electrode, and a third IDT electrode. The first balanced terminal outputs a signal of an opposite phase to that of the unbalanced terminal. The second balanced terminal outputs a signal of the same phase as that of the unbalanced terminal. The first IDT electrode includes first signal electrode fingers electrically connected to the unbalanced terminal and first ground electrode fingers electrically connected to the ground, and the first signal electrode fingers and the first ground electrode fingers are disposed alternately. The second IDT electrode includes second signal electrode fingers electrically connected to the first balanced terminal and second ground electrode fingers electrically connected to the ground, and the second signal electrode fingers and the second ground electrode fingers are disposed alternately. The third IDT electrode includes third signal electrode fingers electrically connected to the second balanced terminal and third ground electrode fingers electrically connected to the ground, and the third signal electrode fingers and the third ground electrode fingers are disposed alternately. A dummy electrode finger electrically connected neither to the unbalanced terminal nor to the ground is provided on the second IDT electrode side in the first IDT electrode. 
     This configuration can improve the degree of balance between a signal output from the first balanced terminal and a signal output from the second balanced terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a configuration of an acoustic wave filter in accordance with an embodiment of the present invention. 
         FIG. 2A  is a graph showing bandpass characteristics of the acoustic wave filter of the embodiment of the present invention and a conventional acoustic wave filter. 
         FIG. 2B  is an enlarged view of a part surrounded by broken line T 1  of  FIG. 2A . 
         FIG. 2C  is a graph showing a phase difference of the acoustic wave filter of the embodiment of the present invention and a phase difference of a conventional acoustic wave filter. 
         FIG. 2D  is a graph showing an amplitude difference of the acoustic wave filter of the embodiment of the present invention and an amplitude difference of a conventional acoustic wave filter. 
         FIG. 3  is a view illustrating a configuration of an acoustic wave filter of a comparative example. 
         FIG. 4A  is a graph showing bandpass characteristics of an acoustic wave filter of the comparative example and a conventional acoustic wave filter. 
         FIG. 4B  is an enlarged view of a part surrounded by broken line T 2  of  FIG. 4A . 
         FIG. 4C  is a graph showing a phase difference of an acoustic wave filter of the comparative example and a phase difference of a conventional acoustic wave filter. 
         FIG. 4D  is a graph showing an amplitude difference of an acoustic wave filter of the comparative example and an amplitude difference of a conventional acoustic wave filter. 
         FIG. 5  is a view illustrating a configuration of another acoustic wave filter of a comparative example. 
         FIG. 6A  is a graph showing bandpass characteristics of another acoustic wave filter of the comparative example and a conventional acoustic wave filter. 
         FIG. 6B  is an enlarged view of a part surrounded by broken line T 3  of  FIG. 6A . 
         FIG. 6C  is a graph showing a phase difference of another acoustic wave filter of the comparative example and a phase difference of a conventional acoustic wave filter. 
         FIG. 6D  is a graph showing an amplitude difference of another acoustic wave filter of the comparative example and an amplitude difference of a conventional acoustic wave filter. 
         FIG. 7  is a view illustrating a configuration of another acoustic wave filter of a comparative example. 
         FIG. 8A  is a graph showing a bandpass characteristic of another acoustic wave filter of the comparative example and a bandpass characteristic of a conventional acoustic wave filter. 
         FIG. 8B  is an enlarged view of a part surrounded by broken line T 4  of  FIG. 8A . 
         FIG. 8C  is a graph showing a phase difference of another acoustic wave filter of the comparative example and a phase difference of a conventional acoustic wave filter. 
         FIG. 8D  is a graph showing an amplitude difference of another acoustic wave filter of the comparative example and an amplitude difference of a conventional acoustic wave filter. 
         FIG. 9  is a view illustrating a configuration of another acoustic wave filter having another configuration in accordance with the embodiment of the present invention. 
         FIG. 10  is a view illustrating a configuration of another acoustic wave filter in accordance with the embodiment of the present invention. 
         FIG. 11  is a view illustrating a configuration of another acoustic wave filter in accordance with the embodiment of the present invention. 
         FIG. 12  is a view illustrating a configuration of another acoustic wave filter in accordance with the embodiment of the present invention. 
         FIG. 13  is a view illustrating a configuration of a conventional acoustic wave filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention is described with reference to the drawings. However, the present invention is not necessarily limited by this embodiment. 
     Embodiment 
       FIG. 1  is a view showing a configuration of acoustic wave filter  10  in accordance with an embodiment of the present invention. This embodiment describes acoustic wave filter  10  that is a longitudinally coupled double mode filter using five IDT electrodes as an example. 
     In  FIG. 1 , acoustic wave filter  10  includes unbalanced terminal  17 , first balanced terminal  18   a,  second balanced terminal  18   b,  and first IDT electrode  11 , fourth IDT electrode  14  and fifth IDT electrode  15  electrically connected to unbalanced terminal  17 . Furthermore, acoustic wave filter  10  includes second IDT electrode  12  electrically connected to first balanced terminal  18   a,  and third IDT electrode  13  electrically connected to second balanced terminal  18   b.  Furthermore, acoustic wave filter  10  includes reflectors  16   a  and  16   b  disposed so as to sandwich first to fifth IDT electrodes  11  to  15  from both sides in the acoustic wave propagation direction. 
     First IDT electrode  11  includes first signal electrode fingers  11   a  electrically connected to unbalanced terminal  17 , and first ground electrode fingers  11   b  electrically connected to the ground. 
     Second IDT electrode  12  includes second signal electrode fingers  12   a  electrically connected to first balanced terminal  18   a  and second ground electrode fingers  12   b  electrically connected to the ground. 
     Third IDT electrode  13  includes third signal electrode fingers  13   a  electrically connected to second balanced terminal  18   b  and third ground electrode fingers  13   b  electrically connected to the ground. 
     Fourth IDT electrode  14  includes fourth signal electrode fingers  14   a  electrically connected to unbalanced terminal  17  and fourth ground electrode fingers  14   b  electrically connected to the ground. 
     Fifth IDT electrode  15  includes fifth signal electrode fingers  15   a  electrically connected to unbalanced terminal  17  and fifth ground electrode fingers  15   b  electrically connected to the ground. 
     Furthermore, first to fifth IDT electrodes  11  to  15  and reflectors  16   a  and  16   b  are provided on a piezoelectric substrate (not shown) along the acoustic wave propagation direction. The piezoelectric substrate is made of a single crystal piezoelectric material having a plate thickness of about 100 μm to 350 μm. For example, the piezoelectric substrate is a substrate of quartz, lithium tantalate, lithium niobate, or potassium niobate. 
     Furthermore, first signal electrode finger  11   a  and second signal electrode finger  12   a  are formed adjacent to each other. First signal electrode finger  11   a  and third ground electrode  13   b  are formed adjacent to each other. 
     This configuration allows a signal input from unbalanced terminal  17  to pass through a range from 2.11 GHz to 2.17 GHz, which is a reception band of Band  1  of the UMTS standard, and also allows first balanced terminal  18   a  and second balanced terminal  18   b  to output signals of opposite phases to each other. 
     However, a configuration other than acoustic wave filter  10  also allows first balanced terminal  18   a  and second balanced terminal  18   b  to output signals of opposite phases to each other. For example, a configuration in which first signal electrode finger  11   a  and second signal electrode finger  12   a  are formed adjacent to each other, and first ground electrode finger  11   b  and third signal electrode  13   a  are formed adjacent to each other may be employed. In this configuration, similar to acoustic wave filter  10 , since first signal electrode finger  11   a  and second signal electrode finger  12   a  are adjacent to each other, the present invention can be applied by providing dummy electrode finger  11   e  described below. 
     In acoustic wave filter  10  of this embodiment, as shown in  FIG. 1 , dummy electrode finger  11   e  electrically connected neither to unbalanced terminal  17  nor to the ground is provided on second IDT electrode  12  side in first IDT electrode  11 . 
     As shown in  FIG. 1 , outermost electrode finger  11   c  (a first outermost electrode finger) on second signal electrode fingers  12   a  side in first signal electrode fingers  11   a  and outermost electrode finger  11   d  (a second outermost electrode finger) on second signal electrode fingers  12   a  side in first ground electrode fingers  11   b  are formed to be short in length so that they are not engaged with each other. Dummy electrode finger  11   e  is formed so as to have electrode parts facing two outermost electrode fingers  11   c  and  11   d,  respectively. 
     Note here that first to fifth signal electrode fingers  11   a  to  15   a , first to fifth ground electrode fingers  11   b  to  15   b  and dummy electrode finger  11   e  have an electrode thickness of about 0.1 μm to 0.5 μm. These are made of, for example, a simple metal from at least one of aluminum, copper, silver, gold, titanium, tungsten, platinum, chromium, and molybdenum, or an alloy including these metals as a main component or a configuration in which these metals are laminated. Note here that dummy electrode finger  11   e  may be made of materials that are different from materials of first to fifth signal electrode fingers  11   a  to  15   a  and first to fifth ground electrode fingers  11   b  to  15   b.    
       FIG. 2A  is a graph showing bandpass characteristic  20  of acoustic wave filter  10  of this embodiment and bandpass characteristic  21  of conventional acoustic wave filter  130 .  FIG. 2B  is an enlarged view of a part surrounded by broken line T 1  of  FIG. 2A .  FIG. 2C  is a graph showing phase difference  22  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  10  and phase difference  23  between first balanced terminal  138   a  and second balanced terminal  138   b  of conventional acoustic wave filter  130 .  FIG. 2D  is a graph showing amplitude difference  24  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  10  and amplitude difference  25  between first balanced terminal  138   a  and second balanced terminal  138   b  in conventional acoustic wave filter  130 . 
     As shown in  FIGS. 2A to 2C , bandpass characteristic  20  and phase difference  22  of acoustic wave filter  10  maintain the same level as bandpass characteristic  21  and phase difference  23  of conventional acoustic wave filter  130 . Furthermore, as shown in  FIG. 2D , amplitude difference  24  is significantly improved as compared with amplitude difference  25  of conventional acoustic wave filter  130 . That is to say, the degree of balance between a signal output from first balanced terminal  18   a  in acoustic wave filter  10  and a signal output from second balanced terminal  18   b  is significantly improved. 
     The cause of deterioration of the degree of balance in conventional acoustic wave filter  130  and cause of improvement of the degree of balance in acoustic wave filter  10  of this embodiment are thought to be as follows. In general, in a longitudinally coupled double mode filter, a propagation path from the unbalanced terminal to the first balanced terminal and a propagation path from an unbalanced terminal to a second balanced terminal are not electrically the same as each other. That is to say, in conventional acoustic wave filter  130 , first signal electrode finger  131   a  in first IDT electrode  131  and second signal electrode finger  132   a  in second IDT electrode  132  are formed adjacent to each other. Furthermore, first signal electrode finger  131   a  in first IDT electrode  131  and third ground electrode finger  133   b  in third IDT electrode  133  are formed adjacent to each other. 
     With this configuration, signals of opposite phase to each other can be output from first balanced terminal  138   a  and second balanced terminal  138   b.  However, difference occurs between propagation paths. In conventional acoustic wave filter  130 , due to the difference in the propagation paths, a phase difference may occur between a signal output from first balanced terminal  138   a  and a signal output from second balanced terminal  138   b.    
     Acoustic wave filter  10  of this embodiment suppresses the difference between the propagation path from unbalanced terminal  17  to first balanced terminal  18   a  and the propagation path from unbalanced terminal  17  to second balanced terminal  18   b  by dummy electrode finger  11   e.    
     Thus, in acoustic wave filter  10 , by providing dummy electrode finger  11   e  in a predetermined position, the degree of balance between a signal output from first balanced terminal  18   a  and a signal output from second balanced terminal  18   b  can be improved. 
     Hereinafter, the fact that the degree of balance between the signal output from first balanced terminal  18   a  and the signal output from second balanced terminal  18   b  cannot be improved when dummy electrode finger  11   e  is provided in a position that is different from the position in acoustic wave filter  10  is described with reference to  FIGS. 3 to 8D . 
       FIG. 3  is a view illustrating a configuration of an acoustic wave filter of a comparative example of the present invention. Acoustic wave filter  30  shown in  FIG. 3  includes dummy electrode finger  31   e  electrically connected neither to unbalanced terminal  17  nor to the ground on third IDT electrode  13  side in first IDT electrode  11 . Acoustic wave filter  30  is different from acoustic wave filter  10  shown in  FIG. 1  in that dummy electrode finger  31   e  is provided on third IDT electrode  13  side. 
       FIG. 4A  is a graph showing bandpass characteristic  40  of acoustic wave filter  30  and bandpass characteristic  21  of conventional acoustic wave filter  130 .  FIG. 4B  is an enlarged view of a part surrounded by broken line T 2  of  FIG. 4A .  FIG. 4C  is a graph showing phase difference  42  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  30  and phase difference  23  between first balanced terminal  138   a  and second balanced terminal  138   b  of conventional acoustic wave filter  130 .  FIG. 4D  is a graph showing amplitude difference  44  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  30  and amplitude difference  25  between first balanced terminal  138   a  and second balanced terminal  138   b  in conventional acoustic wave filter  130 . 
     From the measurement results shown in  FIGS. 4A to 4D , as compared with conventional acoustic wave filter  130 , in acoustic wave filter  30 , phase difference  42  is maintained at the same level as that of conventional phase difference  23  as shown in  FIG. 4C . However, as shown in  FIGS. 4B and 4D , bandpass characteristic  40  and amplitude difference  44  are not improved. 
       FIG. 5  is a view illustrating a configuration of another acoustic wave filter of a comparative example of the present invention. Acoustic wave filter  50  shown in  FIG. 5  includes dummy electrode finger  51   e  electrically connected neither to first unbalanced terminal  18   a  nor to the ground on first IDT electrode  11  side in second IDT electrode  12 . Acoustic wave filter  50  is different from acoustic wave filter  10  shown in  FIG. 1  in that dummy electrode finger  51   e  is provided such that it is included in second IDT electrode  12 . 
       FIG. 6A  is a graph showing bandpass characteristic  60  of acoustic wave filter  50  and bandpass characteristic  21  of conventional acoustic wave filter  130 .  FIG. 6B  is an enlarged view of a part surrounded by broken line T 3  of  FIG. 6A .  FIG. 6C  is a graph showing phase difference  62  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  50  and phase difference  23  between first balanced terminal  138   a  and second balanced terminal  138   b  of conventional acoustic wave filter  130 .  FIG. 6D  is a graph showing amplitude difference  64  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  50  and amplitude difference  25  between first balanced terminal  138   a  and second balanced terminal  138   b  in conventional acoustic wave filter  130 . 
     From the measurement results shown in  FIGS. 6A to 6D , in acoustic wave filter  50 , as shown in  FIGS. 6B ,  6 C and  6 D, bandpass characteristic  60 , phase difference  62  and amplitude difference  64  are not improved as compared with conventional acoustic wave filter  130 . 
       FIG. 7  is a view illustrating a configuration of another acoustic wave filter of a comparative example of the present invention. Acoustic wave filter  70  shown in  FIG. 7  includes dummy electrode finger  71   e  electrically connected neither to second unbalanced terminal  18   b  nor to the ground on first IDT electrode  11  side in third IDT electrode  13 . Acoustic wave filter  70  is different from acoustic wave filter  10  shown in  FIG. 1  in that dummy electrode finger  71   e  is provided such that it is included in third IDT electrode  13 . 
       FIG. 8A  is a graph showing bandpass characteristic  80  of acoustic wave filter  70  and bandpass characteristic  21  of conventional acoustic wave filter  130 .  FIG. 8B  is an enlarged view of region T 4  of  FIG. 8A .  FIG. 8C  is a graph showing phase difference  82  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  70  and phase difference  23  between first balanced terminal  138   a  and second balanced terminal  138   b  of conventional acoustic wave filter  130 .  FIG. 8D  is a graph showing amplitude difference  84  between first balanced terminal  18   a  and second balanced terminal  18   b  in acoustic wave filter  70  and amplitude difference  25  between first balanced terminal  138   a  and second balanced terminal  138   b  in conventional acoustic wave filter  130 . 
     From the measurement results shown in  FIGS. 8A to 8D , in acoustic wave filter  70 , bandpass characteristic  80 , phase difference  82  and amplitude difference  84  are not improved as compared with conventional acoustic wave filter  130  as shown in  FIGS. 8B ,  8 C and  8 D. 
     From the above-mentioned results, as shown in  FIG. 1 , with the configuration in which dummy electrode finger  11   e  is provided on second IDT electrode  12  side in first IDT electrode  11 , the degree of balance between a signal output from first balanced terminal  18   a  and a signal output from second balanced terminal  18   b  can be improved. 
     Note here that in this embodiment, acoustic wave filter  10  that is a longitudinally coupled double mode filter using five IDT electrodes is described as an example. The acoustic wave filter may be a longitudinally coupled double mode filter in which the number of IDT electrodes to be used is other than five.  FIG. 9  is a view illustrating a configuration of an acoustic wave filter having another configuration in accordance with the embodiment of the present invention. Acoustic wave filter  90  shown in  FIG. 9  is a longitudinally coupled double mode filter using three IDT electrodes. By configuring acoustic wave filter  90  as shown in  FIG. 9 , the degree of balance between a signal output from first balanced terminal  18   a  and a signal output from second balanced terminal  18   b  can be improved and the size of the acoustic wave filter can be reduced. 
     Note here that it is more desirable that a dummy electrode finger is provided only on second IDT electrode  12  side as shown in  FIG. 1  rather than a configuration in which a dummy electrode finger is provided at both end portions of first IDT electrode  11 . By providing the dummy electrode finger on only one side, it is possible to suppress the difference between the propagation path from unbalanced terminal  17  to first balanced terminal  18   a  and the propagation path from unbalanced terminal  17  to second balanced terminal  18   b.    
     Note here that the shape of a dummy electrode finger is not necessarily limited to the shape of dummy electrode finger  11   e  shown in  FIG. 1 . The dummy electrode finger has any shapes as long as it can adjust the propagation path of a signal. For example, as acoustic wave filter  100  shown in  FIG. 10 , the width of electrode finger of dummy electrode finger  101   e  may be made to be larger than that of the other electrode fingers. Thus, even when a difference between a propagation path from unbalanced terminal  17  to first balanced terminal  18   a  and a propagation path from unbalanced terminal  17  to second balanced terminal  18   b  is large, the difference of the propagation path can be suppressed easily. On the contrary, when the difference of the propagation paths is small, the width of an electrode finger of dummy electrode finger  101   e  may be made to be smaller than the width of the other electrode fingers. 
     Furthermore, as acoustic wave filter  110  shown in  FIG. 11 , in dummy electrode finger  111   e,  a length facing outermost electrode finger  11   c  and a length facing outermost electrode finger  11   d  may be asymmetric. Also with this configuration, the difference of the propagation paths can be adjusted. 
     Furthermore, as in acoustic wave filter  120  shown in  FIG. 12 , in dummy electrode finger  121   e,  a part facing outermost electrode finger  11   c  and a part facing outermost electrode finger  11   d  may not be connected to each other. Also with this configuration, the difference of the propagation paths can be adjusted. 
     As mentioned above, an acoustic wave filter of the present invention has an effect of improving the degree of balance between signals output from a pair of balanced terminals, and is useful for electronic devices such as mobile telecommunication devices.