Abstract:
In one aspect of the invention, a duplexer including a multilayer substrate with multiple stacked layers and a back surface with a rectangular shape is disclosed. An antenna terminal, a transmission terminal, a reception terminal, a ground terminal, and a conductor are provided in various positions on the multilayer substrate for improved performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-252268, filed on Nov. 10, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     An aspect of the invention discussed herein is related to a duplexer and an electronic device having the same. Another aspect of the invention is related to a duplexer having a multilayer substrate having a plurality of layers stacked and an electronic device having such a duplexer. 
     BACKGROUND 
     Mobile communication device such as cellular phones and portable information terminal equipment are widely spread with the developments in the information-oriented society. For example, the cellular phones use RF bands as high as 800 MHz˜1.0 GHz and 1.5 GHz˜2.0 GHz. In order to cope with such RF bands, duplexers using acoustic waves such as acoustic wave resonators or piezoelectric thin-film resonators are used. 
     Japanese Patent Application Publication No. 2006-60747 (Document 1) discloses an art of using a shield electrode arranged between a transmission filter pattern and a reception filter pattern to improve the isolation between the patterns. Japanese Patent Application Publication No. 2002-76829 (Document 2) discloses an art of arranging external input terminals and external output terminals of a device having multiple filter elements so as to diagonally connect corners of the shape of the device in order to prevent signals from being interfered with each other. 
     Theoretically, an improvement in the isolation between the transmission and reception terminals of the duplexer may be achieved by designing the transmission filter to have an increased amount of attenuation at frequencies located in the passband of the reception filter and designing the reception filter to have an increased amount of attenuation at frequencies located in the passband of the transmission filter. However, actually, the isolation is affected by not only currents but also current that flow through a space due to electromagnetic coupling. Thus, the isolation is saturated at a certain level. 
     The influence of the electromagnetic coupling may be reduced by providing an electrode between the filters to realize electromagnetic shield structure as described in Document 1. Alternatively, the physical distance between the transmission and reception terminals may be increased, as described in Document 2. However, the arts disclosed in Documents 1 and 2 may not reduce the influence of the electromagnetic coupling considerably in the downsizing trend of RF communication devices. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a duplexer including: a multilayer substrate having multiple layers and having a back surface having a rectangular shape; an antenna terminal provided on the back surface so as to be close to a center of a first side of the back surface; a transmission terminal provided on the back surface so as to be close to a second side of the back surface crossing the first side and to be farther away from the antenna terminal than a center of the second side; a first reception terminal provided on the back surface so as to be close to a third side of the back surface opposite to the second side and to be farther away from the antenna terminal than a center of a third side; a first conductor provided on a first surface of a first layer out of the multiple layers so as to surround at least one of a first area defined by projecting the transmission terminal onto the first layer and a second area defined by projecting the first reception terminal thereon; and a ground terminal provided on a second surface of the first layer opposite to the first surface and located between the transmission terminal and a fourth side opposite to the first side and/or between the first reception terminal and the fourth side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a duplexer according to a first embodiment,  FIG. 1B  is a perspective view of the duplexer from which a cap has been removed,  FIG. 1C  is a perspective view of the duplexer from which a transmission filter and a reception filter have been removed,  FIG. 1D  is a cross-sectional view taken along a line A-A in  FIG. 1A , and  FIG. 1E  is a cross-sectional view taken along a line B-B in  FIG. 1A ; 
         FIG. 2A  is a plan view of the transmission filter or the reception filter, and  FIG. 2B  is a circuit diagram of the filter; 
         FIG. 3A  is a plan view of an acoustic wave resonator; and  FIG. 3B  is a cross-sectional view taken along a line A-A in  FIG. 3A ; 
         FIG. 4A  is a view of a cap mount layer and a cavity layer of a first embodiment,  FIG. 4B  is a plan view of a die attach layer of the first embodiment,  FIG. 4C  is a plan view of a line pattern layer of the first embodiment,  FIG. 4D  is a plan view of a line pattern/foot pad layer of the first embodiment, and  FIG. 4E  is a bottom view of the line pattern/foot pad layer; 
         FIG. 5A  is a perspective view of the duplexer of the first embodiment, and  FIG. 5B  is a circuit diagram of the duplexer; 
         FIG. 6A  is a cap mount layer and a cavity layer of a first comparative example,  FIG. 6B  is a plan view of a die attach layer of the first comparative example,  FIG. 6C  is a plan view of a line pattern layer of the first comparative example,  FIG. 6D  is a plan view of a line pattern/foot pad layer of the first comparative example, and  FIG. 6E  is a bottom view of the line pattern/foot pad layer; 
         FIG. 7A  is a perspective view of an electronic device in which the duplexer of the first embodiment is mounted on a printed-circuit board, and  FIG. 7B  is a perspective view of an electronic device in which the duplexer of the first comparative example is mounted on a printed-circuit board; 
         FIG. 8A  illustrates simulation results of passband characteristics of transmission and reception filters,  FIG. 8B  illustrates simulation results of an isolation characteristic between a transmission terminal and a reception terminal; 
         FIG. 9A  is a plan view of a piezoelectric thin-film resonator, and  FIG. 9B  is a cross-sectional view taken along a line A-A in  FIG. 9A ; 
         FIG. 10A  is a perspective view of a duplexer according to a first variation of the first embodiment,  FIG. 10B  is a perspective view of the duplexer from which resin has been removed,  FIG. 10C  is a perspective view of the duplexer from which the transmission and reception filters have been removed,  FIG. 10D  is a cross-sectional view taken along a line A-A in  FIG. 10A , and  FIG. 10E  is a cross-sectional view taken along a line B-B in  FIG. 10A ; 
         FIG. 11A  is a perspective view of a duplexer according to a second variation of the first embodiment,  FIG. 11B  is a perspective view of the duplexer from which a solder film has been removed,  FIG. 11C  is a perspective view of the duplexer from which the transmission and reception filters have been removed,  FIG. 11D  is a cross-sectional view taken along a line A-A in  FIG. 11A , and  FIG. 11E  is a cross-sectional view taken along a line B-B in  FIG. 11A ; 
         FIG. 12A  is a perspective view of a duplexer according to a third variation of the first embodiment,  FIG. 12B  is a perspective view of the duplexer from which resin has been removed,  FIG. 12C  is a perspective view of the duplexer from which a filter chip has been removed,  FIG. 12D  is a cross-sectional view taken along a line A-A in  FIG. 12A , and  FIG. 12E  is a cross-sectional view taken along a line B-B in  FIG. 12A ; 
         FIG. 13A  illustrates an exemplary structure in which a ground terminal is further provided in the center of the back surface of the multilayer substrate, and  FIG. 13B  illustrates an exemplary structure in which ground terminals provided on the back surface of the multilayer substrate are incorporated with each other to form a single piece; 
         FIG. 14  is a bottom view of a line pattern/foot pad layer used in a second embodiment; 
         FIG. 15A  is a perspective view of a duplexer according to the second embodiment, and  FIG. 15B  is a perspective view of an electronic device in which the duplexer of the second embodiment is mounted on a printed-circuit board; 
         FIG. 16A  is a view of a cap mount layer and a cavity layer of a third embodiment,  FIG. 16B  is a plan view of a die attach layer of the third embodiment,  FIG. 16C  is a plan view of a line pattern layer of the third embodiment,  FIG. 16D  is a plan view of a line pattern/foot pad layer of the third embodiment, and  FIG. 16E  is a bottom view of the line pattern/foot pad layer; 
         FIG. 17A  is a perspective view of a duplexer of the third embodiment, and  FIG. 17B  is a perspective view of an electronic device in which the duplexer of the third embodiment is mounted on a printed-circuit board; 
         FIG. 18A  illustrates an exemplary structure in which a ground terminal is further provided in the center of the back surface of the multilayer substrate, and  FIG. 18B  illustrates an exemplary structure in which ground terminals are incorporated with each other to form a single piece; 
         FIG. 19A  is a bottom view of a line pattern/foot pad layer used in a fourth embodiment, and  FIG. 19B  is a bottom view of a line pattern/foot pad layer used in a first variation of the fourth embodiment; 
         FIG. 20A  is a perspective view of a duplexer of the fourth embodiment, and  FIG. 20B  is a perspective view of an electronic device in which the duplexer is mounted on a printed-circuit board; 
         FIG. 21A  is a view of a cap mount layer and a cavity layer of a fifth embodiment,  FIG. 21B  is a plan view of a die attach layer of the fifth embodiment,  FIG. 21C  is a plan view of a line pattern layer,  FIG. 21D  is a plan view of a line pattern/foot pad layer, and  FIG. 21E  is a bottom view of the line pattern/foot pad layer; 
         FIG. 22A  is a perspective view of a duplexer of the fifth embodiment, and  FIG. 22B  is a perspective view of an electronic device in which the duplexer is mounted on a printed-circuit board; 
         FIG. 23A  illustrates passband characteristics of transmission and reception filters, and  FIG. 23B  illustrates isolation characteristics between the transmission and reception terminals; 
         FIG. 24A  is a perspective view of a duplexer according to a sixth embodiment, and  FIG. 24B  is a perspective view of an electronic device in which the duplexer is mounted on a printed-circuit board; 
         FIG. 25A  is a plan view of a reception filter, and  FIG. 25B  is a circuit diagram of the reception filter; 
         FIG. 26A  is a view of a cap mount layer and a cavity layer of a sixth embodiment,  FIG. 26B  is a die attach layer of the sixth embodiment,  FIG. 26C  is a plan view of a line pattern layer of the sixth embodiment,  FIG. 26D  is a plan view of a line pattern/foot pad layer of the sixth embodiment, and  FIG. 26E  is a bottom view of the line pattern/foot pad layer; 
         FIG. 27A  is a perspective view of a duplexer according to a seventh embodiment, and  FIG. 27B  is a perspective view of an electronic device in which the duplexer is mounted on a printed-circuit board; and 
         FIGS. 28A through 28F  illustrate variations of the line pattern/foot pad layer with two reception terminals. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are now described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1A  is a perspective view of a duplexer in accordance with a first embodiment.  FIG. 1B  is a perspective view of the duplexer in  FIG. 1A  from which a cap has been removed.  FIG. 1C  is a perspective view of the duplexer in  FIG. 1B  from which a transmission filter and a reception filter have been removed.  FIG. 1D  is a cross-sectional view taken along a line A-A in  FIG. 1A , and  FIG. 1E  is a cross-sectional view taken along a line B-B in  FIG. 1A . As illustrated in  FIGS. 1A through 1C , the duplexer of the first embodiment has a multilayer substrate  10  having a cavity  18 , a transmission filter  12  and a reception filter  14  housed in the cavity  18 , and a cap  16  made of a metal for sealing the transmission filter  12  and the reception filter  14  in the cavity  18 . Foot pads  36  are provided on the lower surface of the multilayer substrate  10 . A conductive pattern  20  including bump pads and interconnection lines are provided on the bottom plane of the cavity  18 . The lower surface of the multilayer substrate  10  may be 2.5 mm×2.0 mm. 
     As illustrated in  FIGS. 1D and 1E , the multilayer substrate  10  is formed by stacking a plurality of layers, which may be composed of a cap mount layer  22 , a cavity layer  24 , a die attach layer  26 , a line pattern layer  28 , and a line pattern/foot pad layer  30 , for example. The cap mount layer  22  and the cavity layer  24  define the cavity  18  in which the transmission filter  12  and the reception filter  14  are housed. The cap  16  is attached to the upper surface of the cap mount layer  22  and seals the transmission filter  12  and the reception filter  14  in the cavity  18 . Bump pads  32  are provided on the upper surface of the die attach layer  26 . The transmission filter  12  and the reception filter  14  are mounted on the bump pads  32  by using bumps  34 , which may be made of solder. The transmission filter  12  and the reception filter  14  are facedown mounted. Foot pads  36  are provided on the lower surface of the line pattern/foot pad layer  30 . The foot pads  36  may be an antenna terminal, a transmission terminal, a reception terminal, and a ground terminal, for example. Each of the stacked layers may be formed by an insulator such as ceramic or resin. The line pattern  38  and vias  39 , which may be made of an electrically conductive material, are used to electrically interconnect the transmission filter  12 , the reception filter  14  and the foot pads  36 . 
     The transmission filter  12  and the reception filter  14  are now described with reference to  FIGS. 2A and 2B .  FIG. 2A  is a plan view of an exemplary structure of the transmission filter  12  and the reception filter  14 , and  FIG. 2B  is a circuit diagram thereof. Referring to  FIGS. 2A and 2B , each of the transmission filter  12  and the reception filter  14  is of ladder type, and is composed of series resonators S 11 ˜S 14  and parallel resonators P 11 ˜P 14 . Each of those resonators may be a surface acoustic wave resonator, for example. 
     The surface acoustic wave resonator is now described in detail with reference to  FIGS. 3A and 3B .  FIG. 3A  is a plan view of a surface acoustic wave resonator, and  FIG. 3B  is a cross-sectional view taken along a line A-A in  FIG. 3A . Referring to  FIGS. 3A and 3B , the surface acoustic wave resonator is composed of a piezoelectric substrate  40 , a pair of reflection electrodes  42  formed thereon, and an interdigital transducer (IDT)  44  formed on the a piezoelectric substrate  40  and interposed between the pair of reflection electrodes  42 . The IDT  44  has an input electrode and an output electrode, which face each other and electrode fingers thereof are interdigitated. The piezoelectric substrate  40  may be made of lithium tantalate (LT) or lithium niobate (LN), for example. 
     The layers of the multilayer substrate  10  are now described with reference to  FIGS. 4A through 4E . As illustrated in  FIG. 4A , a hole for forming the cavity  18  is formed in the cap mount layer  22  and the cavity layer  24 . The transmission filter  12  and the reception filter  14  are housed in the cavity  18 . 
     As illustrated in  FIG. 4B , an electrically conductive pattern  20  made of an electrically conductive material such as a metal is formed on the upper surface of the die attach layer  26 . The conductive pattern  20  includes bump pads (black circles in  FIG. 4B ), line lines (black lines), and vias (black double circles). The conductive pattern  20  has a thickness of, for example, 15 μm. The conductive pattern  20  may be made of an alloy of aluminum (Al) and copper (Cu). The conductive pattern  20  may be made of another alloy including Al as a main component such as Al—Mg or may be a multilayer film including metals or another multilayer film including a metal and an alloy. Examples of the multilayer film are Al—Cu/Cu/Al—Cu, Al/Cu/Al, Al/Mg/Al, Al—Mg/Mg/Al—Mg. Pads A˜G 2  of the transmission filter  12  and the reception filter  14  illustrated in  FIG. 4A  are connected to bump pads A˜G 2  on the upper surface of the die attach layer  26 . Pads A and C are electrically connected to an antenna terminal  50  on the lower surface of the line pattern/foot pad layer  30 . Pad B is electrically connected to a transmission terminal  52  on the lower side of the layer  30 , and pad D is electrically connected to a first reception terminal  54  thereon. Pads E 1 ˜E 3 , F 1 ˜F 3  and G 1 ˜G 2  are connected to ground terminals  56  on the lower surface of the layer  30 . Vias pass through the die attach layer  26  and are full of an alloy of Al and Cu, for example. The vias electrically connect the conductive pattern  20  formed on the upper surface of the die attach layer  26  and the conductive pattern formed on the upper surface of the line pattern layer  28  together. Vias V 1 ˜V 11  that appear on the upper surface of the die attach layer  26  are connected to vias V 1 ˜V 11  that appear on the upper surface of the line pattern layer  28 . The line pattern includes lines for connecting the bump pads and the vias together. 
     As illustrated in  FIG. 4C , a conductive pattern  20  including lines and vias is formed on the upper surface of the line pattern layer  28 . The lines connect the vias. The vias that pass through the line pattern layer  28  electrically interconnect the conductive pattern form on the upper surface of the line pattern layer  28  and the conductive pattern  20  formed on the upper surface of the line pattern/foot pad layer  30 . Vias V 1 ˜V 4 , V 11  and W 1 ˜W 3  that appear on the upper surface of the line pattern layer  28  are connected to vias V 1 ˜V 4 , V 11  and W 1 ˜W 3  that appear on the upper surface of the line pattern/foot pad layer  30 . 
     As illustrated in  FIG. 4D , a conductive pattern  20  including a line and vias is formed on the upper surface of the line pattern/foot pad layer  30 . A first conductor  46  is provided so as to surround an area virtually formed by projecting the transmission terminal  52  provided on the lower surface of the line pattern/foot pad layer  30  on the upper surface thereof (an area illustrated by a broken line in  FIG. 4D ). The first conductor  46  may be made of the same material as that of the conductive pattern  20  and may be an alloy of Al and Cu, for example. The first conductor  46  may have the same thickness as that of the conductive pattern  20  and is, for example, 15 μm. The first conductor  46  is not connected to the other conductive patterns  20  and is a floating conductor. The vias that pass through the line pattern/foot pad layer  30  electrically interconnect the conductive pattern  20  formed on the upper surface of the layer  30  and the foot pads  56  formed on the lower surface thereof. Vias V 1 ˜V 3 , V 11 , V 12  and W 1 ˜W 3  that appear on the upper surface of the layer  30  are connected to vias V 1 ˜V 3 , V 11 , V 12  and W 1 ˜W 3  that appear on the lower surface thereof. 
     As illustrated in  FIG. 4E , the foot pads  56 , which may be made of a conductor such as cupper plated with gold, are provided on the lower surface of the line pattern/foot pad layer  30 . The foot pads  56  include the antenna terminal  50 , the transmission terminal  52 , the first reception terminal  54  and the ground terminals  56 . The antenna terminal  50  is provided in the vicinity of the center of a first side  58 , which is one of two long sides of a rectangular lower surface of the line pattern/foot pad layer  30 . The transmission terminal  52  is provided close to a second side  60  of the rectangular lower surface orthogonal to the first side  58  and is closer to a fourth side  64  than the first side  58 . The fourth side  64  is opposite to the first side  58 . The first reception terminal  54  is provided close to a third side  62  opposite to the second side  60  and is located in the vicinity of a corner defined by the third side  62  and the fourth side  64 . In  FIG. 4E , six ground pads  56  are illustrated. A first one of the six pads  56  is provided in the vicinity of the center of the second side  60 . A second one of the pads  56  is provided in the vicinity of the center of the third side  62 . A third one of the pads  56  is provided in the vicinity of the center of the fourth side  64 . A fourth one of the pads  56  is provided in the vicinity of a corner defined by the first side  58  and the second side  60 . A fifth one of the pads  56  is provided in the vicinity of a corner defined by the first side  58  and the third side  62 . A sixth one of the pads  56  is provided in the vicinity of a corner defined by the second side  60  and the fourth side  64 . 
       FIG. 5A  is a perspective view of the first embodiment, which is seen through structural components other than the conductive pattern  20 , the foot pads  56  and the first conductor  46  (this holds true for  FIGS. 15A ,  17 A,  20 A and  27 A). As illustrated in  FIG. 5A , the transmission terminal  52  is surrounded by the three ground terminals  56  among the six ground terminals, namely, the above-described first, third and sixth ground pads  56 . That is, the ground terminals  56  are provided in a direction along the fourth side  64  and in another direction along the second side  60 . The two directions are orthogonal to each other. On the lower surface of the multilayer substrate  10 , the transmission terminal  52  is electromagnetically shielded with the ground terminals  56  in the two directions, that is, the direction towards the first reception terminal  54  from the transmission terminal  52  and the direction orthogonal thereto. 
     The first conductor  46  is provided to the upper surface of the line pattern/foot pad layer  30  included in the multilayer substrate  10  so as to surround the area onto which the transmission terminal  52  is projected. The first conductor  46  is arranged around the area onto which the transmission terminal  52  is projected is composed of two portions extending in the direction along the fourth side  64  and a portion extending in the direction orthogonal thereto and connecting the two portions. Thus, the transmission terminal  52  is electromagnetically shielded with the ground terminals  56  in the two directions, that is, the direction towards the first reception terminal  54  from the transmission terminal  52  and the direction orthogonal thereto. 
       FIG. 5B  is a circuit diagram of the duplexer of the first embodiment. Referring to  FIG. 5B , the transmission filter  12  is connected between the antenna terminal  50  and the transmission terminal  52 . The reception filter  14  is connected between the antenna terminal  50  and the first reception terminal  54 . The transmission filter  12  is connected between the antenna terminal  50  and the transmission terminal  52  via the conductive patterns  20  illustrated in  FIGS. 4A through 4E . Similarly, the reception filter  14  is connected between the antenna terminal  50  and the first reception terminal  54  via the conductive patterns  20  illustrated in  FIGS. 4A through 4E . 
     A description is now given of a first comparative example, which is to be compared with the duplexer of the first embodiment in order to describe effects of the first embodiment. A duplexer configured according to the first comparative example differs from the duplexer of the first embodiment only in the pattern formed on the line pattern/foot pad layer  30  of the multilayer substrate  10 . Except the above pattern, the duplexer of the first comparative example is the same as that of the first embodiment already described with reference to  FIG. 1A  through  FIG. 3B .  FIGS. 6A through 6E  illustrate layers of the multilayer substrate  10  employed in the duplexer of the first comparative example. As illustrated in  FIG. 6D , the first conductor  46  is not provided on the upper surface of the line pattern/foot pad layer  30 . As illustrated in  FIG. 6E , the ground terminal  56  is not provided between the transmission terminal  52  and the fourth side  64  on the lower surface of the line pattern/foot pad layer  30 . The transmission terminal  52  is provided in the vicinity of the corner formed by the second side  60  and the fourth side  64 . The other parts of the first comparative example are the same as those of the first embodiment previously described with reference to  FIGS. 4A through 4E . 
       FIG. 7A  is a perspective view of an electronic device in which the duplexer of the first embodiment is mounted on a printed-circuit board, and  FIG. 7B  is a perspective view of an electronic device in which the duplexer of the first comparative example is mounted on a printed-circuit board. In  FIGS. 7A and 7B , the duplexers are seen through the structural parts other than the foot pads  36  and the first conductor  46  (this holds true for  FIGS. 15B ,  17 B,  20 B and  27 B). The electronic device may be a portable information terminal or a mobile communication device, for example. As illustrated in  FIGS. 7A and 7B , a printed-circuit board  66  has an antenna signal line  68 , a transmission signal line  70 , a reception signal line  72 , and a ground line  74 . The antenna terminal provided to the lower surface of the multilayer substrate  10  is connected to the antenna signal line  68  of the printed-circuit board  66  by solder, for example. Similarly, the transmission terminal  52 , the first reception terminal  54  and the ground terminals  56  are respectively connected to the transmission signal line  70 , the reception signal line  72  and the ground line  74  by solder, for example. The heights of the antenna terminal  50 , the transmission terminal  52 , the first reception terminal  54  and the ground terminals  56  may be 30 μm, for example. The above heights correspond to the distance between the printed-circuit board  66  and the lower surface of the multilayer substrate  10 . The ground line  74  is provided so as to surround peripheries of the antenna signal line  68 , the transmission signal line  70  and the reception signal line  72 . 
     The duplexer of the first embodiment differs from that of the first comparative example in that the ground terminal  56  is additionally provided between the transmission terminal  52  and the fourth side  64  and the first conductor  46  is provided so as to surround the area virtually formed by projecting the transmission terminal  52 . Thus, the transmission terminal  52  of the electronic device with the duplexer of the first embodiment is integrally surrounded by the ground terminals  56 , the first conductor  46  and the ground line  74 . 
     The inventors simulated the electronic devices illustrated in  FIGS. 7A and 7B , and measured the passband characteristics of the transmission filter  12  and the reception filter  14 , and the isolation characteristic between the transmission terminal  52  and the first reception terminal  54  in each of the electronic devices.  FIG. 8A  illustrates the simulation results of the passband characteristics between the transmission filter  12  and the reception filter  14  in each of the electronic devices, and  FIG. 8B  illustrates the simulation results of the isolation characteristic between the transmission terminal  52  and the first reception terminal  54  in each of the electronic devices. In  FIGS. 8A and 8B , the horizontal axis denotes the normalized frequency, and the vertical axis denotes the insertion loss. The simulation results of the first embodiment are illustrated by solid lines, and those of the first comparative example are illustrated by broken lines. 
     As illustrated in  FIG. 8A , there is little difference in the passband characteristic of the duplexer between the first embodiment and the first comparative example. As illustrated in  FIG. 8B , the isolation characteristic of the first embodiment is improved by about 10 dB in the transmission band and outside of the passbands, as compared with the first comparative example. The reason why the isolation characteristic of the first embodiment is improved is that the transmission terminal  52  is three-dimensionally shielded by additionally providing the first conductor  46  surrounding the area virtually formed by projecting the transmission terminal  52  and the ground terminal  56  between the transmission terminal  52  and the fourth side  64 . 
     As described above, according to the first embodiment, the first conductor  46  is provided on the upper surface of the line pattern/foot pad layer  30  so as to surround the area virtually formed by projecting the transmission terminal  52  thereon, as illustrated in  FIG. 4D . Further, as illustrated in  FIG. 4E , the ground terminal  56  is provided between the transmission terminal  52  and the fourth side  64  on the lower surface of the multilayer substrate  10  (that is, the lower surface of the line pattern/foot pad layer  30 ) in addition to the ground terminal  56  provided in the vicinity of the center of the second side  60  and the ground terminal provided in the vicinity of the center of the fourth side  64 . Thus, on the lower surface of the multilayer substrate  10  and the upper surface of the line pattern/foot pad layer  30 , the transmission terminal  52  is electromagnetically shielded in both the direction towards the first reception terminal  54  and the direction orthogonal thereto. It is thus possible to block the electromagnetic wave propagating towards the first reception terminal  54  from the transmission terminal  52  and that in the direction orthogonal thereto. It is to be noted that the electromagnetic wave particularly tends to be radiated towards the side of the transmission signal line  70  of the printed-circuit board  66 . Thus, the electromagnetic wave can be blocked by the ground terminal  56  provided between the transmission terminal  52  and the fourth side  64  in addition to the ground terminal  56  provided in the vicinity of the center of the second side  60 . It is thus possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and to improve the isolation characteristic as illustrated in  FIG. 8B . 
     As illustrated in  FIG. 4E , the antenna terminal  50  is provided in the vicinity of the center of the first side  58 , which is one of the four sides of the lower surface of the multilayer substrate  10  having a rectangular shape. The transmission terminal  52  is provided in the vicinity of the second side  60  so as to be farther away from the antenna terminal  50  than the center of the second side  60 . The first reception terminal  54  is provided in the vicinity of the third side  62  so as to be farther away from the antenna terminal  50  than the center of the third side  62 . It is thus possible to increase the physical distances between the antenna terminal  50 , the transmission terminal  52  and the first reception terminal  54  and to reduce the electromagnetic couplings between these terminals more effectively. The lower surface (back surface) of the multilayer substrate  10  may be not limited to the rectangular shape but may be shaped into a square. 
     In the above-described first embodiment, the first conductor  46  surrounds the area virtually formed by projecting the transmission terminal  52 , and the ground terminal  56  is provided between the transmission terminal  52  and the fourth side  64 . However, the first embodiment is not limited to the above but may have another arrangement. The first conductor  46  may be provided so as to surround an area virtually formed by projecting the first reception terminal  54 , and the ground terminal  56  may be provided between the first reception terminal  54  and the fourth side  64 . That is, the first conductor  46  may be provided so as to surround the area virtually formed by projecting the transmission terminal  52  or the first reception terminal  54 , and the ground terminal  56  may be located below the first conductor  46  and may be interposed between the transmission terminal  52  and the fourth side  64  or between the first reception terminal  54  and the fourth side  64 . It is thus possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and improve the isolation characteristic. When downsizing of the duplexer is considered, it is preferable that the ground terminal  56  is provided between the transmission terminal  52  and the fourth side  64  or between the first reception terminal  54  and the fourth side  64  and is located just below the first conductor  46 . 
     In order to block the electromagnetic wave radiated via the transmission terminal  52 , it is preferable that the first conductor  46  surrounds the area virtually formed by projecting the transmission terminal  52  in the direction towards the first reception terminal  54  and the direction orthogonal thereto. However, even when the first conductor  46  partly surrounds the area virtually formed by projecting the transmission terminal  52 , the electromagnetic wave from the transmission terminal  52  may be blocked. 
     The first conductor  46  is not limited to the floating conductor but may be grounded. That is, the first conductor  46  is not connected to the signal line connected to any of the antenna terminal  50 , the transmission terminal  52  and the first reception terminal  54 . In order to improve the effect of shielding the transmission terminal  52 , it is preferable that the ground terminal  56  provided between the transmission terminal  52  and the fourth side  64  and the ground terminal  56  provided in the vicinity of the center of the second side  60  extend beyond the transmission terminal  52  in the direction in which the fourth side  64  extends. That is, it is preferable that the whole side of the transmission terminal  52  is surrounded by the ground terminal  56 . However, even when at least a part of the side of the transmission terminal  52  is surrounded by the ground terminal  56 , the electromagnetic wave may be blocked. 
     The transmission filter  12  and the reception filter  14  of the first embodiment are not limited to the ladder type filters but may be of another type and may be a multi-mode filter. The resonators that form the ladder type filter are not limited to the surface acoustic wave resonators but may be acoustic wave resonators such as piezoelectric thin-film resonators or boundary acoustic wave filters. Yet another type of filter may be employed. It is particularly preferable to use resonators suitable for RF communications. Now, the piezoelectric thin-film resonator is described with reference  FIGS. 9A and 9B .  FIG. 9A  is a plan view of a piezoelectric thin-film resonator, and  FIG. 9B  is a cross-sectional view taken along a line A-A in  FIG. 9A . Referring to  FIGS. 9A and 9B , the piezoelectric thin-film resonator has a lower electrode  78 , a piezoelectric film  80  and an upper electrode  82 , which are stacked in this order on a substrate  76 . A resonator part  84  is formed by portions of the lower electrode  78  and the upper electrode  82  that face each other through the piezoelectric film  80 . A cavity  18  is formed on the substrate  76  below the lower electrode  78  so as to include the resonator part  84  horizontally. 
     In the first embodiment, as illustrated in  FIGS. 1A through 1E , the transmission filter  12  and the reception filter  14  are arranged in the cavity  18  of the multilayer substrate  10 , and are enclosed in the cavity  18  by the cap  16 . However, the first embodiment is not limited to the above arrangement. For example, the first embodiment has a first variation illustrated in  FIGS. 10A through 10E , a second variation illustrated in  FIGS. 11A through 11E , a third variation illustrated in  FIGS. 12A through 12E , and may have a further variation. The first through third variations are now described. 
       FIGS. 10A through 10C  are perspective views of the first variation of the first embodiment,  FIG. 10D  is a cross-sectional view taken along a line A-A illustrated in  FIG. 10A , and  FIG. 10E  is a cross-sectional view taken along a line B-B in  FIG. 10A . Referring to  FIGS. 10A through 10E , the multilayer substrate  10  is composed of the die attach layer  26 , the line pattern layer  28  and the line pattern/foot pad layer  30 . The cavity  18  is not formed in the multilayer substrate  10 . The transmission filter  12  and the reception filter  14  are mounted on the upper surface of the die attach layer  26 , and are sealed with resin  86  by transfer molding. The other structures of the first variation are the same as those of the first embodiment. 
       FIGS. 11A through 11C  are perspective views of the second variation of the first embodiment,  FIG. 11D  is a cross-sectional view taken along a line A-A illustrated in  FIG. 11A , and  FIG. 11E  is a cross-sectional view taken along a line B-B in  FIG. 11A . Referring to  FIGS. 11A through 11E , the multilayer substrate  10  is composed of the die attach layer  26 , the line pattern layer  28  and the line pattern/foot pad layer  30 . The cavity  18  is not formed in the multilayer substrate  10 . The transmission filter  12  and the reception filter  14  are mounted on the upper surface of the die attach layer  26 . A seal ring  88  made of a metal or the like is provided on peripheral portions on the upper surface of the die attach layer  26  so as to surround the transmission filter  12  and the reception filter  14 . A solder film  90  is connected to the seal ring  88  and is formed to enclose the transmission filter  12  and the reception filter  14 . The transmission filter  12  and the reception filter  14  are hermetically sealed with the solder film  90  that contacts the seal ring  88 . The other structures of the second variation are the same as those of the first embodiment illustrated in  FIGS. 1A through 1E . 
       FIGS. 12A through 12C  are perspective views of the third variation of the first embodiment,  FIG. 12D  is a cross-sectional view taken along a line A-A illustrated in  FIG. 12A , and  FIG. 12E  is a cross-sectional view taken along a line B-B in  FIG. 12A . Referring to  FIGS. 12A through 12E , the multilayer substrate  10  is composed of the die attach layer  26 , the line pattern layer  28  and the line pattern/foot pad layer  30 . The cavity  18  is not formed in the multilayer substrate  10 . A filter chip  92  on which the transmission filter  12  and the reception filter  14  are formed is mounted on the upper surface of the die attach layer  26 . The seal ring  88  is provided on the peripheral portions on the upper surface of the die attach layer  26  so as to surround the filter chip  92 . The filter chip  92  is sealed with resin  94  by a laminate method using a resin sheet. The filter chip  92  can be hermetically sealed with the resin  94  that contacts the seal ring  88 . The other structures of the third variation are the same as those of the first embodiment illustrated in  FIGS. 1A through 1E . 
     The ground terminals  56  provided on the back surface of the multilayer substrate  10  of the first embodiment are not limited to the arrangement illustrated in  FIG. 4E . This arrangement is briefly described here again. Three ground terminals  56  are respectively arranged in the vicinities of the centers of the second side  60 , the third side  62  and the fourth side  64 . One ground terminal  56  is provided in the vicinity of the corner defined by the first side  58  and the second side  60 . One ground terminal  56  is provided in the vicinity of the corner defined by the third side  58  and the third side  62 . One ground terminal  56  is provided between the transmission terminal  52  and the fourth side  64 .  FIG. 13A  illustrates another arrangement of the ground terminals  56 . Referring to  FIG. 13A , one ground terminal  56  is provided in the center of the back surface of the multilayer substrate  10 .  FIG. 13B  illustrates yet another arrangement of the ground terminals  56 . Referring to  FIG. 13B , the ground terminals  56  in  FIG. 13A  are connected together so that a single ground terminal  56  is formed on the back surface of the multilayer substrate  10 . The single ground terminal  56  is formed so as to surround the antenna terminal  50 , the transmission terminal  52  and the first reception terminal  54 . The arrangements in  FIGS. 13A and 13B  surround the transmission terminal  52  more widely than the arrangement illustrated in  FIG. 4E . It is thus possible to more effectively reduce the influence of the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . 
     Second Embodiment 
     A second embodiment is an exemplary duplexer in which the ground terminal  56  is provided so as to surround the periphery of the transmission terminal  52  in four directions. The second embodiment is the same as the first embodiment illustrated in  FIGS. 1A through 5  except that the second embodiment employs a pattern of the boot pads  36  on the lower surface of the line pattern/foot pad layer  30 , which pattern is different from that of the first embodiment. Thus, a description is mainly given of the lower surface of the line pattern/foot pad layer  30 . 
       FIG. 14  is a diagram that illustrates an exemplary back or lower surface of the line pattern/foot pad layer  30 . Referring to  FIG. 14 , there are provided five ground terminals  56 . One of the ground terminals  56  is provided in the vicinity of the center of the fourth side  64  and is interposed between the transmission terminal  52  and the first reception terminal  54 . Another one of the ground terminals  56  has a portion provided between the transmission terminal  52  and the fourth side  64 , another portion provided between the transmission terminal  52  and the second side  60 , and yet another portion provided in the vicinity of the center of the second side  60 . Thus, the transmission terminal  52  is surrounded in the four directions. 
       FIG. 15A  is a perspective view of the duplexer of the second embodiment, and  FIG. 15B  is a perspective view of an electronic device in which the duplexer of the second embodiment is mounted on a printed-circuit board. Referring to  FIG. 15A , the ground terminal  56  is provided between the transmission terminal  52  and the second side  60 , so that the transmission terminal  52  is surrounded in the four directions. The other structures of the second embodiment are the same as those of the first embodiment illustrated in  FIG. 5 , and a description thereof is omitted here. As illustrated in  FIG. 15B , since the ground terminal  56  is provided between the transmission terminal  52  and the second side  60 , the ground line  74  on the printed-circuit board  66  is provided around the transmission signal line  70  to which the transmission terminal  52  is connected. Thus, the transmission signal line  70  is provided within the printed-circuit board  66  except the portion thereof to which the transmission terminal  52  is connected. The other structures of the second embodiment are the same as those of the first embodiment illustrated in  FIG. 7A , and a description thereof is omitted here. 
     As described above, according to the second embodiment, the ground terminal  56  is provided between the transmission terminal  52  and the first reception terminal  54  in addition to the ground terminal  56  in the vicinity of the center of the second side  60 , the ground terminal  56  in the vicinity of the center of the fourth side  64  and interposed between the transmission terminal  52  and the first reception terminal  54 , and the ground terminal  56  between the transmission terminal  52  and the second side  60 . Thus, on the back surface of the line pattern/foot pad layer  30 , the transmission terminal  52  is electromagnetically shielded in the direction towards the first reception terminal  54 , two directions orthogonal thereto, and the direction opposite to the direction towards the first reception terminal  54 . It is thus possible to block the electromagnetic waves radiated in the direction towards the first reception terminal  54 , the two directions orthogonal thereto, and the direction opposite to the direction towards the first reception terminal  54  and to more effectively shield the transmission terminal  52 . It is thus possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and further improve the isolation characteristic. 
     The first conductor  46  surrounds the area formed by projecting the transmission terminal  52  in the direction towards the first reception terminal  54  and the two directions orthogonal thereto, as illustrated in  FIG. 15A . Preferably, the area formed by projecting the transmission terminal  52  is surrounded in the four directions by providing the ground terminal  56  between the transmission terminal  52  and the second side  60 . In this case, the area is surrounded in the direction towards the first reception terminal  54 , the two directions orthogonal thereto, and the direction opposite to the direction towards the first reception terminal  54 . It is thus possible to more effectively shield the transmission terminal  52  and further reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . 
     The second embodiment is not limited to the above structure. For example, in the structure in which the first conductor  46  surrounds the area defined by projecting the first reception terminal  54 , an additional ground terminal  56  may be provided between the first reception terminal  54  and the third side  62  in addition to the ground terminal between the first reception terminal  54  and the fourth side  64 . Thus, the peripheries of the first reception terminal  54  are surrounded by the ground terminals  56  in the four directions. Therefore, it is possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and improve the isolation characteristic. Preferably, as described above, the first conductor  46  surrounds the area defined by projecting the transmission terminal  52  or the area defined by projecting the first reception terminal  54 . The ground terminal  56  is positioned below the first conductor  46  and is interposed between the transmission terminal  52  and the second side  60  or between the first reception terminal  54  and the fourth side  64 . 
     The second embodiment may be varied so that the single-piece ground terminal  56  is divided into a separate ground terminal in the vicinity of the center of the second side  60 , another separate ground terminal between the transmission terminal  52  and the second side  60  and yet another separate ground terminal between the transmission terminal  52  and the fourth side  64 . However, it is preferable to employ the single-piece ground terminal  56  in order to shield the transmission terminal  52  more effectively. 
     The second embodiment may be varied so that the additional ground terminal  56  is provided in the center of the back surface of the multilayer substrate  10  as in the case of the first embodiment. The ground terminals  56  on the back surface of the multilayer substrate  10  may be incorporated with each other to form a single-piece ground terminal. 
     Third Embodiment 
     A duplexer of a third embodiment has an exemplary structure in which the first conductor  46  surrounds both the area defined by projecting the transmission terminal  52  and the area defined by projecting the first reception terminal  54 . The duplexer of the third embodiment is the same as the duplexer of the first embodiment illustrated in  FIGS. 1A through 5  except the pattern of the first conductor  46  formed on the upper surface of the line pattern/foot pad layer  30  and the pattern of the foot pads  36  formed on the lower or back surface thereof. 
     The multilayer substrate  10  of the third embodiment  3  is described with reference to  FIGS. 16A through 16E . As illustrated in  FIG. 16D , the first conductor  46  provided on the upper surface of the line pattern/foot pad layer  30  surrounds both the area defined by projecting the transmission terminal  52  thereon (an area defined by a broken line) and the area defined by projecting the first reception terminal  54  thereof (another area defined by another broken line). 
     As illustrated in  FIG. 16E , the ground terminal  56  provided on the back surface of the line pattern/foot pad layer  30  is provided between the transmission terminal  52  and the fourth side  64  and between the first reception terminal  54  and the fourth side  64 . The ground terminal  56  is not limited to a single-piece terminal as illustrated in  FIG. 16E  but may be divided into separate parts. 
       FIG. 17A  is a perspective view of the duplexer of the third embodiment, and FIG.  17 B is a perspective view of an electronic device in which the duplexer of the third embodiment is mounted on a printed-circuit board. As illustrated in  FIG. 17A , the first conductor  46  is provided on the upper surface of the line pattern/foot pad layer  30  included in the multilayer substrate  10  so as to surround both the area defined by projecting the transmission terminal  52  and the area defined by projecting the first reception terminal  54 . The ground terminals  56  are provided between the transmission terminal  52  and the fourth side  64  and between the first reception terminal  54  and the fourth side  64 . As illustrated in  FIG. 17B , the duplexer of the third embodiment is mounted on the printed-circuit board  66  as in the case of the first embodiment illustrated in  FIG. 7A . 
     According to the third embodiment, the transmission terminal  52  is shielded in both the direction towards the first reception terminal  54  and the direction orthogonal thereto on the back surface of the multilayer substrate  10  and on the upper surface of the line pattern/foot pad layer  30 . It is thus possible to block the electromagnetic waves emitted from the transmission terminal  52  in the above two directions. Similarly, the first reception terminal  54  is shielded in both the direction towards the transmission terminal  52  and the direction orthogonal thereto. It is thus possible to block the electromagnetic waves emitted from the transmission terminal  52  in the above two directions before the electromagnetic waves arrive at the first reception terminal  54 . It is thus possible to further reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . 
     The third embodiment may be varied to have the ground terminal  56  in the center of the back surface of the multilayer substrate  10  as in the case of the first embodiment. It is also possible to incorporate the ground terminals  56  with each other to form a single-piece ground terminal on the whole back surface of the multilayer substrate  10 .  FIG. 18A  illustrates an exemplary structure in which the ground terminal  56  is provided in the center of the back surface of the multilayer substrate  10 .  FIG. 18B  illustrates a single-piece ground terminal  56  provided on the whole back surface of the multilayer substrate  10 . 
     Fourth Embodiment 
     A fourth embodiment has an exemplary structure in which the first conductor  46  surrounds both the area defined by projecting the transmission terminal  52  and the area defined by projecting the first reception terminal  54 , and the ground terminal  56  surrounds the peripheries of the transmission terminal  52  and the first reception terminal  54  in the four directions. A duplexer of the fourth embodiment is the same as that of the third embodiment illustrated in  FIGS. 16A through 16E  except that the foot pad  36  formed on the lower surface of the line pattern/foot pad layer  30  of the multilayer substrate  10  has a different pattern. Thus, a description is given of only the back surface of the line pattern/foot pad layer  30 . 
       FIG. 19A  is a plan view of a back surface of the line pattern/foot pad layer employed in the fourth embodiment. Referring to  FIG. 19A , the ground terminals  56  includes a unique ground terminal  56 , which is provided between the transmission terminal  52  and the fourth side  64  and between the first reception terminal  54  and the fourth side  64 . This unique ground terminal  56  is further located between the transmission terminal  52  and the second side  60  and between the first reception terminal  54  and the third side  62 . The peripheries of the transmission terminal  52  are surrounded by the ground terminals  56  in the four directions, and the peripheries of the first reception terminal  54  are surrounded by the ground terminals  56  in the four directions. The unique ground terminal  56  is a single-piece pattern formed by joining the ground terminal located in the center of the second side  60 , the ground terminal between the transmission terminal  52  and the second side  60 , the ground terminal between the transmission terminal  52  and the fourth side  64 , the ground terminal between the first reception terminal  54  and the fourth side  64 , the ground terminal between the first reception terminal  54  and the third side  62 , and the ground terminal located in the center of the third side  62 . The unique ground terminal  56  may be varied so as to be divided into the separate ground terminals. However, it is preferable to use the single-piece ground terminal  56  because the single-piece ground terminal  56  surrounds the transmission terminal  52  and the first reception terminal  54  more sufficiently and improves the effect of shielding. The ground terminal  56  provided in the center of the back surface of the line pattern/foot pad layer  30  may be omitted. However, the presence of the center ground terminal  56  shields the transmission terminal  52  and the first reception terminal  54  more effectively.  FIG. 19B  illustrates a first variation of the fourth embodiment. Referring to  FIG. 19B , the ground terminals  56  are joined together so as to surround the transmission terminal  52  and the reception terminal  54  so as to form a single-piece ground terminal  56  on the whole back surface of the line pattern/foot pad layer  30 . This single-piece ground terminal  56  shields the transmission terminal  52  and the first reception terminal  54  more effectively. 
       FIG. 20A  is a perspective view of a duplexer according to the fourth embodiment, and  FIG. 20B  is a perspective view of an electronic device in which the duplexer of the fourth embodiment is mounted on a printed-circuit board. Referring to  FIG. 20A , the first conductor  46  is provided on the upper surface of the line pattern/foot pad layer  30  included in the multilayer substrate  10  so as to surround both the area defined by projecting the transmission terminal  52  and the area defined by projecting the first reception terminal  54 . The single-piece ground terminal  56  is located between the transmission terminal  52  and the fourth side  64 , between the first reception terminal  54  and the fourth side  64 , and is further located between the transmission terminal  52  and the second side  60  and between the first reception terminal  54  and the third side  62 . Since the ground terminal  56  is provided between the transmission terminal  52  and the second side  60  and between the first reception terminal  54  and the third side  62 , the ground line  74  of the printed-circuit board  66  is provided so as to surround the periphery of a portion of the transmission signal line  70  connected to the transmission terminal  52  and that of a portion of the reception signal line  72  connected to the first reception terminal  54 . The other portions of the transmission signal line  70  and the reception signal line  72  may be provided within the printed-circuit board  66 . The remaining structures of the fourth embodiment are the same as those of the first embodiment, and a description thereof is omitted here. 
     In the duplexer of the fourth embodiment, the transmission terminal  52  is electromagnetically shielded in the direction towards the first reception terminal  54  and the direction orthogonal thereto on the upper surface of the line pattern/foot pad layer  30 . Further, on the back surface of the multilayer substrate  10 , the transmission terminal  52  is electromagnetically shielded in the direction towards the first reception terminal  54 , the direction orthogonal thereto, the direction opposite to the first reception terminal  54  and the direction orthogonal thereto. Similarly, the first reception terminal  54  is electromagnetically shielded in the direction towards the transmission terminal  52  and the direction orthogonal thereto on the upper surface of the line pattern/foot pad layer  30 . Further, the first reception terminal  54  is electromagnetically shielded in the direction towards the transmission terminal  52 , the direction orthogonal thereto, and the direction opposite to the transmission terminal  52  on the back surface of the multilayer substrate  10 . It is thus possible to further improve the effect of shielding the transmission terminal  52  and the first reception terminal  54 . Thus, the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  can be further reduced, and the isolation characteristic can be further improved. 
     Fifth Embodiment 
     A duplexer of a fifth embodiment has an exemplary structure having a second conductor that covers the area defined by projecting the transmission terminal  52 . The duplexer of the fifth embodiment is the same as that of the first embodiment illustrated in  FIGS. 1A through 5  except a pattern formed on the upper surface of the line pattern layer  28  included in the multilayer substrate  10 . 
     A description is now given of a structure of the multilayer substrate  10  employed in the fifth embodiment with reference to  FIGS. 21A through 21E . Referring to  FIG. 21C , a second conductor  96  is provided on the upper surface of the line pattern layer  28 , and is formed so as to cover the area defined by projecting the transmission terminal  52  onto the line pattern layer  28 . The second conductor  96  may be made of the same material as that of the conductive pattern  20  and may be an alloy including, for example, Al and Cu. The second conductor  96  is not electrically connected to the other conductive pattern  20  but is a floating conductor. The second conductor  96  may have the same thickness as the conductive pattern  20  and may be 15 μm thick, for example. 
       FIG. 22A  is a perspective view of the duplexer of the fifth embodiment, and  FIG. 22B  is a perspective view of an electronic device in which the duplexer of the fifth embodiment is mounted on a printed-circuit board. The view of  FIG. 22A  is seen through structural components other than the conductive pattern  20 , the foot pads  56 , the first conductor  46  and the second conductor  96  (this holds true for  FIG. 24A ). Similarly, the view of  FIG. 22B  is seen through structural components other than the foot pads  36 , the first conductor  46  and the second conductor  96  (this holds true for  FIG. 24B ). As illustrated in  FIG. 22A , the second conductor  96  is provided on the upper surface of the line pattern layer  28  that is the upper surface of the line pattern/foot pad layer  30  included in the multilayer substrate  10  so as to cover the area defined by projecting the transmission terminal  52  onto the upper surface of the line pattern layer  28 . As illustrated in  FIG. 22B , the duplexer of the fifth embodiment is mounted on the printed-circuit board  66  as in the case of the first embodiment. 
     The electronic device illustrated in  FIG. 22B  is simulated to measure the passband characteristics of the transmission filter  12  and the reception filter  14  and the isolation characteristics between the transmission terminal  52  and the first reception terminal  54 . Simulation results of the electronic device illustrated in  FIG. 7B  are illustrated for the purpose of comparison with the electronic device illustrated in  FIG. 7B .  FIG. 23A  illustrates simulation results of passband characteristics of the transmission filter  12  and the reception filter  14  of the first comparative example and the fifth embodiment, and  FIG. 23B  illustrates simulation results of the isolation characteristics between the transmission terminal  52  and the first reception terminal  54  of the first comparative example and the fifth embodiment. The simulation results of the fifth embodiment are illustrated by solid lines, and those of the first comparative example are illustrated by broken lines. 
     As illustrated in  FIG. 23A , the fifth embodiment and the first comparative example have little difference in the passband characteristics of the transmission filter  12  and the reception filter  14 . In contrast, as illustrated in  FIG. 23B , the fifth embodiment has improved isolation characteristics between the transmission terminal  52  and the first reception terminal  54  in the transmission band and outsides of the passband, as compared with the first comparative example. 
     According to the fifth embodiment, the upper side of the transmission terminal  52  is electromagnetically shielded, and electromagnetic waves emitted upwards from the transmission terminal  52  are blocked. It is thus possible to improve the effect of shielding the transmission terminal  52  and to further reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . Thus, the isolation characteristic can be further improved, as illustrated in  FIG. 23B . 
     It is preferable that the second conductor  96  completely covers the area defined by projecting the transmission terminal  52  in order to improve the effect of blocking the electromagnetic waves emitted from the transmission terminal  52 . However, a similar effect of blocking may be obtained even by partly covering the area defined by projecting the transmission terminal  52 . 
     The second conductor  96  is not limited to the above arrangement but may be varied. For example, it is preferable that the second conductor  96  covers the area defined by projecting the first reception terminal  54  in a case where the first conductor  46  covers this area. It is thus possible to further reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . 
     The second conductor  96  may be provided to cover the area defined by projecting the transmission terminal  52  in a case where the first conductor  46  is provided to surround the area defined by projecting the transmission terminal  52  and the ground terminal  56  is provided between the transmission terminal  52  and the second side  60  and between the transmission terminal  52  and the fourth side  64 . Such a case is employed in the second embodiment. Similarly, the second conductor  96  may be provided to cover the area defined by projecting the first reception terminal  54  in the case where the first conductor  46  surrounds the area defined by projecting the first reception terminal  54  and the ground terminal  56  is provided between the first reception terminal  54  and the third side  62  and between the first reception terminal  54  and the fourth side. In the cases described above, the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  can be further reduced. 
     The second conductor  96  is not limited to the floating conductor but may be grounded. The second conductor  96  is not connected to signal lines connected to the antenna terminal  50 , the transmission terminal  52  and the first reception terminal  54 . 
     Sixth Embodiment 
     A duplexer of a sixth embodiment has an exemplary structure in which the second conductor  96  is provided so as to cover both the area defined by projecting the transmission terminal  52  and the area defined by projecting the first reception terminal  54 .  FIG. 24A  is a perspective view of the duplexer of the sixth embodiment, and  FIG. 24B  is a perspective view of an electronic device in which the duplexer of the sixth embodiment is mounted on a printed-circuit board. As illustrated in  FIG. 24A , the first conductor  46  is provided so as to surround both the area defined by projecting the transmission terminal  52  onto the upper surface of the line pattern/foot pad layer  30  and the area defined by projecting the first reception terminal  54  onto the upper surface of the line pattern/foot pad layer  30 . The second conductor  96  is provided so as to cover both the area defined by projecting the transmission terminal  52  onto the upper surface of the line pattern layer  28  and the area defined by projecting the first reception terminal  54  onto the upper surface of the line pattern layer  28 . The ground terminal  56  is provided on the back surface of the multilayer substrate  10  so as to be located between the transmission terminal  52  and the fourth side  64  and between the first reception terminal  54  and the fourth side  64 . As illustrated in  FIG. 24B , the duplexer of the sixth embodiment is mounted on the printed-circuit board  66  as in the case of the first embodiment illustrated in  FIG. 7A . 
     According to the sixth embodiment, the upper sides of both the transmission terminal  52  and the first reception terminal  54  are electromagnetically shielded. It is thus possible to block electromagnetic waves emitted upwards from the transmission terminal  52  and blocks the waves before the electromagnetic waves arrive at the first reception terminal  54  from the upper side. It is thus possible to further reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and to further improve the isolation characteristic therebetween. 
     The second conductor  96  may be provided so as to cover the areas respectively defined by projecting the transmission terminal  52  and the first reception terminal  54  in the case where the ground terminal  56  is provided between the transmission terminal  52  and the second side  60  and between the first reception terminal  54  and the third side  62 . It is possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54 . 
     Seventh Embodiment 
     A duplexer of a seventh embodiment has an exemplary structure in which the transmission filter  12  is of ladder type like the first embodiment and the reception filter  14  is a double-mode filter.  FIG. 25A  is a plan view of the reception filter  14 , and  FIG. 25B  is a circuit diagram of the reception filter  14 . Referring to  FIGS. 25A and 25B , the reception filter  14  has double mode type surface acoustic wave resonators DMS 1  and DMS 2  connected in parallel. Each of DMS 1  and DMS 2  has three interdigital transducers (IDT) interposed between a pair of reflection electrodes. The IDT located in the center of each of DMS 1  and DMS 2  is coupled to the antenna terminal  50  via a resonator S 21 . The antenna terminal  50  is an unbalanced terminal. Two IDTs located at both sides of the center IDT of DMS 1  are coupled to a second reception terminal  55  via a resonator S 23 . Similarly, two IDTs located at both sides of the center IDT of DMS 2  are coupled to the first reception terminal  54  via a resonator S 22 . The first and second reception terminals  54  and  55  are balanced terminals. The duplexer of the seventh embodiment is a unbalanced-balanced type duplexer. 
     A description is given of each layer of the multilayer substrate  10  with reference to  FIGS. 26A through 26E . As illustrated in  FIG. 26A , holes for defining the cavity  18  are formed in the cap mount layer  22  and the cavity layer  24 . The transmission filter  12  of the ladder type and the reception filter  14  of the double mode type are housed in the cavity  18 . 
     As illustrated in  FIGS. 26B and 26C , the conductive patterns  20  are provided on the upper surface of the die attach layer  26  and the upper surface of the line pattern layer  28 . 
     As illustrated in  FIG. 26D , in addition to the conductive pattern  20 , the first conductor  46  is provided on the upper surface of the line pattern/foot pad layer  30  so as to surround the area defined by projecting the transmission terminal  52  than the area defined by projecting the first reception terminal  54  closer to the transmission terminal  52  than the second reception terminal  55 . 
     As illustrated in  FIG. 26E , the first reception terminal  54  is provided close to the third side  62  and is located farther away from the antenna terminal  50  than the center of the third side  62 . The second reception terminal  55  is provided close to the center of the third side  62  and is next to the first reception terminal  54 . Thus, the ground terminal  56  is not provided close to the center of the third side  62 . This is different from the arrangements illustrated in  FIGS. 4E and 16E . The other structures of the seventh embodiment are the same as those of the third embodiment illustrated in  FIG. 16E , and a description thereof is omitted here. 
       FIG. 27A  is a perspective view of the duplexer of the seventh embodiment, and  FIG. 27B  is a perspective view of an electronic device in which the duplexer is mounted on a printed-circuit board. As illustrated in  FIG. 27A , the first reception terminal  54  is provided close to the third side  62  and is located farther away from the antenna terminal  50  than the center of the third side  62 . The second reception terminal  55  is provided close to the center of the third side  62  and is next to the first reception terminal  54 . The first conductor  46  surrounds the areas respectively defined by projecting the transmission terminal  52  and the first reception terminal  54 . As illustrated in  FIG. 27B , reception signal lines  72  are provided in association with the first and second reception terminals  54  and  55 . 
     In the duplexer of the seventh embodiment with the reception filter of the double mode type, the first conductor  46  surrounds the area defined by projecting the transmission terminal  52  and the area defined projecting the first reception terminal  54  closer to the transmission terminal  52  than the second reception terminal  55 . Further, the peripheries of the transmission terminal  52  and the first reception terminal  54  are surrounded by the ground terminals  56  in the three directions. It is thus possible to reduce the electromagnetic coupling between the transmission terminal  52  and the first reception terminal  54  and to improve the isolation characteristic. 
     The isolation characteristic may be further improved by providing the first conductor  46  so as to surround both the areas defined by projecting the first and second reception terminals  54  and  55 . 
       FIGS. 28A through 28F  illustrate exemplary pattern variations of the lower surface of the line pattern/foot pad layer  30  having the first and second reception terminals  54  and  55 . The ground terminals  56  depicted in  FIGS. 28A through 28F  improve the isolation characteristic. 
     The present invention is not limited to the specifically described embodiments but includes other embodiments and variations within the scope of the claimed invention.