Abstract:
A multilayer directional coupler which is easy to mass-produce and reduce in size, in which fine setting of the degree of electromagnetic coupling is facilitated, and which has a large bandwidth ratio, includes magnetic substrates, a laminate including first and second transformers, and external electrodes. One external electrode, which is connected to two ends of a primary coil of the first transformer, defines an input end for a main signal, and another external electrode defines an output end. A further external electrode, which is connected to two ends of a secondary coil of the second transformer, defines an output end for a sub-signal. A winding length ratio N 1  of the secondary coil to primary coil of the first transformer and a winding length ratio N 2  of the primary coil to secondary coil of the second transformer are each set to a value that is greater than about 1 and not greater than about 10. A ratio of the winding length ratio N 2  to the winding length ratio N 1  is greater than about 0.5 and less than about 2.0.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a multilayer directional coupler used in a mobile communication apparatus such as a cellular phone or the like.  
         [0003]     2. Description of the Related Art  
         [0004]     Multilayer directional couplers include, for example, the technologies disclosed in Japanese Unexamined Patent Application Publication No. 05-152814 and Japanese Unexamined Patent Application Publication No. 05-160614. These directional couplers have a structure in which strip-line transmission lines, each having a length of a quarter wavelength or not greater than a quarter wavelength, are laminated and formed in a dielectric substrate. On the basis of this structure, these directional couplers can be easily mass-produced and reduced in size. Accordingly, these multilayer directional couplers are widely used in mobile communication apparatuses, etc.  
         [0005]     However, in these directional couplers, a bandwidth at which a stable degree of electromagnetic coupling can be obtained is represented by a bandwidth ratio of 50% or less. Therefore, these multilayer directional couplers cannot be used in apparatuses that need a signal having a bandwidth ratio of 90% or greater, such as a television signal.  
         [0006]     Conversely, a directional coupler capable of realizing a band with ratio of 99% or greater is disclosed in Michael G. Ellis, “RF Directional Couplers”, Electronic System Products, Searched on May 20, 2005, Internet, http://members.tripod.com/michaelgellis/direct.html.  
         [0007]     As shown in  FIG. 14 , this directional coupler  100  is constructed by winding an electrode  102  around a binocular-shaped magnetic core  101 , and its circuit configuration is as shown in  FIG. 15 .  
         [0008]     In this configuration, by appropriately setting a winding ratio (a ratio of the numbers of turns) N 1  of a coil  112  to coil  111  of a transformer  110 , and appropriately setting a winding ratio (a ratio of the numbers of turns) N 2  of a coil  121  to coil  122  of a transformer  120 , for example, a main signal input from a port  110   a  (port  110   b ) of the transformer  110  can be output to a port  110   b  (port  110   a ) and a port  120   a  (port  120   b ) of the transformer  120  at a distribution ratio corresponding to the winding ratios.  
         [0009]     However, since, as shown in  FIG. 14 , for the directional coupler  100 , an operation of manually winding the electrode  102  around the magnetic core  101  is needed, mass productivity is low compared with the above multilayer directional couplers. In addition, this type of directional coupler needs at least a volume of 80 mm 3 , and is accordingly not suitable in terms of size reduction. In addition, with the directional coupler  100 , it is difficult to finely and accurately set the degree of electromagnetic coupling between the transformers because the degree of electromagnetic coupling between the transformers must be set on the basis of a winding ratio (a ratio of the numbers of turns) of the electrode  102 . Accordingly, a main signal input to the port  110   a  (the port  110   b ) may not be output to the port  110   b  (the port  110   a ) and the port  120   a  (the port  120   b ) of the transformer  120  at a desired distribution ratio.  
       SUMMARY OF THE INVENTION  
       [0010]     In order to overcome the problems described above, preferred embodiments of the present invention provide a multilayer directional coupler that is easy to mass-produce and reduce in size, in which fine setting of the degree of electromagnetic coupling is facilitated, and which has a large bandwidth ratio.  
         [0011]     A multilayer directional coupler according to a preferred embodiment of the present invention includes a first magnetic substrate, a laminate laminated on the first magnetic substrate, first and second transformers provided in the laminate, and a second magnetic substrate provided on the laminate. Two ends of a primary coil of the first transformer defines input and output terminals for a main signal, one end of a secondary coil of the first transformer defines a ground terminal, and the other end of the secondary coil is connected to one terminal of a secondary coil of the second transformer. One end of a primary coil of the second transformer is connected to one terminal of the first transformer, the other end of the primary coil of the second transformer is connected to the ground terminal, and one terminal of the secondary coil of the second transformer is used as an output terminal for outputting a sub-signal. The winding length ratio N 1  of the secondary coil to primary coil of the first transformer and the winding length ratio N 2  of the primary coil to secondary coil of the second transformer are each preferably set to a value that is greater than 1 and not greater than 10.  
         [0012]     The above-described configuration allows the multilayer directional coupler to have a multilayer structure including the first magnetic substrate, the laminate having the first and second transformers located therein, and the second magnetic substrate. Thus, by using a known production technology such as photolithography, micro-multilayer directional couplers can be mass-produced.  
         [0013]     When the main signal is input to the input terminal of the primary coil of the first transformer, the main signal is distributed and output from the output terminal of the primary coil and the output terminal of the secondary coil of the second transformer. In this case, the distribution ratio is determined depending on the degree of electromagnetic coupling between the first and second transformers. The degree of electromagnetic coupling is determined on the basis of a winding length ratio N 1  of the secondary coil to first coil of the first transformer and a winding length ratio N 2  of the primary coil to secondary coil of the second transformer. Accordingly, in a preferred embodiment of the present invention, the winding length ratios N 1  and N 2  are each set to a value that is greater than about 1 and not greater than about 10.  
         [0014]     In addition, since the first and second transformers can be formed by a known technology such as photolithography, the lengths of the coils of the first and second transformers can be pattern-formed so as to have preferable values. Accordingly, differently from the directional coupler  100  of the related art, the winding length ratios N 1  and N 2  of the first and second transformers can be finely and accurately set, thus enabling fine setting of the degree of electromagnetic coupling.  
         [0015]     Further, the two ends of the primary coil of the first transformer define the input and output terminals for the main signal, one end of the secondary coil of the first transformer defines the ground terminal, and the other end of the secondary coil is connected to one terminal of the secondary coil of the second transformer. One end of a primary coil of the second transformer is connected to one terminal of the first transformer, the other end of the primary coil of the second transformer is connected to the ground terminal, and one terminal of the secondary coil of the second transformer defines the output terminal for outputting the sub-signal. Thus, the multilayer directional coupler can be used even for an apparatus that needs a signal having a bandwidth ratio equal to or greater than 90%, such as a television signal.  
         [0016]     The ratio of the winding length ratio N 2  to the winding length ratio N 1  is preferably set to a value that is greater than about 0.5 and less than about 2.0.  
         [0017]     The laminate is preferably formed by covering the first and second transformers with a non-magnetic body.  
         [0018]     As described above in detail, preferred embodiments of the present invention provide a multilayer directional coupler which is easy to mass-produce and reduce in size, in which fine setting of the degree of electromagnetic coupling between the first and second transformers is facilitated, and which has a large bandwidth ratio.  
         [0019]     Since the ratio of the winding length ratio N 2  to the winding length ratio N 1  is preferably set to a value that is greater than about 0.5 and less than about 2.0, an advantage is obtained in that a multilayer directional coupler in which the degree of electromagnetic coupling and directionality between the first and second transformers can be finely and accurately adjusted.  
         [0020]     Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is an exploded perspective view of a multilayer directional coupler according to a preferred embodiment of the present invention.  
         [0022]      FIG. 2  is an exterior view of the multilayer directional coupler.  
         [0023]      FIG. 3  is a plan view of a conductor pattern of a bottom layer.  
         [0024]      FIG. 4  is a plan view of a non-magnetic layer.  
         [0025]      FIG. 5  is a plan view of a conductor pattern of the second layer from the bottom.  
         [0026]      FIG. 6  is an exploded perspective view showing a connection structure of the conductor pattern of the bottom layer and the conductor pattern of the second layer.  
         [0027]      FIG. 7  is a plan view of a conductor pattern of the third layer from the bottom.  
         [0028]      FIG. 8  is a plan view of a non-magnetic layer.  
         [0029]      FIG. 9  is a plan view of a conductor pattern of a top layer.  
         [0030]      FIG. 10  is an exploded perspective view showing a connection structure of the conductor pattern of the third layer and the conductor pattern of the top layer.  
         [0031]      FIG. 11  is a schematic diagram showing an electrical structure of first and second transformers.  
         [0032]      FIG. 12  is a graph showing a winging length ratio of coils and the degree of coupling between the first and second transformers.  
         [0033]      FIG. 13  is a perspective view showing a mounting state of a multilayer directional coupler.  
         [0034]      FIG. 14  is a perspective view showing an example of a directional coupler of the related art.  
         [0035]      FIG. 15  is an equivalent circuit diagram of the directional coupler in  FIG. 14 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Preferred embodiments of the present invention are described below with reference to the drawings.  
         [0037]      FIG. 1  is an exploded view of a multilayer directional coupler according to a preferred embodiment of the present invention.  FIG. 2  is an exterior view of the multilayer directional coupler.  
         [0038]     As shown in  FIG. 2 , a multilayer directional coupler  1  according to the present preferred embodiment preferably includes a magnetic substrate  2 - 1  as a first magnetic substrate, a laminate  3  laminated on the magnetic substrate  2 - 1 , a magnetic substrate  2 - 2  bonded as a second magnetic substrate to the laminate  3 , and external electrodes  4 - 1  to  4 - 6 .  
         [0039]     As shown in  FIG. 1 , the laminate  3  includes a first transformer  5 , a second transformer  6 , and a non-magnetic body  7  (see  FIG. 2 ) that externally, completely covers the first and second transformers  5  and  6 .  
         [0040]     The non-magnetic body  7  preferably is, for example, formed of a dielectric, and formed by laminating non-magnetic layers  71  to  75 . The first and second transformers  5  and  6  are pattern-formed on the non-magnetic layers  71  to  74 .  
         [0041]     Specifically, the first transformer  5  includes a primary coil  5 - 1  and a secondary coil  5 - 2  above it. The primary coil  5 - 1  is defined by conductor patterns  51  and  52 , and the secondary coil  5 - 2  is defined by conductor patterns  53  and  54 .  
         [0042]     In addition, the second transformer  6  includes a primary coil  6 - 1  and a secondary coil  6 - 2  above it. The primary coil  6 - 1  is defined by conductor patterns  63  and  64 , and a secondary coil  6 - 2  is defined by conductor patterns  61  and  62 .  
         [0043]     Here, structures of the first and second transformers  5  and  6  are specifically described.  
         [0044]     The conductor patterns  51  and  64  are pattern-formed on the non-magnetic layer  71  laminated on the magnetic substrate  2 - 1  by a photolithography method or the like. After the non-magnetic layer  72  is laminated on the conductor patterns  51  and  64 , the conductor patterns  52  and  63  are pattern-formed on the non-magnetic layer  72 .  
         [0045]      FIG. 3  is a plan view of the conductor patterns  51  and  64 .  FIG. 4  is a plan view of the non-magnetic layer  72 .  FIG. 5  is a plan view of the conductor patterns  52  and  63 .  FIG. 6  is an exploded perspective view showing a connection structure of the conductor patterns  51  and  64 , and the conductor patterns  52  and  63 .  
         [0046]     As shown in  FIG. 3 , the conductor pattern  51  includes an internal electrode  51   a  leading from the inside and an end portion  51   b  inside the pattern. As shown in  FIG. 5 , the conductor pattern  52  includes an internal electrode  52   a  leading to the outside and an inside end portion  52   b.    
         [0047]     As shown in  FIG. 6 , the end portion  51   b  of the conductor pattern  51  is connected to the end portion  52   b  of the conductor pattern  52  by a through hole  72   b  in the non-magnetic layer  72 , which is shown also in  FIG. 4 . This defines the primary coil  5 - 1 , which is spiral, and which uses the internal electrodes  51   a  and  52   a  as two ends of the primary coil  5 - 1 .  
         [0048]     In addition, as shown in  FIG. 3 , the conductor pattern  64  includes an internal electrode  64   a  leading to an outside central portion (a position corresponding to the internal electrode  52   a ) of the conductor pattern  51 , which is next to the conductor pattern  64 . Left-pointing end portions  64   b  to  64   d  and right-pointing end portions  64   e  to  64   h  are alternately arranged on a side opposite to a side on which the internal electrode  64   a  leads. As shown in  FIG. 5 , the conductor pattern  63  includes an internal electrode  63   a  leading from the inside up to the center between the conductor patterns  52  and  63 . Pattern end portions  63   b  to  63   d  are arranged to the left of a lead of the internal electrode  63   a , and end portions  63   e  to  63   h  are arranged to the right of the lead. As shown in  FIG. 6 , end portions  64   b  to  64   d  of the conductor pattern  64  are respectively connected to end portions  63   b  to  63   d  of the conductor pattern  63  by through holes  72   b ′ to  72   d ′ of the non-magnetic layer  72 , which are shown also in  FIG. 4 . Also, end portions  64   e  to  64   h  of the conductor pattern  64  are respectively connected to end portions  63   e  to  63   h  of the conductor pattern  63  by through holes  72   e ′ to  72   h ′. This defines the primary coil  6 - 1 , which is spiral, and which uses the internal electrodes  64   a  and  63   a  as two ends of the primary coil  6 - 1 .  
         [0049]     Two leads of the internal electrodes  52   a  and  64   a  are connected by a through hole  72   j  in the non-magnetic layer  72 .  
         [0050]     In addition, as shown in  FIG. 1 , the conductor patterns  53  and  62  are pattern-formed on the non-magnetic layer  73 , which is laminated on the conductor patterns  52  and  63 . After the non-magnetic layer  74  is laminated on the conductor patterns  53  and  62 , the conductor patterns  54  and  61  are pattern-formed on the non-magnetic layer  74 .  
         [0051]      FIG. 7  is a plan view of the conductor patterns  53  and  62 .  FIG. 8  is a plan view of the non-magnetic layer  74 .  FIG. 9  is a plan view of the conductor patterns  54  and  61 .  FIG. 10  is an exploded perspective view showing a connection structure of the conductor patterns  53  and  62 , and the conductor patterns  54  and  61 .  
         [0052]     As shown in  FIG. 7 , the conductor pattern  53  includes an internal electrode  53   a  leading from the inside up to the center between the conductor patterns  53  and  62 . End portions  53   b  to  53   d  are arranged to the right of a lead of the internal electrode  53   a , and end portions  53   e  to  53   h  are arranged to the left of the lead. In addition, as shown in  FIG. 9 , the conductor pattern  54  includes an internal electrode  54   a  leading to an outside central portion (a position corresponding to an internal electrode  62   a ) of the conductor pattern  61 , which is next to the internal electrode  54 . Right-pointing end portions  54   b  to  54   d  and left-pointing end portions  54   e  to  54   h  are alternately arranged on a side opposite to a side on which the internal electrode  54   a  leads.  
         [0053]     As shown in  FIG. 10 , the end portions  53   b  to  53   d  of the conductor pattern  53  are respectively connected to the end portions  54   b  to  54   d  by through holes  74   b  to  74   d  in the non-magnetic layer  74 , which is shown also in  FIG. 8 . Also, the end portions  53   e  to  53   h  of the conductor pattern  53  are respectively connected to the end portions  54   e  to  54   h  of the conductor pattern  54  by through holes  74   e  to  74   h . This defines the secondary coil  5 - 2 , which is spiral, and which uses the internal electrodes  53   a  and  54   a  as two ends of the secondary coil  5 - 2 .  
         [0054]     In addition, as shown in  FIG. 7 , the conductor pattern  62  includes an internal electrode  62   a  leading to the outside and an end portion  62   b  that is inwardly positioned. Also, as shown in  FIG. 9 , the conductor pattern  61  includes an internal electrode  61   a  leading from the inside and an inward end portion  61   b . As shown in  FIG. 10 , the end portion  62   b  of the conductor pattern  62  is connected to the end portion  61   b  of the conductor pattern  61  by a through hole  74   b ′ in the non-magnetic layer  74 , which is shown also in  FIG. 8 . This defines the secondary coil  6 - 2 , which is spiral, and which uses the internal electrodes  62   a  and  61   a  as two ends of the secondary coil  6 - 2 .  
         [0055]     In addition, two leads of the internal electrodes  54   a  and  62   a  are connected by a through hole  74   j  in the non-magnetic layer  72 .  
         [0056]     As shown in  FIG. 1 , the non-magnetic layer  75  is laminated on the conductor patterns  54  and  61 , and the magnetic substrate  2 - 2  is bonded to the non-magnetic layer  75 .  
         [0057]     External electrodes  4 - 1  to  4 - 6  are disposed outside the laminate  3  having the above-described structure.  
         [0058]     This allows the external electrode  4 - 1  to electrically connect to both the internal electrodes  52   a  and  64   a  of the conductor patterns  52  and  64 , and allows the external electrode  4 - 2  to electrically connect to the internal electrode  51   a  of the conductor pattern  51 . Also, the external electrode  4 - 3  electrically connects to the internal electrode  61   a  of the conductor pattern  61 . The external electrode  4 - 4  electrically connects to both the internal electrodes  53   a  and  63   a  of the conductor patterns  53  and  63 . The external electrode  4 - 5  electrically connects to both the internal electrodes  54   a  and  62   a  of the conductor patterns  54  and  62 .  
         [0059]      FIG. 11  is a schematic diagram showing an electrical structure of the first and second transformers  5  and  6 .  
         [0060]     On the basis of the above connections between the conductor patterns and the above connections between the external electrodes  4 - 1  to  4 - 6  and the internal electrodes, the electrical structure has the circuit structure shown in  FIG. 11 .  
         [0061]     In other words, the external electrode  4 - 2  connected to the internal electrode  51   a  of the primary coil  5 - 1  of the first transformer  5  can be used as an input terminal for a main signal, and the external electrode  4 - 1  connected to the internal electrode  52   a  can be used as an output terminal for the main signal. The external electrode  4 - 4  connected to the internal electrode  53   a  of the secondary coil  5 - 2  can be used as a ground terminal. The internal electrode  64   a  of the primary coil  6 - 1  of the second transformer  6  is connected to the internal electrode  52   a  of the primary coil, and the internal electrode  63   a  is connected to the internal electrode  53   a  of the secondary coil  5 - 2  of the first transformer  5 . The internal electrode  62   a  of the secondary coil  6 - 2  is connected to the internal electrode  54   a  of the secondary coil  5 - 2  of the first transformer  5 . Accordingly, the external electrode  4 - 3  connected to the internal electrode  61   a  of the secondary coil  6 - 2  can be used as an output terminal for a sub-signal, and the external electrode  4 - 5  connected to both the internal electrodes  54   a  and  62   a  can be used as a terminating end by a terminating resistor or the like, which is not shown.  
         [0062]     This circuit structure is preferably identical to a circuit structure of the directional coupler  100  shown in  FIG. 15 . This multilayer directional coupler has a function of distributing and outputting the main signal input from the external electrode  4 - 2  to the external electrode  4 - 1  and  4 - 3 . Needless to say, when inputting the main signal from the external electrode  4 - 3 , the multilayer directional coupler has a function of distributing and outputting the input main signal to the external electrodes  4 - 5  and  4 - 2 .  
         [0063]     As described above, the multilayer directional coupler has a function of distributing and outputting the main signal. The distribution ratios are determined according to the degree of electromagnetic coupling occurring between the first and second transformers, such as electromagnetic coupling between the primary and secondary coils  5 - 1  and  5 - 2  of the first transformer  5 , and electromagnetic coupling between the primary and secondary coils  6 - 1  and  6 - 2  of the second transformer  6 . The degree of electromagnetic coupling is dependent on the winding length ratio N 1  of the secondary coil  5 - 2  to the primary coil  5 - 1  of the first transformer  5  and the winding length ratio N 2  of the primary coil  6 - 1  to the secondary coil  6 - 2  of the second transformer  6 .  
         [0064]      FIG. 12  is a graph showing the winding length ratios (N 1 , N 2 ) and the degree of coupling of the multilayer directional coupler. As a result of performing simulation in order to identify a range of the winding length ratios (N 1 , N 2 ) at which the first and second transformers have a good degree of coupling therebetween, the present inventors obtained the results shown in  FIG. 12 . Specifically, it was discovered that, as indicated by curve S in  FIG. 12 , the degree of coupling between the first and second transformers  5  and  6  had good values of approximately 2 dB to 20 dB in a range in which a winding length ratio (1N 2 ) was not less than about 1 and not greater than about 10. Accordingly, the winding length ratios N 1  and N 2  are preferably set so that 1&lt;N 1 ≦10 and 1&lt;N 2 ≦10.  
         [0065]     Specifically, in this example of preferred embodiments of the present invention, the primary coil  5 - 1 , whose winding length was about “4.2 mm” was pattern-formed, and the secondary coil  5 - 2 , whose winding length was about “10.5 mm” was pattern-formed, with the winding length ratio N 1  set to about “2.5”. Also, the primary coil  6 - 1 , whose winding length was about “4.2 mm”, was pattern-formed, and the secondary coil  6 - 2 , whose winding length was about “10.5 mm”, was pattern-formed, with the winding length ratio N 2  set to about “2.5”. In addition, by setting a ratio (N 2 /N 1 ) of the winding length ratio N 2  to the winding length ratio N 1  so that 0.5&lt;N 2 /N 1 &lt;2.0, an impedance of each port can be improved. Accordingly, in this example of the preferred embodiments, the ratio N 2 /N 1  was set to approximately “1”.  
         [0066]     In this example of the preferred embodiments, since the first and second transformers  5  and  6  are formed by a known laminating technology such as photolithography, the primary and secondary coils  5 - 1  and  5 - 2  of the first transformer  5  and the primary and secondary coils  6 - 1  and  6 - 2  of the second transformer  6  can be pattern-formed so as to have preferable winding lengths. Accordingly, the winding length ratios N 1  and N 2  of the first and second transformers can be finely and accurately set.  
         [0067]     Next, the operation and advantages exhibited by the multilayer directional coupler according to the preferred embodiments are described.  
         [0068]     As shown in  FIG. 13 , the external electrodes  4 - 1  and  4 - 2  are connected to end portions of a main line  200 , with the external electrode  4 - 2  used as an input terminal for main signal S, and the external electrode  4 - 1  used as an output terminal. The external electrode  4 - 4  is set to a ground state. The external electrode  4 - 5  is grounded via a terminating resistor or the like, and the external electrode  4 - 3  is connected to a sub-line  201  and is used as an output terminal for sub-signal S 2 .  
         [0069]     Accordingly, by transmitting main signal S through the main line  200 , main signal S is input from the external electrode  4 - 2  to the multilayer directional coupler  1 . Then, main signal S 1  is output from the external electrode  4 - 1  to the main line  200 , and sub-signal S 2  is output from the external electrode  4 - 3  to the sub-line  201 . In other words, main signal S input to the multilayer directional coupler  1  is distributed and output to the main line  200  and the sub-line  201  at an optimal distribution ratio corresponding to the winding length ratios N 1  and N 2  of the first and second transformers  5  and  6  and the ratio N 2 /N 1 .  
         [0070]     With the first and second transformers  5  and  6  that are identical in circuit structure to the multilayer directional coupler  100  having the circuit structure in  FIG. 15 , main signal S can be distributed as main signal S 1  and sub-signal S 2  in a wide band. Thus, the multilayer directional coupler  1  can be used even for an apparatus that needs a signal having a bandwidth ratio equal to or greater than 90%, such as a television signal.  
         [0071]     The present invention is not limited to the above-described preferred embodiments and may be variously modified and altered within the sprit of the present invention.  
         [0072]     For example, although, in the foregoing preferred embodiments, the winding length ratio N 1  (winding length ratio N 2 ) of the secondary coil  5 - 2  (the primary coil  6 - 1 ) to the primary coil  5 - 1  (the secondary coil  6 - 2 ) of the first transformer  5  (the second transformer  6 ) is preferably set to about “2.5”, each of the winding length ratios N 1  and N 2  may be a value that is greater than about 1 and not greater than about 10, and is not limited the value set in the foregoing preferred embodiments. In addition, although N 2 /N 1  is preferably set to about “1”, this ratio is not limited to this value since this ratio may be a value that is greater than about 0.5 and less than about 2.0.  
         [0073]     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.