Abstract:
SAW filter package which can reduce an electromagnetic feed-through for improving an SAW filter performance including a base having a frame around a periphery, a ground part formed of a metal plating on an inside surface of the base for attaching an acoustic surface wave filter thereto, cut-away portions formed by cutting the metal plating of the ground part in two steps, and a metal piece electrically separated from the metal plating by the cut-away portions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an SAW(Surface Acoustic Wave) filter package, and more particularly, to an SAW filter package which can reduce an electromagnetic feed-through for improving an SAW filter performance. 
     2. Background of the Related Art 
     Being a filter of a piezoelectric material, the SAW filter is widely used as an intermediate frequency filter because the SAW filter permits multipoint drive and reception, has excellent earthquake and impact resistances as being supported from a substrate, and permits independent control of amplitude and phase, and has an excellent filter performance. 
     Referring to FIG. 1, a related art SAW filter is provided with an input terminal IDT (Inter-Digital Transducer)  12  for providing an input signal Vs with an impedance Rs, an output terminal IDT  13  connected to an output impedance R 0 , and absorber  14  for absorbing a reflected wave. 
     Shown in FIG. 2 are details of the input terminal IDT  12  and the output terminal IDT  13 , both are of a withdrawal weighting type, respectively having one side with an input port  15  and the other side with a ground terminal  17 , and one side with a ground terminal  17  and the other side with an output port  16 . And, there is a slightly tilted blocking bar  18  disposed between the input terminal IDT  12  and the output terminal IDT  13  having both ends connected to the ground terminal  17  for reducing the electromagnetic feed-through. 
     FIGS.  3 A˜ 3 D illustrate details of a general SMD(Surface Mounted Device) package for mounting the SAW filter, wherein FIG. 3A illustrates a back of the package, FIG. 3B illustrates a side of the package, FIG. 3C illustrates a plan view of the package with a cover closed, and FIG. 3D illustrates a plan view of the package with a cover opened. 
     Referring to FIGS.  3 A˜ 3 D, an SMD package is provided with a base  20  of ceramic, a metal plated frame  21  projected vertical from the base  20  along a periphery of the base  20 , a plurality of gold plated pads  22  formed in sides of the base  20 , a ground terminal  23  of a gold plating formed on an inside surface of the base  20 , and a cover  24  for sealing the mounted filter. And, the SAW filter is mounted on the ground terminal  23  of the SMD package. The reference numeral  25  is a pad connection terminal. 
     Upon application of a high frequency electric field to the input terminal IDT  12 , a surface acoustic wave is excited by a piezoelectric action in the related art SAW filter, transmitted to the output terminal IDT  13  through a surface of the piezoelectric material  11 , and converted into the high frequency electric field again by a reverse piezoelectric action, to generate an electric signal. When the surface acoustic wave, generated upon application of a high frequency electric field to the input terminal IDT  12 , is transmitted through surface of the piezoelectric material  11 , there is an electromagnetic feed-through directly transmitted from the input terminal IDT  12  to the output terminal IDT  13  in a form of electric or emissive energy. This is caused by the close proximity of the input terminal IDT  12  and the output terminal IDT  13 , which is increased as the applied frequency is increased and makes interference with the surface acoustic wave to deteriorate an SAW filter performance because the electromagnetic feed-through is transmitted at a speed of light different from an energy carried by the surface acoustic wave that is the basic operation principle of the SAW filter. 
     The electromagnetic feed-through is caused by the following major two reasons. 
     The first one is that, as in general upper and lower surfaces of the package are formed of conductive material and the piezoelectric material of the SAW filter is a ferroelectric material, the package with the SAW filter mounted thereon forms a dielectric waveguide because a ferroelectric material present between the input/output terminal IDTs, which are formed on surfaces of the piezoelectric material, and the package. In this instance, even though a wave fed through a dielectric waveguide is leaky, an energy transmitted by the electromagnetic feedthrough is relatively great in comparison to other secondary effect because the input terminal IDT and the output terminal IDT are at a close proximity. 
     Another one is that emission occurs between the input/output terminal IDTs and bonding wires. Since the input/output terminal IDTs and the bonding wires actually act as antennas, they emits electromagnetic waves within the SAW filter package, causing the electromagnetic feed-through. As shown in FIG. 2, in the related art, in order to reduce the electromagnetic feed-through, the metallic conductive blocking bar  18  is placed on the surface of the piezoelectric material  11  between the input terminal IDT  12  and the output terminal IDT  13 , with the bar  18  tilted at a given angle and both ends of the bar  18  grounded. This configuration can reduce the electromagnetic feed-through because the electromagnetic feed-through from the input terminal IDT  12  to the output terminal IDT  13  through the surface of the piezoelectric material  11  is absorbed by the blocking bar  18  and discharged through the ground terminals at both ends thereof. 
     However, as there is only the blocking bar provided in the related art SAW filter package, the related art SAW filter package can not but remove a portion of the electromagnetic feed-through flowing through the surface of the piezoelectric material, allowing the other portion to cause interference with the surface acoustic wave, that deteriorates the filter performance. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an SAW filter package that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an SAW filter package which can improve an SAW filter package performance. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the SAW filter package includes a base having a frame around a periphery, a ground part formed of a metal plating on an inside surface of the base for attaching an acoustic surface wave filter thereto, cut-away portions formed by cutting the metal plating of the ground part in two places, and a metal piece electrically separated from the metal plating by the cut-away portions. 
     In a case of the SAW filter package of the present invention which is provided with the two cut-away portions and the center grounded metal piece in the ground portion, the negative current at the output terminal is very weak because the negative current generated at the input terminal is, not only blocked by the cut-away portions to be reduced significantly, but also becomes very weak at the output terminal even if a weak negative current passes through the cutaway portion by the waveguide effect as the weak negative current is drained to the ground, thereby significantly reducing an unwanted electromagnetic feed-through. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a related art surface acoustic wave filter, schematically; 
     FIG. 2 illustrates details of an input terminal IDT and an output terminal IDT on the related art SAW filter; 
     FIGS.  3 A˜ 3 D illustrate details of a related art SAW filter package structure; 
     FIGS.  4 A˜ 4 D illustrate details of an SAW filter package structure in accordance with a preferred embodiment of the present invention; and, 
     FIGS. 5A and 5B illustrate negative current flows in lower surfaces of the related art SAW filter package structure and the SAW filter package structure of the present invention, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIGS.  4 A˜ 4 D schematically illustrate details of an SAW filter package structure in accordance with a preferred embodiment of the present invention, wherein FIG. 4A illustrates a back of the package, FIG. 4B illustrates a side of the package, FIG. 4C illustrates a plan view of the package, and FIG. 4D illustrates a plan view of the package with a cover opened without the SAW filter mounted thereon. 
     Referring to FIGS.  4 A˜ 4 D, the SAW filter package in accordance with a preferred embodiment of the present invention includes a base  30  of dielectric, such as ceramic, a frame  31  plated with a metal, such as gold, and projected vertical from the base  30  along a periphery of the base  30 , a plurality of metal pads  32  in sides of the base  30  for grounding and signal connection, a ground part  33  of a metal plating on an inside surface of the base  30  having an SAW filter(not shown) attached thereto, pad connections  35  for connecting the ground part  33  to the pads  32  in sides of the base  30 , and a cover  34  for sealing the SAW filter. The metal plating of the ground part  33  is provided with two cut-away parts  36  and  36 ′ for electric separation of the ground part  33  at a position corresponding to a position at which the blocking bar  18 (see FIG. 2) is mounted between the input terminal IDT  12  and the output terminal IDT  13  in the related art SAW filter, preferably leaving a metal piece  37  at a center between the two cut-away portions  36  and  36 ′ for more effective reduction of the electromagnetic feed-through provided that the metal piece  37  is grounded. And, by attaching the SAW filter  10  on a surface of the ground part  33  and covering the SAW filter  10  with the cover  34 , packing of the SAW filter is completed. In the SAW filter package of the present invention, because not only the blocking bar  18  absorbs the electromagnetic feed-through in the middle of transmission from the input terminal IDT  12  to the output terminal IDT  13 , but also the cut-away portions  36  and  36 ′ and the grounded metal piece  37 , both are formed beneath the blocking bar  18 , for two step electrical separation of the metal plating of the ground part  18 , which forms a portion of the waveguide, breaks the dielectric waveguide structure formed by the SAW filter package and the piezoelectric material of the SAW filter, not only the electromagnetic feed-through, an electromagnetic wave energy generated at the input terminal IDT  12 , is not transmitted to the output terminal IDT  13 , but also the electromagnetic feed-through is blocked by the grounded center metal piece  37  even if a weak electromagnetic feed-through is emitted along the dielectric waveguide. That is, the electromagnetic feed-through generated at an upper portion of the SAW filter is blocked by the blocking bar on the piezoelectric material and the electromagnetic feed-through generated at a lower portion of the piezoelectric material is blocked by the package of the present invention. 
     FIGS. 5A and 5D illustrate negative current flows in lower surfaces of the related art SAW filter package structure and the SAW filter package structure of the present invention, respectively. 
     The unwanted negative current in the lower surface, which corresponds to the electromagnetic feed-through, lessens the reduction effect at the blocking region. As shown in FIG. 5A, in the case of the related art, very great negative current  40  generated at the input terminal flows toward the output terminal. However, as shown in FIG. 5B, in a case of the SAW filter package of the present invention which is provided with the two cut-away portions  36  and  36 ′ the center grounded metal piece  37  in the ground portion  33 , the negative current  43  at the output terminal is very weak because the negative current  41  generated at the input terminal is, not only blocked by the cut-away portions  36  and  36 ′ to be reduced significantly, but also becomes very weak at the output terminal even if a weak negative current  42  passes through the cut-away portion  36  by the waveguide effect as the weak negative current  42  is drained to the ground. Thus, the unwanted electromagnetic feed-through can be removed, significantly. 
     As has been explained, the SAW filter package of the present invention can reduce the electromagnetic feed-through, permitting a greater reduction at a filter stop band and reduces ripples in a pass band, whereby providing an SAW filter of an excellent performance. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the SAW filter package of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.