Ladder filter with piezoresonators and folded electrode terminals stacked in a case
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to, a ladder filter, and more particularly, to a ladder filter for use in a communication device, such as a portable telephone or the like. 
     2. Description of Related Art 
     A conventional type of ladder filter has a structure wherein piezoresonators and terminals are stacked and contained in a box-type case. At least one of the terminals has a first electrode portion and a second electrode portion which are mutually folded, and the first electrode portion and the second electrode portion each have a projection for supporting a piezoresonator. 
     A possible way of downsizing the ladder filter is to use a thin plate as the material of the terminals. However, the terminals must have a strength over a certain degree, and the plate must have a thickness which guarantees the strength. 
     Another way of downsizing the ladder filter is to provide low projections on the first and the second electrode portions. In this case, however, if a piezoresonator is placed in the case in a slanted position, the peripheral portion of the piezoresonator may come into contact with the first and/or the second electrode portions of the terminal. Then, the piezoresonator may be influenced by damping, and the electrical characteristics of the ladder filter may be lowered. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a compact ladder filter, wherein a piezoresonator of the ladder filter is unlikely to come into contact with an electrode of a terminal. 
     In order to attain the object, a ladder filter according to the present invention comprises a terminal which has a first electrode portion and a second electrode portion, which is folded back on the first electrode portion. The first electrode portion and the second electrode portion have a first projection and a second projection, respectively, which are in contact with a respective one of the piezoresonators. The folding position of the second electrode portion is inside peripheral sides of the piezoresonators, and the whole second electrode portion is inside the peripheral sides of the piezoresonators. Further, the second projection is lower in height than the first projection. 
     In the structure described above, the sum of the thickness of the second electrode portion and the height of the second projection is the substantial height (the height from the first electrode portion) of the second projection, that is, the second electrode portion acts as part of the substantial height of the second projection. Thereby, the height of the second projection itself can be reduced, and this will not cause a piezoresonator which is supported by the second projection to come into contact with the first and/or the second electrode portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a fragmentary sectional view of a ladder filter which is an embodiment of the present invention; 
     FIG. 2 is a perspective view of an output terminal which is a component of the ladder filter shown in FIG. 1; 
     FIG. 3 is a plan view of the output terminal shown FIG. 2; 
     FIG. 4 is a side view of the output terminal shown in FIG. 2; 
     FIG. 5 is a perspective view of a relay terminal which is a component of the ladder filter shown in FIG. 1; 
     FIG. 6 is a perspective view of a grounding terminal which is a component of the ladder filter shown in FIG. 1; and 
     FIG. 7 is a perspective view of a modification of a terminal to be employed in a ladder filter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary ladder filter according to the present invention is hereinafter described with reference to the accompanying drawings. 
     As shown in FIG. 1, the ladder filter comprises a box-type case 1, four piezoresonators 5, 6, 7 and 8, an input terminal 10, an output terminal 11, a relay terminal 12 and a grounding terminal 13. The piezoresonators 5, 6 and 8 include peripherals sides 5a , 6a and 8a , respectively. The piezoresonators 5, 6, 7 and 8 are of a conventional type which has oscillation electrodes on the front and the back sides and operates in a stretch vibration mode. 
     The case 1 has an opening 1a at the right side of FIG. 1. The case 1 is made of resin, ceramics or the like. 
     The input terminal 10 has an electrode portion 10a, a connecting portion 10b and a projection 10c. The projection 10c is pressed against the lower surface of the piezoresonator 5. The pressure on the projection 10c is generated by the elasticity of a spring member 20 which is provided under the electrode portion 10a. 
     The output terminal 11 has electrode portions 11a and 11b, projections 11c and 11d, and a connecting portion 11e. The projections 11c and 11d are disposed on the electrode portions 11a and 11b , respectively, such that the projection 11c is in contact with the upper surface of the piezoresonator 7 and the projection 11d is in contact with the lower surface of the piezoresonator 8. As can be seen in FIGS. 2 through 4, the electrode portion 11b is folded back on the electrode portion 11a, and the connecting portion 11e extends from the electrode portion 11a. The folding position 111 is inside peripheral sides 8a of the piezoresonator 8, and the whole electrode portion 11b is inside the peripheral sides 8a of the piezoresonator 8 (see FIGS. 3 and 4). A cutout 113 (see FIGS. 2 and 3) is made in the electrode portion 11a, and the folding position 111 is at the bottom of the cutout 113. 
     The output terminal 11 is made of a plate which has a thickness of t (see FIG. 4). The electrode portion 11a has dimensions of L 1  by L 2  and the electrode portion 11b has dimensions of l 1  by l 2  as seen in FIG. 3. The projection lid has a height of C 1 , and the projection 11c has a height of C 2  (as seen in FIG. 4) which is larger than C 1 . The values t, L 1 , L 2 , l 1 , l 2  and C 1  are set such that the peripheral sides 8a of the piezoresonator 8, even if the piezoresonator 8 is held by the projection 11d in a slanted position, will not come into contact with peripheral sides 112 (see FIG. 4) of the electrode portion 11b. In this embodiment, as seen in FIG. 4 the thickness t of the plate of the output terminal 11, the height C 2  of the projection 11c and the height C 1  of the projection 11 d meet the condition: C 2  =t+C 1 . The addition of the thickness t of the electrode portion 11b and the height C 1  of the projection 11d is the substantial height H (the height from the electrode portion 11a as seen in FIG. 4) of the projection 11d, that is, the electrode portion 11b acts as part of the substantial height H of the projection 11d. Conventionally, the projections 11c and 11d have the same height. However, in this embodiment, the height of the projection 11d, which is disposed on the electrode portion 11b with a thickness of t, can be reduced by the thickness t, and this will not cause contact of the piezoresonator 8 with the electrode portions 11a and/or 11b. 
     As shown in FIG. 1, the relay terminal 12 has electrode portions 12a, 12b and 12c, projections 12d, 12e and 12f, and a relay portion 12g. The projections 12d, 12e and 12f are disposed on the electrode portions 12a, 12b and 12c, respectively, such that the projection 12d is in contact with the upper surface of the piezoresonator 8, the projection 12e is in contact with the lower surface of the piezoresonator 6 and the projection 12f is in contact with the upper surface of the piezoresonator 5. FIG. 5 shows the relay terminal 12 having the electrode portions 12a, 12b, and 12c, projections 12d, 12e (not shown) and 12f, and a relay portion 12g. As can be seen in FIG. 5, the electrode portion 12c is folded back on electrode portion 12b. The folding position 121 and the electrode portion 12c are inside peripheral sides 5 a of the piezoresonator 5. A cutout 123 is made in the electrode the portion 12b, and the folding position 121 is at the bottom of the cutout 123. 
     Length of the projection 12f is less than the projection 12e. As seen in FIG. 1, the measurements of the electrode portions 12b and 12c are set such that the peripheral sides 5a of the piezoresonator 5, even if the piezoresonator 5 is held by the projection 12f on a slant, will not come into contact with peripheral sides 122 (see FIG. 5) of the electrode portion 12c. In this embodiment, the addition of the height of the projection 12f and the thickness of the plate of the relay terminal 12 is the height of the projection 12e. Thereby, the addition of the thickness of the electrode portion 12c and the height of the projection 12f is the substantial height H (the height from the electrode portion 12b) of the projection 12f, that is, the electrode portion 12c acts as part of the substantial height H of the projection 12f. Thus, the projection 12f can be made lower in height than the projection 12e, and this will not cause contact of the piezoresonator 5 with the electrode portions 12b and/or 12c. 
     As shown in FIG. 1, the grounding terminal 13 has electrode portions 13a and 13b, projections 13c and 13d, and a connecting portion 13e. The projections 13c and 13d are disposed on the electrode portions 13a and 13b respectively such that the projection 13c is in contact with the lower surface of the piezoresonator 7 and the projection 13d is in contact with the upper surface of the piezoresonator 6. FIG. 6 shows the grounding terminal 13 having projection 13d and the connecting portion 13e. As can be seen in FIG. 6, the electrode portion 13b is folded back on the electrode portion 13a. The folding position 131 and the electrode portion, 13b are inside peripheral sides 6a(see FIG. 1) of the piezoresonator 6. A cutout 133 is made in the electrode portion 13a, and the folding position 131 is at the bottom of the cutout 133. 
     As seen in FIG. 1, the height of the projection 13d is less than the that of projection 13c. The measurements of the electrode portions 13a and 13b are set such that the peripheral sides 6a of the piezoresonator 6, even if the piezoresonator 6 is held by the projection 13d on a slant, will not come into contact with peripheral sides 132 of the electrode portions 13b. In this embodiment, the addition of the height of the projection 13d and the thickness of the plate of the grounding terminal 13 is equal to the height of the projection 13c. Thereby, the addition of the thickness of the electrode portion 13b and the height of the projection 13d is the substantial height H (the height from the electrode portion 13a) of the projection 13d, that can be, the electrode portion 13b acts as part of the substantial height H of the projection 13d. Thus, the projection 13d is made lower than the projection 13c, and this will not cause contact of the piezoresonator 6 with the electrode portions 13a and/or 13b. 
     As seen in FIG. 1, the piezoresonators 5 through 8 and the terminals 10 through 13 are inserted into the case 1 via the opening 1a. Then, an insulating sheet 30 is set on a step made in the opening 1a. A sealing material 31 such as resin is potted in the opening 1a so as to seal the case 1. Further, the connecting portions 10b, 11e, 13e of the terminals 10, 11 and 13, respectively, may be bent after hardening of the sealing material 31 so as to finish the ladder filter as a surface mounting type. 
     The ladder filter obtained in this way is smaller than a conventional one because the projections 11d, 12f and 13d of the terminals 11, 12 and 13, respectively, are lower than those of the conventional one. 
     In the present invention, the terminals may have any other configuration. A terminal 40 illustrated in FIG. 7 is an example. The terminal 40 has electrode portions 40a and 40b, two projections 40c (only one of them is shown), and a connecting portion 40e. The electrode portion 40b is folded back on the electrode portion 40a. A hole 143 is made in the electrode portion 40a, and the folding position 141 is at the edge of the hole 143. 
     Although the present invention has been described in connection with the preferred embodiment, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.