Abstract:
A ladder filter includes two piezoelectric resonators which operate in a stretch vibration mode and stacked on top of each other with a grounding terminal located therebetween. Two piezoelectric resonators which operate in a length vibrating mode are provided at both sides of the piezoelectric resonators which operate in a stretch vibration mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a ladder filter, and more particularly to a ladder filter which is preferably used for a transceiver, a cordless telephone and so on. 
     2. Description of Related Art 
     Conventionally, a ladder filter, as shown in FIG. 6, which is assembled by stacking a plurality of piezoelectric resonators and terminals and inserting the stack of resonators and terminals into a case is well known. The ladder filter of FIG. 6 is formed by laminating piezoelectric resonators 3, 4, 5 and 6, terminals 11, 12, 13 and 14 and a dummy terminal 15 and inserting the resonators and terminals into a resin case 1 which has an opening at the left side in FIG. 6. The piezoelectric resonators of this filter form an equivalent circuit as shown in FIG. 5. Since FIG. 5 shows a preferred embodiment of the present invention, the reference numerals of the preferred embodiment are indicated. 
     However, in the conventional ladder filter, both the resonators 3 and 4 connected in series and the resonators 5 and 6 connected in parallel operate in a stretch vibration mode. Thereby, the area and the thickness of each resonator 3, 4, 5 and 6 are to a specified size, and the height of each of the resonators becomes large. In addition, because the resonators 3, 4, 5 and 6 having a relatively large height are stacked vertically, the stack of resonators also has a great height thereby preventing the size of the filter from being reduced. 
     SUMMARY OF THE INVENTION 
     To solve the disadvantages of the prior art, the preferred embodiments of the present invention provide a short thin ladder filter having an overall size that is substantially reduced as compared to the prior art. 
     A a ladder filter according to the preferred embodiments of the present invention comprises a piezoelectric resonator which operates in a stretch vibration mode and a piezoelectric resonator which operates in a length vibration mode. The resonator which operates in the stretch vibration mode requires a specified area, but is relatively thin. On the other hand, the resonator which operates in the length vibration mode requires a specified thickness but requires only a small area. Thus, the resonator which operates in a stretch vibration mode is set in a lateral position, and the resonator which operates in a length vibration mode is provided at a side of the resonator which operates in a stretch vibration mode. 
     In the above structure, since the number of stacked resonators is decreased, a short thin ladder filter can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the preferred embodiments of the present invention will become apparent from the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is an exploded perspective view of a ladder filter which is a preferred embodiment of the present invention; 
     FIG. 2 is a perspective view of the ladder filter which is a finished product; 
     FIG. 3 is a sectional view of the ladder filter, taken along the line III--III, only a case being cut; 
     FIG. 4 is a sectional view of the ladder filter, taken along the line IV--IV, only the case and a sealing material being cut; 
     FIG. 5 is an equivalent circuit diagram of the ladder filter; and 
     FIG. 6 is a sectional view which shows an inner structure of a conventional ladder filter. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The description of preferred embodiments according to the present invention is given below, referring to the accompanying drawings. 
     In FIGS. 1 through 4, a ladder filter is composed of piezoelectric resonators 21, 22, 23 and 24, an input terminal 25, an output terminal 26, a junction terminal 27, a grounding terminal 28, an inner case 30 and an outer case 31. 
     The piezoelectric resonators 21 and 22 are preferably rectangular parallelepiped. Electrodes preferably made of silver are formed on two opposite sides (upper and bottom sides in FIG. 3) of the piezoelectric resonators 21 and 22. The piezoelectric resonators 21 and 22 operate in a length vibration mode and are connected in series (refer to FIG. 5). The piezoelectric resonators 23 and 24 are preferably plate-shaped. Electrodes preferably made of silver are formed on both sides of the piezoelectric resonators 23 and 24. The piezoelectric resonators 23 and 24 operate in a stretch vibration mode and are connected in parallel. 
     The input terminal 25 has a contact portion 25a and an outer connecting portion 25b. The output terminal 26 has contact portions 26a and 26b and an outer connecting portion 26c. The junction terminal 27 has a contact portion 27a at the center and contact portions 27b and 27c at both ends. The grounding terminal 28 is formed by folding a plate and has contact portions 28a and 28b and an outer connecting portion 28c. 
     The inner case 30 is a frame type which has a center opening 30a and side openings 30b and 30c and is preferably integrally molded out of resin. The outer case 31 is preferably integrally molded out of resin into a box and has an opening 31a at a side and a slit 31b at the opposite side. 
     Next, an assembling method will be explained. 
     First, the junction terminal 27 is placed on an assembling line (not shown), and the inner case 30 is set thereon. Next, the resonator 23, the grounding terminal 28 and the resonator 24 are stacked in the center opening 30a of the inner case 30, and at the same time, the resonators 21 and 22 are placed in the side openings 30b and 30c. At that time, the outer connecting portion 28c of the grounding terminal 28 is protruded from the slit 30d (refer to FIG. 4) which is made in the inner case 30. Then, the input terminal 25 is set on the resonator 21, and the output terminal 26 is set on the resonators 22 and 24. 
     A unit which is assembled in accordance with the above method is inserted into the outer case 31 from the opening 31a. At that time, the outer connecting portion 28c of the grounding terminal 28 is protruded outward from the slit 31b. Also, the outer connecting portions 25b and 26c of the terminals 25 and 26 are also protruded outward from the opening 31a. Next, as shown in FIG. 4, the opening 31a of the outer case 31 is sealed with a sealing material 32 (e.g. epoxy resin). After hardening of the sealing material 32, the outer connecting portions 25b, 26c and 28c are bent. 
     Thereby, the surface mounting type ladder filter with three terminals is finished. 
     In the present preferred embodiment, only two piezoelectric resonators 23 and 24 which operate in a stretch vibration mode are stacked, and the piezoelectric resonators 21 and 22 which operate in a length vibration mode are provided at the sides of the piezoelectric resonators 23 and 24. Thus, the height of the filter is about a half of the height of the conventional ladder filter of FIG. 6. Furthermore, the resonators 21 and 22 which operate in a length vibration mode do not occupy a large area. 
     Also, it is possible to use resonators which operate in a stretch vibration mode for series connection and resonators which operate in a length vibration mode for parallel connection. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are apparent to a person skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.