JP3480891B2 - Multi-element ladder type piezoelectric filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a portable mobile radio device,
The present invention relates to a ladder-type piezoelectric filter used in a filtering circuit of a wireless communication device such as a car phone, and particularly to a ladder-type piezoelectric filter most suitable for digital communication.

[0002]

2. Description of the Related Art Conventionally, this type of ladder type piezoelectric filter is
Between the input terminal, the output terminal, and the ground terminal, unit filtering circuits each having a series resonator and a parallel resonator connected in an L shape are arranged in multiple stages. Then, the resonance frequency of the series resonator and the anti-resonance frequency of the parallel resonator are matched, and the difference between the resonance frequency and the anti-resonance frequency is made constant. For example, the resonance frequency of three series resonators is about 455kHz, the anti-resonance frequency is about 477kHz, and the difference Δf is set to 22kHz, and the resonance frequency of three parallel resonators is about 433kHz. The anti-resonance frequency is set to about 455 kHz, and the difference Δf is set to 22 kHz. However, this configuration has a problem that the flatness is poor due to the group delay characteristic.

Therefore, in order to improve the group delay characteristic, a set of a series resonator and two parallel resonators inserted adjacent to one side of the series resonator between the input terminal and the output terminal. A piezoelectric filter having a circuit structure in which two or more stages are connected together as a basic unit filtering circuit has been proposed.

[0004]

On the other hand, the conventional multi-element ladder type piezoelectric filter has an opening on the side surface of the case, and the resonator and the terminal plate are parallel to the mounting surface (bottom surface) of the filter. I was trying to insert it. However,
As the piezoelectric filter is required to have a low profile, the height of the opening is correspondingly reduced, and the insertion of the resonator and the terminal plate becomes troublesome.

Further, as described above, a multi-element ladder piezoelectric filter having a large number of resonators, such as two parallel resonators used for one series resonator and a complicated structure, is used. As a result, the arrangement structure for forming the predetermined circuit has become complicated, and the assembling work has become even more troublesome.

Therefore, a shallow bottom rectangular case made of synthetic resin having an opening to which a cover plate is attached is used, and inside thereof, two internal connecting ends of an input terminal and an output terminal are provided on opposite two sides. And further expose the internal connection end of the ground terminal to the locking step formed on the other side part orthogonal to the side part, and integrate each terminal with the case by insert molding to connect the respective external connection ends. Along with extending to the outer surface of the case, a plurality of series resonators and parallel resonators are vertically inserted between the internal connection ends and arranged in an overlapping manner, and one electrode of the parallel resonators is grounded. It is connected to the internal connection end of the ground terminal via a connection piece, and further, a seating groove is formed on the inner bottom surface of the case along the stacking direction of the terminals, and the U-shaped connecting terminal is formed in the seating groove. One end of the terminal piece is connected to the electrodes of the adjacent series resonator. And, multi-element ladder type piezoelectric filter having the configuration connecting the electrodes mutual are proposed (Japanese Patent Application No. 9
-33219).

In such a structure, since the opening is formed on the upper surface, there is an advantage that a wide opening area can be secured and the mounting is easy without going against the height reduction of the case. However, in such a configuration, the opposing electrodes of the series resonators are connected to each other via the U-shaped connection connecting piece, and the number of parts increases and the seat to which the U-shaped connection connecting piece fits is increased. Since the constant groove is formed on the bottom of the case, it is necessary to increase the thickness of the bottom of the case.
Therefore, there was a problem that the height of the case could not be reduced. Moreover, the U-shaped connection connecting piece is fitted into the seating groove, which requires a process, which makes the assembly troublesome. It is an object of the present invention to provide a multi-element ladder type piezoelectric filter that is easy to assemble and does not have the above-mentioned problems.

[0008]

DISCLOSURE OF THE INVENTION The present invention is directed to a synthetic resin rectangular case having an opening on the upper surface of which a cover plate is attached, and inside each of the two sides, facing each other, an input terminal and an output terminal. The internal connection end is exposed, and further, connecting step portions are formed on the other two side portions orthogonal to the side portion, and the internal connecting end of the ground terminal is exposed on the step surface of one connecting step portion and the other connecting side portion is formed. A plurality of connection electrode layers are formed on the step surface of the connection step portion or the lower surface portion of the cover plate which is attached to the step surface, and further, each terminal is integrated with the case by insert molding to connect each external connection end to the case. While extending to the outer surface, between the internal connection end of the input terminal and the output terminal,
Insert a plurality of series resonators and parallel resonators perpendicularly to the apertures and arrange them in a stacked manner, and connect the electrodes of each series resonator to each parallel resonator via the input / output connection piece. Then, by connecting the other side electrode of the parallel resonator to the ground connection piece and connecting each input / output connection piece to each connection electrode layer, each series resonator is connected in series between the input terminal and the output terminal. Thus, each parallel resonator is connected in parallel to the ground terminal by connecting each ground connection piece to the internal connection end of the ground terminal on the other connection step portion. It is an element ladder type piezoelectric filter.

In this structure, the connection step portions are formed on the two sides of the case, the internal connection end of the ground terminal is exposed on the step surface of one connection step portion, and the connection step portion of the other connection step portion is formed. A plurality of connection electrode layers are formed on the lower surface portion of the step surface or the lid plate to be attached to the step surface, and a series of resonators and a parallel resonator are superposed on each other from a resonator row to each side. Since the connecting piece and the input / output connecting piece are projected so as to be connected to the internal connecting end of the ground terminal and the connecting electrode layer, there is no need to form a seating groove at the bottom of the case, The case can be made low in height without requiring thickening of the case. Further, the ground connection piece and the input / output connection piece can have the same shape.

In such a structure, in order to improve the group delay characteristic, a series resonator inserted between the input terminal and the output terminal and two sheets inserted adjacent to one side of the series resonator. It is most suitable for a circuit configuration in which a unit filtering circuit configured by combining the parallel resonator of (1) is used as a basic section and two or more stages are connected.

In this structure, one series resonator and two parallel resonators are set as one unit between the internal connection ends of the input terminal and the output terminal, and a plurality of units are inserted vertically to the opening. And the electrodes of each series resonator are connected to each parallel resonator via the input / output connection piece, and the ground connection piece is inserted between two adjacent parallel resonators. Then
By connecting each input / output connection piece to each connection electrode layer,
Make sure that each series resonator is connected in series between the input terminal and the output terminal, and connect each ground connection piece to the internal connection end of the ground terminal on the other connection step so that each parallel resonator can connect to the ground terminal. A multi-element ladder-type piezoelectric filter is proposed, which is characterized in that it is connected in parallel with. In this circuit configuration, the number of resonators is large and the connection form is complicated, but the above configuration makes it easy to organize,
Can be stored in the case.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 14 shows three series resonators S.
₁ , S ₂ , S ₃ (see FIG. 8) are input terminals I and output terminals O
Six parallel resonators P ₁ , P ₂ , P ₃ , P ₄ , P ₅ , P ₆ connected in series to the input / output line L ₁ connecting them (see FIG. 9).
FIG. 3 is a circuit diagram of a ladder type filter in which is connected in parallel between the input / output line L ₁ and the ground line L ₂ .

That is, the series resonator S ₁ (S ₂ , S ₃ ) inserted between the input terminal I and the output terminal O and the series resonator S ₁ (S ₂ , S ₃ ) inserted adjacent to one side of the series resonator S _1. A unit filtering circuit configured by combining a plurality of parallel resonators P ₁ and P ₂ (P ₃ , P ₄ , P ₅ , and P ₆ ) is used as a basic section, and this is referred to as 3
It is designed to be connected in stages. As described above, in the configuration in which two or more unit filtering circuits each including two parallel resonators are connected to each series resonator, the group delay is hardly deteriorated and the group delay is reduced. There is an advantage that the characteristics are greatly improved.

1 to 11 show the present invention applied to a ladder type filter having such a circuit configuration.

Here, the series resonators S ₁ , S ₂ , and S ₃ are
As shown in FIG. 8, it has a strip shape made of lead zirconate titanate or the like, and is formed by forming electrodes 2a and 2b on the front and back sides of a strip piezoelectric ceramic plate 1 having a predetermined thickness. Further, as shown in FIG. 9, the parallel resonators P ₁ , P ₂ , P ₃ , P ₄ , P ₅ , and P ₆ are the same as the series resonators S ₁ , S ₂ , and S _3, and lead zirconate titanate. The strip-shaped piezoelectric ceramic plate 4 has electrodes 5a and 5b formed on the front and back sides thereof, and the thickness thereof is reduced to increase the capacitance. In this way, by making the thickness of the parallel resonator smaller than that of the series resonator, the capacitance ratio of the two becomes large,
The amount of attenuation can be increased.

Next, a rectangular case 1 in which each resonator is housed
0 is made of synthetic resin and has a shallow shape. The rectangular case 10 has an opening 11 formed in its upper part,
A lid plate 13 is attached to the opening 11. This square case 1
Inside the 0, along the inner wall of the opening 11, connection step portions 12a, 12b are formed on two opposite sides, and further, at the center position of the inner surface of the two side portions orthogonal to the connection step portions 12a, 12b, Internal connection ends 15a, 1 of the input terminal I and the output terminal O
6a is exposed. Further, the internal connection end 17a of the ground terminal E is exposed on the upper surface of the connection step portion 12a. In addition, 3 is provided on the step surface of the connection step portion 12b.
One connection electrode layer 8a, 8b, 8c is formed separately by means such as vapor deposition. The terminals I, O and E are formed in a rectangular case 1 by insert molding in a process described later.
It is assembled by inserting it integrally with 0.

Instead of the connection electrode layers 8a, 8b and 8c, as shown in FIG. 13, the connection step portion 12b of the cover plate 13 is formed.
The connection electrode layers 8a ′, 8b ′,
8c 'may be formed.

The rectangular case 10 and the terminals I, O and E can be insert-molded by the means shown in FIG. 12 with good mass productivity. That is, first, the thickness is 0.05 mm
A long thin plate 40 obtained by plating PbSn on the phosphor bronze is punched into the shape shown in FIG. Then, as shown by the position a, the portions (see chain lines) corresponding to the internal connection ends 15a and 16a of the punched portions 41 formed at predetermined intervals are bent at a right angle in advance. Then, as shown by the position B, it is intermittently transferred to the molding die, and the internal connection ends 15a, 16a, 17a are positioned in the die as described above, and the rectangular case 10 is molded. Next, as shown in position C,
The terminals I, O and E are separated from the long thin plate 40 by cutting and bent at the same time so that the terminals I and O are located on the lower bottom of the rectangular case 10 and the lower bottom ends 15b and 16b are located.
Both ends of the lower bottom ends 15b and 16b are bent at a right angle and folded along the outer side surface to form external connection ends 15c and 16c. Further, the ground terminal E projects laterally from the internal connection end 17a and is folded along the side surface to form the external connection end 17c. Further, the end portion thereof is folded along the bottom surface, and this is The lower bottom end 17b.

Thus, the rectangular case 1 having a structure in which the external connection ends 15c, 16c, 17c of the terminals I, O, E are exposed on the peripheral surface and the lower bottom ends 15b, 16b, 17b are exposed on the lower bottom.
0 is completed. Then, as will be described later, when the ladder type filter using the rectangular case 10 is mounted on the printed circuit board, the external connection ends 15c, 16c and 17c are connected to a required electric circuit on the printed circuit board. Become. The lower bottom ends 15b, 16b, 17b may be connected to a required electric circuit.

In such a structure, as in the conventional structure, the input terminal plate, the input / output terminal plate, and the ground terminal plate are separately inserted into the case, or the insert molding is performed without bending after the insertion. As a result of the series of processes up to bending, the rectangular case 10 with the external connection ends 15c, 16c, 17c exposed to the outer surface is formed, so that the assembly is easy.

Next, the circuit configuration in the rectangular case 10 will be described. In the rectangular case 10, as shown in FIG. 1, the internal connection ends 15a, 16 of the input terminal I and the output terminal O are connected.
Between a, one series resonator and two parallel resonators are set as one unit, and a plurality of units are arranged in an overlapping manner. That is, between the internal connecting ends 15a and 16a, from the internal connecting end 15a side, the series resonator S ₁ , the parallel resonators P ₁ and P ₂ , and the second stage unit filtering circuit that form the first-stage unit filtering circuit are arranged. Of the series resonator S ₂ , the parallel resonators P ₃ and P ₄ and the series resonator S ₃ and the parallel resonators P ₅ and P ₆ of the unit filter circuit of the third stage are sequentially superposed, and Opening 1
Inserted perpendicular to 1.

The outer electrode of the series resonator S ₁ is closely connected to the internal connection end 15a, and the series resonator S 1
Input / output connection piece 2 connected to the opposing electrodes of ₁ and S ₂ , respectively
1, 21 are connected to the connection electrode layer 8a on the step surface of the connection step portion 12b, and the input / output connection pieces 21 and 21 connected to the opposing electrodes of the series resonators S ₂ and S ₃ are the connection electrode layers 8 similarly.
Similarly, the series resonator S ₃ and the input / output connection pieces 21 and 21 connected to the internal connection end 16a are connected to the connection electrode layer 8b.
connected to c. Thus, the series resonators S ₁ and S are connected between the internal connection ends 15a and 16a of the input terminal I and the output terminal O.
₂ and S ₃ are connected in series via the connection electrode layers 8a, 8b and 8c, respectively. The input / output connection piece 21 and the connection electrode layer 8
The connection with a, 8b, and 8c is performed by connecting the connection branch portion 33 formed on the input / output connection piece 21 to the connection electrode layers 8a and 8c as described later.
It is carried out by placing the solder on b and 8c and soldering.

Furthermore, the parallel resonator P ₁ , between the input / output connecting pieces 21 and 21 located between the series resonators S ₁ and S ₂ .
P ₂ is, series resonators S _2, is between output connection piece 21 located between the S ₃ parallel resonators P _3, P ₄ is the series resonator S ₃
Between the internal connection terminal 16a, the parallel resonator P _5, P ₆
Are arranged respectively, and the parallel resonators P ₁ and P ₂ and the parallel resonator P are arranged.
₃ , ₃ and P ₄ and the parallel resonators P ₅ and P ₆ are respectively provided with the ground connection piece 25, and the ground connection piece 25 connects the connection branch portion 33 to the internal connection of the ground terminal E on the connection step portion 12a. Connection is made by placing the solder on the end 17a and soldering. Thus, each of the parallel resonators P _{1 to} P ₆ has its one side electrode grounded, and in each unit stage, the series resonators S ₁ to P _6.
It will be electrically connected in parallel to S ₃ .

Here, the input / output connection piece 21 and the ground connection piece 25 can be formed of the same part as shown in FIGS. That is, as shown in FIG. 10, a metal piece 30 formed by punching a metal plate material is bent, and the metal piece 30 has a main plate portion 31 and
Upper and lower rectangular bent portions 3 extending from the upper and lower edges of the main plate portion 31.
2, 32 and a connecting branch portion 33 projecting from the front edge of the main plate portion 31. Then, the rectangular bent portions 32, 32 are folded into the main plate portion 31 in an overlapping shape as shown by an arrow in FIG. 10, and the thickness is three times the plate thickness in contact with the electrodes of each resonator. The thick connecting portion 34 is formed. The rectangular bent portions 32, 32 are formed on the upper and lower edges of the main plate portion 31, but only one of the upper and lower edges is formed to have a double length, and the rectangular bent portions 32, 32 are continuously wound so as to be wound around the central portion of the main plate portion. It may be bent and formed. Connection pieces 21 and 2 having such a configuration
5, the thickness example of the metal plate material is about 0.05 mm, the width of the rectangular bent portions 32, 32 is about 0.4 mm, and the connecting branch portion 33 has The height is about 0.2 mm. Note that this size is merely an example, and the size is not limited to this size.

In the connection piece 21 and the connection piece 25 having such a structure, the thick connection portion 34 ensures the connection with the adjacent resonator without the need to bulge the center as in the conventional terminal board. To be done. Moreover, since the thickness is defined by the material thickness, the thickness is always constant, a uniform contact pressure can be secured, and stable filtering characteristics can be achieved.

Then, as described above, the connecting branch portion 33.
To the internal connection end 17a of the ground terminal E and the connection electrode layers 8a, 8b, 8c on the connection step portions 12a, 12b, and soldering the connection pieces 21 and 25, respectively, so that the connection pieces 21 and 25 have the required connection described above. Then, the cover plate 13 is attached to the opening 11 and its peripheral edge is pressed against the connection step portions 12a and 12b.
At this time, since the height of the connecting branch portions 33 is as small as about 0.2 mm, each of the connecting branch portions 33 is connected to the internal connecting end 1.
7a and the connection electrode layers 8a, 8b, 8c, and the attachment of the cover plate 13 is not hindered.

On the other hand, as shown in FIG. 13, the connection electrode layers 8a ', 8b', 8 are formed on the lower surface of the cover plate 13 which is attached to the connection step 12b by means such as vapor deposition or screen printing.
You may make it form c '. In this case,
By attaching the cover plate 13, the connecting branch portion 33 becomes the connecting step portion 12
b and the cover plate 13, and the connection electrode layers 8a ′,
The connection with 8b 'and 8c' is secured.

Thus, in the rectangular case 10, the parallel resonators P ₁ and P ₂ , the parallel resonators P ₃ and P ₄ and the parallel resonator P are arranged.
₅ , P ₆ has one side electrode for each series resonator S ₁ , S ₂ , S ₃
The other side electrode is connected to the ground, and each of them is connected in parallel, while the series resonators S ₁ and S are connected.
₂ and S ₃ are connected in series between the internal connection ends 15a and 16a to form the input / output line L ₁ , and are unit filtering circuits composed of one series resonator and two parallel resonators. As a basic division, the circuit of FIG. 14 is configured by connecting these in three stages.

In this embodiment, since the resonators are connected by the input / output connection piece 21 and the ground connection piece 25 which have the same shape, the number of parts is small and the rectangular case 10 has a small number of parts. After inserting the members in a superposed manner from the opening 11 which is largely opened on the upper surface, the connecting branch portion 33 is connected to the internal connecting end 1 on the connecting step portions 12a and 12b.
7a and the connecting electrode layers 8a, 8b, 8c are connected only by soldering, respectively. Therefore, it is not necessary to connect each electrode or the like with a lead wire, and the assembling is easy. The shapes of the input / output connection piece 21 and the ground connection piece 25 may be different.

Further, in such a structure, the rectangular case 1
Since it is not necessary to form a seating groove or the like on the bottom surface of 0, it is not necessary to increase the bottom thickness of the rectangular case 10, which contributes to a reduction in height.

In this structure, the piezoelectric filter F
An example of the shape is a flat square with a height of 1.6 mm and a side length of 5.0 mm. The above embodiment has been described by taking a unit type filtering circuit consisting of one series resonator and two parallel resonators as a basic section and using a ladder type piezoelectric filter in which these are connected in three stages as an example. , It can also be applied to multi-element ladder-type piezoelectric filters connected in multiple stages such as four stages. Furthermore, the unit filtering circuit consisting of one series resonator and one parallel resonator can be used as a basic section to connect to other stages. It can also be applied to the multi-element ladder type piezoelectric filter.

[0032]

As described above, according to the present invention, the inside of the rectangular case made of synthetic resin having the opening to which the cover plate is attached is provided with the input terminal and the output terminal on the opposite two sides. Each internal connection end is exposed, and a connection step is formed on each of the other two sides, the internal connection end of the ground terminal is exposed on the step surface of one connection step, and the step surface of the other connection step. Or, by forming a plurality of connection electrode layers on the lower surface portion adhered to the stepped surface of the lid plate,
Between the internal connection ends of the input and output terminals, multiple series resonators and parallel resonators are inserted vertically and arranged in a stack, and each resonator row is grounded to each side. The piece and the input / output connection piece are projected so as to be connected to the internal connection end of the ground terminal of each connection step and the connection electrode layer, respectively. For this reason, (a) the opening is formed on the upper surface, so that a wide opening area can be secured and the mounting is facilitated without going against the height reduction of the case. (B) It is not necessary to form a seating groove in the bottom of the case, the thickness of the case does not need to be increased, and the height of the case can be reduced. C) The ground connection piece and the input / output connection piece can have the same shape, in which case the number of parts is significantly reduced.

[Brief description of drawings]

FIG. 1 is a plan view of a multi-element ladder type piezoelectric filter of the present invention. Only each resonator is shown in cross section.

FIG. 2 is a bottom view of the multi-element ladder type piezoelectric filter of the present invention.

FIG. 3 is a rear view of the multi-element ladder type piezoelectric filter of the present invention.

FIG. 4 is an exploded perspective view of the rectangular case 10.

5 is a plan view of the rectangular case 10. FIG.

6 is a cross-sectional view taken along the line AA of FIG.

7 is a sectional view taken along line BB of FIG.

FIG. 8 is a perspective view of series resonators S _{1 to} S ₃ .

FIG. 9 is a perspective view of parallel resonators P _{1 to} P ₆ .

FIG. 10 is a perspective view of the connection pieces 21 and 25 before bending.

FIG. 11 is a perspective view of the connection pieces 21 and 25 after bending.

FIG. 12 is a plan view of the long thin plate 40.

FIG. 13 is a separated vertical side view showing a modified example of the connecting means of the input / output connecting piece 21.

FIG. 14 is a circuit diagram of the piezoelectric filter according to the present embodiment.

[Explanation of symbols]

S _{1 to} S ₃ series resonator P _{1 to} P ₆ parallel resonator I input terminal O output terminal E ground terminal 8a, 8b, 8c, 8a ', 8b', 8c connection electrode layer 10 square case 12a, 12b connection step portion 13 cover plates 15a, 16a, 17a internal connection ends 15c, 16c, 17c external connection end 21 input / output connection piece 25 ground connection piece 33 connection branch portion

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-18383 (JP, A) JP-A-6-334474 (JP, A) JP-A-7-58575 (JP, A) Actual Kaihei 5- 11544 (JP, U) Actually open 61-90317 (JP, U) Actually open 61-184320 (JP, U) Actually open 62-53819 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/58

Claims (2)

(57) [Claims] 1. A piezoelectric device having a circuit structure in which two or more unit filtering circuits each including a series resonator and one or more parallel resonators are connected between an input terminal and an output terminal. In the filter, the internal connection ends of the input terminal and the output terminal are exposed at the two opposite sides inside the rectangular case made of synthetic resin having an opening to which the lid plate is attached, and further, Connecting step portions are formed on the other two side portions orthogonal to the side portion, the internal connection end of the ground terminal is exposed on the step surface of one connecting step portion, and the step surface of the other connecting step portion or the lid plate is formed. A plurality of connection electrode layers are formed on the lower surface part attached to the step surface, and each terminal is integrated with the case by insert molding to extend each external connection end to the outer surface of the case, and to connect with the input terminal. Connect multiple series resonators and parallel resonators between the internal terminals of the output terminals. Inserted perpendicularly to the mouth and arranged in a stack, and connect the electrodes of each series resonator to each parallel resonator via the input / output connection piece, and connect the other side electrode of the parallel resonator to ground. By connecting each input / output connection piece to each connection electrode layer,
Make sure that each series resonator is connected in series between the input terminal and the output terminal, and connect each ground connection piece to the internal connection end of the ground terminal on the other connection step so that each parallel resonator can connect to the ground terminal. A multi-element ladder-type piezoelectric filter characterized in that it is connected in parallel with. 2. A series resonator between the input terminal and the output terminal,
A combination with two parallel resonators inserted adjacent to one side of the series resonator is used as a basic unit filtering circuit,
In a piezoelectric filter with a circuit configuration with more than one stage connected, in a synthetic resin rectangular case with an opening on the top surface of which a cover plate is attached, inside the two sides, the input terminal and the output terminal Exposing each internal connection end, further forming a connection step on each of the other two sides orthogonal to the side, exposing the internal connection end of the ground terminal on the step surface of one connection step, A plurality of connection electrode layers are formed on the step surface of the other connection step portion or the lower surface portion of the lid plate which is attached to the step surface, and further, each terminal is integrated with the case by insert molding to form each external connection end. While extending to the outer surface of the case, one series resonator and two parallel resonators are used as one unit between the internal connection ends of the input terminal and the output terminal. Inserted and arranged in a stack, and connecting the electrodes of each series resonator to input and output By connecting to each parallel resonator via to connect by inserting a ground connecting piece between adjacent two of the parallel resonator, connecting each output connecting pieces to each connecting electrode layers,
Make sure that each series resonator is connected in series between the input terminal and the output terminal, and connect each ground connection piece to the internal connection end of the ground terminal on the other connection step so that each parallel resonator can connect to the ground terminal. A multi-element ladder-type piezoelectric filter characterized in that it is connected in parallel with.