CN111557076B - Piezoelectric filter - Google Patents

Abstract

On the inner surface of a package accommodating two piezoelectric filter elements, a shielding electrode is formed that is opposite to the input electrode and the output electrode of each piezoelectric substrate of each piezoelectric filter element, and the opposing gap between the two electrodes of the piezoelectric substrate and the shielding electrode on the inner surface of the package is set to be greater than 100μm.

Description

Piezoelectric Filters

Technical Field

The present invention relates to a piezoelectric filter used in communication equipment such as a mobile phone, electronic equipment, and the like.

Background technique

For example, a piezoelectric filter using a piezoelectric substrate such as an AT-cut quartz plate generally has a structure in which an input electrode and an output electrode are formed on one surface of the piezoelectric substrate, and a common electrode (ground electrode) corresponding to the input and output electrodes is formed on the other surface. This structure is called a bipolar piezoelectric filter because it has two electrode pairs sandwiching the piezoelectric substrate. In order to improve the characteristics of the piezoelectric filter, there is a quadrupole piezoelectric filter formed by cascading such bipolar piezoelectric filters.

Devices using such piezoelectric filters, particularly portable communication devices, are being miniaturized, and further miniaturization of piezoelectric filters used in communication devices and the like is also being sought.

When a piezoelectric filter is miniaturized, leakage current is easily generated due to electromagnetic coupling between the input electrode and the output electrode of the piezoelectric filter element housed in the package, resulting in a significant reduction in the stopband attenuation (guaranteed attenuation).

To this end, conventionally, a shield electrode opposed to the input electrode and the output electrode is formed on the inner surface of the package to perform electromagnetic shielding, thereby preventing a reduction in stopband attenuation (for example, refer to Patent Document 1).

Patent Document 1: Japanese Patent No. 3239769

In the prior art such as the above-mentioned Patent Document 1, although it is possible to prevent the reduction of the stopband attenuation, if the miniaturization of the piezoelectric filter is promoted, the background level (background noise level) of the stopband attenuation characteristics will be tilted, so sometimes a frequency band that cannot meet the required stopband attenuation characteristics will be generated.

In Patent Document 1, no particular consideration is given to the inclination of the background level of the stopband attenuation characteristic.

Summary of the invention

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a piezoelectric filter having a flat background level of stop-band attenuation characteristics and high stop-band attenuation characteristics.

In order to achieve the above-mentioned object, the present invention is constructed as follows.

That is, the piezoelectric filter of the present invention is formed by cascading two piezoelectric filter elements, each of which has an input electrode and an output electrode formed on one main surface of a piezoelectric substrate and a common electrode formed on the other main surface, and the two piezoelectric filter elements are housed in a package, wherein:

A shield electrode is formed on the inner surface of the package, the shield electrode facing at least the input electrode and the output electrode on the one main surface of each piezoelectric substrate of each piezoelectric filter element, and a facing gap between the shield electrode and the two electrodes is greater than 100 μm.

According to the present invention, since the shielding electrode is opposed to at least the input electrode and the output electrode, the electromagnetic coupling between the input electrode and the output electrode can be suppressed to increase the stopband attenuation. Moreover, since the opposing gap between the shielding electrode and the two electrodes is set to be greater than 100 μm, the above-mentioned opposing gap is too narrow and the background level tilt of the stopband attenuation characteristics can be suppressed to achieve flattening.

Moreover, since the input electrodes, output electrodes and common electrodes are formed on separate piezoelectric substrates to constitute the piezoelectric filter elements respectively, they are acoustically separated from each other. Compared with the structure in which the input electrodes, output electrodes and common electrodes of two piezoelectric filter elements are formed on a common piezoelectric substrate, it is possible to suppress stray electricity and improve filter characteristics.

Preferably, the opposing gap is smaller than 360 μm.

According to the above configuration, since the facing gap between the shield electrode and the input electrode and the output electrode of each piezoelectric substrate is set to less than 360 μm, electromagnetic coupling between the input electrode and the output electrode can be effectively suppressed.

Preferably, each of the piezoelectric substrates of each of the piezoelectric filter elements has a rectangular shape in a plan view, and each of the piezoelectric substrates is held by the package at three corners of the rectangle.

According to the above structure, since each piezoelectric substrate having a rectangular shape in plan view is held in the package at the three corners of the rectangle, the piezoelectric substrate does not tilt and can be held stably. Thus, the opposing gap between the shielding electrode on the inner surface of the package and the input electrode and the output electrode of the piezoelectric substrate can be stably maintained.

Preferably, the input electrode, the output electrode, and the common electrode of each piezoelectric substrate of each piezoelectric filter element are individually connected to external connection terminals independently provided in the package body.

According to the above structure, since each electrode of each piezoelectric substrate is individually connected to an independent external connection terminal of the package, that is, each electrode of each piezoelectric substrate is individually connected to each external connection terminal in the package, it is possible to suppress the mutual influence of each electrode, thereby increasing the stopband attenuation.

It may also be that: the package body has a rectangular storage recess in a top view for storing the two piezoelectric filter elements, and input electrode pads and output electrode pads are respectively formed at opposite corners of the storage recess, and the input electrode pads and the output electrode pads are respectively connected to the external connection terminals for input of the piezoelectric filter and the external connection terminals for output of the piezoelectric filter among the external connection terminals, and the input electrode of the piezoelectric substrate of one piezoelectric filter element and the output electrode of the piezoelectric substrate of the other piezoelectric filter element are respectively led out to the opposite corners and respectively connected to the input electrode pads and the output electrode pads.

According to the above structure, since the input electrode of the piezoelectric substrate of one of the two piezoelectric filter elements accommodated in the accommodation recess of the package body and the output electrode of the piezoelectric substrate of the other piezoelectric filter element are respectively led to the input electrode pad and the output electrode pad at the opposite corners of the accommodation recess which is rectangular when viewed from above, and connected to the input external connection terminal and the output external connection terminal, the input external connection terminal and the output external connection terminal of the piezoelectric filter can be arranged at the farthest opposite corners. As a result, the mutual influence can be suppressed and the stopband attenuation can be improved.

Preferably, the two piezoelectric filter elements are juxtaposed in the housing recess of the package so that the output electrode of the piezoelectric substrate of the one piezoelectric filter element is close to the input electrode of the piezoelectric substrate of the other piezoelectric filter element.

According to the above configuration, the piezoelectric filter elements can be isolated from each other to suppress mutual influence, and the piezoelectric filter elements can be arranged close to each other to achieve miniaturization.

Preferably, the input electrodes and the output electrodes of the two piezoelectric filter elements are respectively formed on the one main surface of each piezoelectric substrate with a gap between them, and the shielding electrode is continuously formed on the inner surface of the package in a manner opposite to the two electrodes on the one main surface and opposite to the area with the gap between them, and the inner surface of the package on which the shielding electrode is continuously formed is flat.

According to the above configuration, since the shield electrode facing the region with a gap between the input electrode and the output electrode is continuously formed on the flat inner surface of the package, the structure is simplified compared with a configuration in which a protrusion is formed in the region facing the gap.

Preferably, the piezoelectric substrate is a quartz substrate.

According to the above structure, the frequency-temperature characteristic is improved.

As described above, according to the present invention, since the shielding electrode is opposed to at least the input electrode and the output electrode, the electromagnetic coupling between the input electrode and the output electrode can be suppressed and the stopband attenuation can be increased, and since the opposing gap between the shielding electrode and the two electrodes is set to be greater than 100 μm, the opposing gap is too narrow and the background level tilt of the stopband attenuation characteristics can be suppressed and flattened.

Moreover, since the input electrodes, output electrodes and common electrodes are formed on separate piezoelectric substrates to constitute the piezoelectric filter elements respectively, they are acoustically separated from each other. Compared with the structure in which the input electrodes, output electrodes and common electrodes of two piezoelectric filter elements are formed on a common piezoelectric substrate, it is possible to suppress stray electricity and improve filter characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a piezoelectric filter according to one embodiment of the present invention.

FIG. 2 is a plan view of the piezoelectric filter of FIG. 1 with a cover removed.

FIG. 3 is a top view of a base of the piezoelectric filter of FIG. 1 .

FIG. 4 is a schematic diagram showing the structure of the piezoelectric filter of FIG. 1 .

FIG. 5 is a plan view corresponding to FIG. 2 of a piezoelectric filter according to a comparative example.

FIG. 6 is a diagram showing filter characteristics of the piezoelectric filter of FIG. 5 .

FIG. 7 is a longitudinal sectional view corresponding to FIG. 1 of a piezoelectric filter according to another comparative example.

FIG. 8 is a diagram showing filter characteristics of the piezoelectric filter of FIG. 7 .

FIG. 9 is a diagram showing filter characteristics of a piezoelectric filter according to another comparative example.

FIG. 10 is a diagram showing filter characteristics of the piezoelectric filter of the example according to the embodiment of FIG. 1 .

FIG. 11 is a diagram showing filter characteristics of a piezoelectric filter according to another embodiment.

Detailed ways

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Figure 1 is a longitudinal sectional view of a piezoelectric filter according to an embodiment of the present invention, Figure 2 is a top view of the piezoelectric filter of Figure 1 with a cover removed, Figure 3 is a top view of a base of the piezoelectric filter of Figure 1, and Figure 4 is a schematic diagram showing the structure of the piezoelectric filter of Figure 1.

The piezoelectric filter 1 of this embodiment includes: a base 2 made of ceramic; two piezoelectric filter elements, namely a first piezoelectric filter element 3 and a second piezoelectric filter element 4, which are accommodated in the base 2; and a cover 5 for airtightly sealing the base 2. The cover 5 is made of a metal plate such as Kovar. The base 2 and the cover 5 constitute a package that accommodates the two piezoelectric filter elements 3 and 4.

The base 2 of this embodiment is composed of a ceramic multilayer substrate with alumina or the like as a base material. The base 2 has a three-layer structure as follows: a flat lower layer 2a, which is roughly rectangular when viewed from above; a middle layer 2b, which constitutes a step portion formed on the upper part of the lower layer 2a; and an upper layer 2c, which constitutes a frame portion formed on the middle layer 2b. In the base 2, a rectangular storage recess 6 is formed by these three layers 2a to 2c when viewed from above, and the storage recess 6 has a step portion for storing and holding the first piezoelectric filter element 3 and the second piezoelectric filter element 4.

The piezoelectric filter 1 of this embodiment is a small piezoelectric filter. The size of the base 2 is, for example, 6.0 mm×3.5 mm, and the height of the base 2 and the cover 5 combined is, for example, 1.0 mm.

The first piezoelectric filter element 3 and the second piezoelectric filter element 4 include, respectively, a first piezoelectric substrate 7 and a second piezoelectric substrate 8 which are made of, for example, AT-cut quartz plates and are rectangular in plan view.

On the upper surface of the middle layer 2b of the base 2, i.e., the step portion of the receiving recess 6 of the base 2, there are formed a first input pad 9, a first output pad 10, and a first ground pad 11 for electromechanically holding and mounting the first piezoelectric substrate 7. Similarly, there are formed a second input pad 12, a second output pad 13, and a second ground pad 14 for electromechanically holding and mounting the second piezoelectric substrate 8.

The three corners of the rectangle of the first piezoelectric substrate 7, which is rectangular when viewed from above, are held by the first input pad 9, the first output pad 10, and the first ground pad 11 of the base 2 via the conductive adhesive 15. Similarly, the three corners of the rectangle of the second piezoelectric substrate 8 are held by the second input pad 12, the second output pad 13, and the second ground pad 14 via the conductive adhesive 15.

Since the piezoelectric substrates 7 and 8 of the rectangular shape are held at the three corners of the rectangle in this way, the piezoelectric substrates 7 and 8 do not tilt as when they are held at two corners, for example, but can be stably held in a horizontal state. As a result, the opposing gap G between the first shielding electrode 22 and the second shielding electrode 23, which are formed on the surface of the lower layer 2a of the base 2 constituting the inner bottom surface of the package and are described later, and the input electrodes 71 and 81 and the output electrodes 72 and 82, which are described later, of the piezoelectric substrates 7 and 8 can be stably maintained within a desired range. In addition, compared with the case where the piezoelectric substrates 7 and 8 are held at all four corners, the influence of the stress applied from the base 2 can be reduced.

Among the pads 9 to 14 , the first input pad 9 as an input electrode pad and the second output pad 13 as an output electrode pad are formed at obliquely opposite corners of the rectangular housing recess 6 in plan view, that is, at the most distant positions.

Rectangular input electrodes 71 and output electrodes 72 are formed in parallel with each other at intervals on one main surface, that is, the back surface (lower surface) of the first piezoelectric substrate 7 of the first piezoelectric filter element 3. These electrodes 71 and 72 are led out to the opposite corners of the first piezoelectric substrate 7, and are electrically connected to the first input pads 9 and the first output pads 10 respectively through the conductive adhesive 15 applied under the primer. Since the input electrode 71 and the output electrode 72 of the first piezoelectric substrate 7 are led out to the opposite corners of the rectangular first piezoelectric substrate 7, that is, the most distant positions of the rectangle, the mutual influence between the two electrodes 71 and 72 can be suppressed, thereby improving the stopband attenuation.

The input electrode 71 of the first piezoelectric substrate 7 is led out and electrically connected to the first input pad 9 at one of the opposing corners of the accommodation recess 6 having a rectangular shape in plan view through the conductive adhesive 15 .

A rectangular grounding electrode, i.e., a common electrode 73, is formed on the other main surface, i.e., the surface (upper surface), of the first piezoelectric substrate 7, corresponding to the input electrode 71 and the output electrode 72. The common electrode 73 is led out to the other corner of the first piezoelectric substrate 7, and is electrically connected to the first grounding pad 11 through the conductive adhesive 15 applied under the primer and the re-applied primer.

In the second piezoelectric substrate 8 of the second piezoelectric filter element 4, a rectangular input electrode 81 and an output electrode 82 are formed on the back surface, similarly to the first piezoelectric substrate 7. These electrodes 81 and 82 are led out to the opposite corners of the second piezoelectric substrate 8, and are electrically connected to the second input pad 12 and the second output pad 13 through the conductive adhesive 15 applied under the primer. Since the input electrode 81 and the output electrode 82 of the second piezoelectric substrate 8 are led out to the opposite corners of the rectangular second piezoelectric substrate 8, that is, the most distant positions of the rectangle, the mutual influence between the two electrodes 81 and 82 can be suppressed, thereby improving the stopband attenuation.

The output electrode 82 of the second piezoelectric substrate 8 is led out and electrically connected to the second output pad 13 at the other of the opposing corners of the rectangular accommodation recess 6 in plan view through the conductive adhesive 15 .

A common electrode 83, which is a rectangular grounding electrode corresponding to the input electrode 81 and the output electrode 82, is formed on the surface of the second piezoelectric substrate 8. The common electrode 83 is led out to another corner of the second piezoelectric substrate 8 and is electrically connected to the second grounding pad 14 through the conductive adhesive 15 applied under the primer and re-applied.

In this way, the two piezoelectric substrates 7 and 8 are arranged in isolation in the base 2 in such a way that the output electrode 72 of the first piezoelectric substrate 7 and the input electrode 81 of the second piezoelectric substrate 8 are close to each other, thereby achieving miniaturization while suppressing the mutual influence of the piezoelectric filter elements 3 and 4.

The two opposite corners of the first piezoelectric substrate 7, which is rectangular in plan view, are held on the first input pad 9 and the first output pad 10 by the conductive adhesive 15 applied under the primer. The corner between the two corners of the first piezoelectric substrate 7 is held on the first ground pad 11 by the conductive adhesive 15 applied under the primer and reapplied under the primer. In this way, the two opposite corners of the first piezoelectric substrate 7 are respectively held on the first input pad 9 and the first output pad 10 by the conductive adhesive 15 applied under the primer, and the first ground pad 11 between the two corners is held by the conductive adhesive 15 applied under the primer in addition to the conductive adhesive 15 applied under the primer, so that the first piezoelectric substrate 7 can be effectively prevented from tilting.

Similarly, the two opposing corners of the second piezoelectric substrate 8 are held on the second input pad 12 and the second output pad 13 by the primer-coated conductive adhesive 15. Since the corner between the two corners of the second piezoelectric substrate 8 is held on the second ground pad 14 by the primer-coated and re-coated conductive adhesive 15, the second piezoelectric substrate 8 can be effectively prevented from tilting.

Six external connection terminals for surface mounting, namely, the first to sixth external connection terminals 16 to 21, are independently formed on the outer bottom surface of the base 2 of the piezoelectric filter 1 as shown in FIG. 4. That is, the first to sixth external connection terminals 16 to 21 of the base 2 are separate terminals. These external connection terminals 16 to 21 correspond to the pads 9 to 14 of the storage recess 6 of the base 2, and are electrically connected to the corresponding pads 9 to 14 via wiring (wiring pattern and through hole, etc.) formed inside the base 2 (not shown). In addition, these external connection terminals 16 to 21 are formed below the corresponding pads 9 to 14 of the storage recess 6 of the base 2.

Since the pads 9 to 14 of the storage recess 6 of the base 2 are respectively connected to the electrodes 71 to 73 of the first piezoelectric substrate 7 and the electrodes 81 to 83 of the second piezoelectric substrate 8 as described above, the first to sixth external connection terminals 16 to 21 are respectively connected to the electrodes 71 to 73 of the first piezoelectric substrate 7 and the corresponding electrodes 81 to 83 of the second piezoelectric substrate 8 via their respective corresponding pads 9 to 14.

Specifically, as shown in Figure 4, the input electrode 71 of the first piezoelectric substrate 7 and the first external connection terminal 16 are connected via the first input pad 9 (not shown), the common electrode 73 of the first piezoelectric substrate 7 and the second external connection terminal 17 are connected via the first ground pad 11 (not shown), and the output electrode 72 of the first piezoelectric substrate 7 and the third external connection terminal 18 are connected via the first output pad 10 (not shown).

In addition, the output electrode 82 of the second piezoelectric substrate 8 and the fourth external connection terminal 19 are connected via the second output pad 13 (not shown), the common electrode 83 of the second piezoelectric substrate 8 and the fifth external connection terminal 20 are connected via the second ground pad 14 (not shown), and the input electrode 81 of the second piezoelectric substrate 8 and the sixth external connection terminal 21 are connected via the second input pad 12 (not shown).

As described above, the input electrodes 71, 81, the output electrodes 72, 82, and the common electrodes 73, 83 of each piezoelectric substrate 7, 8 are individually connected to the independent first external connection electrode 16, the sixth external connection electrode 21, the third external connection terminal 18, the fourth external connection terminal 19, the second external connection terminal 17, and the fifth external connection terminal 20 of the base 2. That is, since the electrodes 71 to 73, 81 to 83 of each piezoelectric substrate 7, 8 are individually connected to the independent external connection terminals 16 to 21, the electrodes 71 to 73, 81 to 83 of each piezoelectric substrate 7, 8 can be suppressed from influencing each other, thereby improving the stopband attenuation.

Furthermore, one piezoelectric filter element 3 (or 4) can be adjusted independently without being affected by another piezoelectric filter element 4 (or 3).

The first external connection terminal 16 connected to the input electrode 71 of the first piezoelectric substrate 7 is an external connection terminal for the input of the piezoelectric filter 1, that is, the input terminal, and the fourth external connection terminal 19 connected to the output electrode 82 of the second piezoelectric substrate 8 is an external connection terminal for the output of the piezoelectric filter 1, that is, the output terminal.

The first external connection terminal 16 for input is formed on the outer bottom surface below the first input pad 9 at one of the opposite corners of the storage recess 6 which is rectangular when viewed from above, and the fourth external connection terminal 19 for output is formed on the outer bottom surface below the second output pad 13 at the other of the opposite corners of the storage recess 6.

The second external connection terminal 17 connected to the common electrode 73 of the first piezoelectric substrate 7 and the fifth external connection terminal 20 connected to the common electrode 83 of the second piezoelectric substrate 8 are grounded as GND terminals.

The third external connection terminal 18 connected to the output electrode 72 of the first piezoelectric filter element 3 and the sixth external connection terminal 21 connected to the input electrode 81 of the second piezoelectric filter element 4 are commonly connected to the coupling capacitor C1 .

Thus, the output electrode 72 of the first piezoelectric filter element 3 and the input electrode 81 of the second piezoelectric filter element 4 are connected to each other, thereby forming a quadrupole piezoelectric filter 1 in which two bipolar piezoelectric filter elements, namely the first piezoelectric filter element 3 and the second piezoelectric filter element 4, are cascade-connected.

As described above, the input electrode 71 of the first piezoelectric substrate 7 is led out to one of the opposite corners of the storage recess 6 which is rectangular in plan view, connected to the first input pad 9 via the conductive adhesive 15, and electrically connected to the input external connection terminal 16 on the outer bottom surface below the first input pad 9. In addition, the output electrode 82 of the second piezoelectric substrate 8 is led out to the other of the opposite corners of the storage recess 6, connected to the second output pad 13 via the conductive adhesive 15, and connected to the output external connection terminal 19 on the outer bottom surface below the second output pad 13.

In this way, since the first input pad 9 and external connection terminal 16 for input and the second output pad 13 and external connection terminal 19 for output of the piezoelectric filter 1 are respectively arranged at the farthest positions of the rectangular storage recess 6 of the base 3 when viewed from above, that is, the opposite corners, it is possible to suppress mutual influence and increase the stopband attenuation.

Generally speaking, at low frequencies below about 60 MHz, there are many spurious signals and their levels are also high. In this embodiment, the aspect ratio of the rectangular input electrode 71 and the output electrode 72 of the first piezoelectric filter element 3 is made different from the aspect ratio of the rectangular input electrode 81 and the output electrode 82 of the second piezoelectric filter element 4. As shown in FIG. 2, since the input electrode 71 and the output electrode 72 are set to a shape close to a rectangle and the input electrode 81 and the output electrode 82 are set to a shape close to a square, spurious signals can be suppressed.

In addition, as another embodiment of the present invention, the electrode shapes of the two piezoelectric filter elements 3 and 4 may be the same.

In this embodiment, the first shielding electrode 22 and the second shielding electrode 23 corresponding to the first piezoelectric substrate 7 and the second piezoelectric substrate 8 are formed on the flat surface of the flat plate-shaped lower layer 2 a of the base 2 constituting the inner bottom surface of the package.

Each of the shield electrodes 22 and 23 has a gold-plated layer formed on the upper surface of a metallization layer made of tungsten or the like.

The shield electrodes 22 and 23 are commonly connected to the GND terminal, ie, the second external connection terminal 17 and the fifth external connection terminal 20 , via wiring (wiring pattern, via hole, etc.) not shown formed inside the base 2 , and are grounded.

The shield electrodes 22 and 23 are opposed to each other with a facing gap G therebetween at least at the input electrodes 71 and 81 and the output electrodes 72 and 82 on the back surfaces of the piezoelectric substrates 7 and 8 .

That is, the first shielding electrode 22 is formed to overlap at least the input electrode 71 and the output electrode 72 of the first piezoelectric substrate 7 in a plan view, and the second shielding electrode 23 is formed to overlap at least the input electrode 81 and the output electrode 82 of the second piezoelectric substrate 8 in a plan view.

The first shielding electrode 22 is continuously formed so as to oppose not only the two electrodes 71 and 72 of the first piezoelectric substrate 7 but also the region between the input electrode 71 and the output electrode 72. The second shielding electrode 23 is also continuously formed so as to oppose not only the two electrodes 81 and 82 of the second piezoelectric substrate 8 but also the region between the input electrode 81 and the output electrode 82.

Therefore, the area of the region where the first shielding electrode 22 is formed is larger than the total area obtained by adding the area of the region where the input electrode 71 is formed and the area of the region where the output electrode 72 is formed of the first piezoelectric substrate 7. Similarly, the area of the region where the second shielding electrode 23 is formed is larger than the total area obtained by adding the area of the region where the input electrode 81 is formed and the area where the output electrode 82 is formed of the second piezoelectric substrate 8.

The area of the region where the first shielding electrode 22 is formed is preferably within, for example, about 4 times the area of the region where the input electrode 71 and the output electrode 72 are formed on the first piezoelectric substrate 7. Similarly, the area of the region where the second shielding electrode 23 is formed is preferably within, for example, about 4 times the area of the region where the input electrode 81 and the output electrode 82 are formed on the second piezoelectric substrate 8.

In this way, since the shield electrodes 22 and 23 are opposed to at least the input electrodes 71 and 81 and the output electrodes 72 and 82 of the piezoelectric substrates 7 and 8, respectively, electromagnetic coupling between the input electrodes 71 and 81 and the output electrodes 72 and 82 can be prevented, thereby improving the stopband attenuation.

Fig. 5 is a plan view corresponding to Fig. 2 of a piezoelectric filter ₁₁ of a comparative example in which the area where the shielding electrodes 22 ₁ and 23 ₁ are formed is small. In the piezoelectric filter ₁₁ of the comparative example, the first shielding electrode 22 ₁ faces the output electrode 72 of the first piezoelectric substrate 7 but does not face the input electrode 71. In addition, the second shielding electrode 23 ₁ faces the input electrode 81 of the second piezoelectric substrate 8 but does not face the output electrode 82.

The piezoelectric filter ₁₁ of the comparative example has a shield electrode formed in a half area compared to the piezoelectric filter 1 of the embodiment shown in Fig. 2. The piezoelectric filter ₁₁ is similar to the piezoelectric filter 1 of the embodiment shown in Fig. 2 except for the formation area of the shield electrodes 22 ₁ and 23 ₁ , and the opposing gap G between each shield electrode 22 and 23 and each input electrode 71 and 81 and each output electrode 72 and 82 of each piezoelectric substrate 7 and 8 is 250 μm.

FIG6 is a graph showing the filter characteristics of the piezoelectric filter ₁₁ of the comparative example, wherein the horizontal axis represents the frequency (kHz) and the vertical axis represents the stopband attenuation (dB), and the center frequency _f0 is 73.35 MHz.

As shown in FIG. 6 , in the frequency range from the center frequency f ₀ (73.35 MHz) to 4500 kHz, the required stopband attenuation, for example, 90 dB or more, cannot be satisfied.

As described above, in the piezoelectric filter ₂₁ of FIG. 5 in which each shielding electrode 22 ₁ , 23 ₁ is opposed to only one electrode 72 , 81 of each piezoelectric substrate 7 , 8 , each shielding electrode 22 ₁ , 23 ₁ cannot sufficiently suppress electromagnetic coupling generated between the input electrodes 71 , 81 and the output electrodes 72 , 82 in the region where the shielding electrodes 22 1 , 23 1 are not opposed to the input electrodes 71 , 81 and the output electrodes 72 , 82 of the piezoelectric substrates 7 , 8 , resulting in a decrease in the stopband attenuation characteristics.

In the piezoelectric filter 1 of the present embodiment, as described above, the shield electrodes 22 and 23 face at least the input electrodes 71 and 81 and the output electrodes 72 and 82 of the piezoelectric substrates 7 and 8 .

Furthermore, in the piezoelectric filter 1 of the present embodiment, the gap G between the shield electrodes 22 and 23 and the input electrodes 71 and 81 and the output electrodes 72 and 82 of the piezoelectric substrates 7 and 8 is set to be 100 μm or more.

If the facing gap G is smaller than 100 μm, the floor level (background noise level) of the stopband attenuation characteristic will be inclined, and a frequency band that cannot satisfy the required stopband attenuation characteristic may be generated.

In order to effectively suppress electromagnetic coupling between the input electrodes 71, 81 and the output electrodes 72, 82 of the piezoelectric substrates 7, 8, the facing gap G is preferably less than 360 μm. That is, the facing gap G is 100 μm or more and less than 360 μm, preferably 200 μm or more and 300 μm or less.

7 is a longitudinal sectional view corresponding to FIG. 1 of a piezoelectric filter ₁₂ of a comparative example in which the base ₂₂ has a two-layer structure and the facing gap G is set to be less than 100 μm, specifically, to be 40 μm.

Although the shielding electrodes 22, 23, the input electrodes 71, 81, and the output electrodes 72, 82 are not shown in FIG7, the piezoelectric filter ₁₂ has a two-layer structure in order to narrow the opposing gap G. In addition, the shielding electrodes 22, ₂₃ are opposed to the input electrodes 71, 81 and the output electrodes 72, 82 of the piezoelectric substrates 7, 8, as in the embodiment shown in FIG1.

FIG8 is a graph showing filter characteristics of the piezoelectric filter ₁₂ of the comparative example, wherein the horizontal axis represents frequency (kHz) and the vertical axis represents stopband attenuation (dB), and the center frequency _f0 is 73.35 MHz.

In the piezoelectric filter ₁₂ of the comparative example, since the shielding electrodes 22, 23 are respectively opposed to the input electrodes 71, 81 and the output electrodes 72, 82 of the piezoelectric substrates 7, 8, the electromagnetic coupling between the input electrodes 71, 81 and the output electrodes 72, 82 can be suppressed. As shown in FIG8, within the frequency range from the center frequency _f0 (73.35MHz) to -4500kHz, the required stopband attenuation, for example, about 90dB or more, can be met.

However, since the opposing gap G between each shield electrode 22, 23 and each input electrode 71, 81 and each output electrode 72, 82 of each piezoelectric substrate 7, 8 is made close to 40 μm, which is less than 100 μm, the opposing gap G is too narrow, and the background level of the stopband attenuation characteristic is inclined as shown by the dotted line L1 as shown in Fig. 8. Since the inclination angle is unevenly distributed, a frequency band that cannot satisfy the required stopband attenuation is generated.

On the contrary, if the opposing gap G between each shielding electrode 22, 23 and each input electrode 71, 81 and each output electrode 72, 82 of each piezoelectric substrate 7, 8 is too wide, the electromagnetic coupling generated between the input electrodes 71, 81 and the output electrodes 72, 82 of each piezoelectric substrate 7, 8 cannot be effectively suppressed.

Fig. 9 is a diagram showing the filter characteristics of a piezoelectric filter of a comparative example in which the opposing gap G is set to 360 μm. The piezoelectric filter of the comparative example showing the filter characteristics of Fig. 9 is similar to the embodiment shown in Fig. 1 except that the opposing gap G is widened to 360 μm, and each shielding electrode 22, 23 is opposed to each input electrode 71, 81 and each output electrode 72, 82 of each piezoelectric substrate 7, 8.

As shown in FIG. 9 , in the piezoelectric filter of the comparative example in which the facing gap G is set to 360 μm, the required stopband attenuation, for example, 90 dB or more, cannot be satisfied in the frequency range from the center frequency f ₀ (73.35 MHz) to 4500 kHz.

In contrast, in the present embodiment, as shown in FIG. 2 , each shield electrode 22 , 23 faces at least each input electrode 71 , 81 and each output electrode 72 , 82 of each piezoelectric substrate 7 , 8 , respectively, and the facing gap G is set to be greater than 100 μm and less than 360 μm.

FIG. 10 is a diagram showing filter characteristics of the piezoelectric filter 1 of the embodiment in which the facing gap G is set to 250 μm.

In the piezoelectric filter 1 of the present embodiment, since the shield electrodes 22 and 23 are opposed to at least the input electrodes 71 and 81 and the output electrodes 72 and 82 of the piezoelectric substrates 7 and 8, respectively, and the opposing gaps G between the shield electrodes 22 and 23 and the input electrodes 71 and 81 and the output electrodes 72 and 82 are set to appropriate opposing gaps so as not to be too close, the electromagnetic coupling between the input electrodes 71 and 81 and the output electrodes 72 and 82 can be sufficiently suppressed, and as shown in the area surrounded by the dotted line in FIG _{. 10} , the required stopband attenuation can be satisfied, for example, about 90 dB or more, and can even be improved to about 100 dB, in the frequency range of ±4500 kHz from the center frequency f 0 (73.35 MHz). Furthermore, the background level of the stopband attenuation characteristic does not incline, but exhibits a flat characteristic.

FIG. 11 is a diagram showing filter characteristics of a piezoelectric filter 1 according to another embodiment in which the facing gap G is set to 220 μm.

The piezoelectric filter of this embodiment can also satisfy the required stopband attenuation, for example, about 90 dB or more, in the frequency range from the center frequency f ₀ (73.35 MHz) to 4500 kHz, and the background level of the stopband attenuation characteristic does not tilt but exhibits a flat characteristic.

In the above embodiment, a quartz substrate is used as the piezoelectric substrate, but a ceramic substrate or a polycrystalline substrate having piezoelectric properties may also be used.

Symbol Description

1, _{1 1} , 1 ₂ Piezoelectric filter

2, 2 ₁ , 2 ₂ base

3. First piezoelectric filter element

4 Second piezoelectric filter element

6 Storage recess

7. First piezoelectric substrate

8 Second piezoelectric substrate

22, 22 ₁ first shielding electrode

23, 23 ₁ Second shielding electrode

71, 81 Input electrode

72, 82 output electrodes

Claims (8)

1. A piezoelectric filter, comprising two piezoelectric filter elements connected in cascade, each piezoelectric filter element having an input electrode and an output electrode formed on one main surface of a piezoelectric substrate and a common electrode formed on the other main surface, the two piezoelectric filter elements being housed in a package, wherein: A shield electrode is formed on the inner surface of the package, and the shield electrode faces at least two electrodes, the input electrode and the output electrode, on the one main surface of each piezoelectric substrate of each piezoelectric filter element. The opposing gap between the shielding electrode and the two electrodes is greater than 100 μm. The aspect ratio of the rectangular input electrode and the output electrode of one of the two piezoelectric filter elements is different from the aspect ratio of the rectangular input electrode and the output electrode of the other piezoelectric filter element. The common electrode of the two piezoelectric filter elements has a rectangular shape. 2. The piezoelectric filter according to claim 1, wherein The opposing gap is smaller than 360 μm. 3. The piezoelectric filter according to claim 1 or 2, wherein: The piezoelectric substrates of the piezoelectric filter elements are rectangular in plan view. Each of the piezoelectric substrates is held by the package at three corners of the rectangle. 4. The piezoelectric filter according to claim 3, wherein: The input electrode, the output electrode, and the common electrode of each piezoelectric substrate of each piezoelectric filter element are individually connected to external connection terminals independently provided in the package body. 5. The piezoelectric filter according to claim 4, wherein The package has a rectangular housing recess for housing the two piezoelectric filter elements in a plan view. An input electrode pad and an output electrode pad are formed at opposite corners of the storage recess, and the input electrode pad and the output electrode pad are connected to an input external connection terminal of the piezoelectric filter and an output external connection terminal of the piezoelectric filter among the external connection terminals, respectively. The input electrode of the piezoelectric substrate of one piezoelectric filter element and the output electrode of the piezoelectric substrate of the other piezoelectric filter element are respectively led out to the opposing corners and connected to the input electrode pad and the output electrode pad, respectively. 6. The piezoelectric filter according to claim 5, wherein The two piezoelectric filter elements are juxtaposed in the housing recess of the package so that the output electrode of the piezoelectric substrate of the one piezoelectric filter element and the input electrode of the piezoelectric substrate of the other piezoelectric filter element are close to each other. 7. The piezoelectric filter according to claim 6, wherein The input electrodes and the output electrodes of the two piezoelectric filter elements are formed on the one main surface of each piezoelectric substrate at a distance from each other. The shield electrode is continuously formed on the inner surface of the package so as to face the two electrodes on the one main surface and face the region separated by the gap. The inner surface of the package body on which the shield electrode is continuously formed is flat. 8. The piezoelectric filter according to claim 7, wherein The piezoelectric substrate is a quartz substrate.