JP3408350B2 - 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 piezoelectric filter mainly used in radio equipment, and more particularly to a ladder type piezoelectric filter used as an intermediate frequency filter in which a plurality of piezoelectric vibrators are combined in series and parallel.

[0002]

2. Description of the Related Art Generally, as an intermediate frequency filter used for a wireless device or the like, a piezoelectric filter utilizing a spreading vibration of a disk-type or square-plate type piezoelectric vibrator is widely used. That is, these piezoelectric vibrators are configured in a ladder type, the size of the series element and the size of the parallel element are adjusted, the resonance frequency of the series element and the antiresonance frequency of the parallel element are matched, and the pass band is formed. It is known that steep filter characteristics can be realized by forming attenuation poles on both sides of the filter. At this time, the matching impedance of the filter is the matching impedance at the center frequency of the filter. In recent years, along with digitalization in the field of mobile communication, not only attenuation characteristics but also group delay time characteristics are becoming more important. Here, the group delay time is obtained by differentiating the phase with respect to the frequency. By the way, since the digital signal is modulated and demodulated due to the phase deviation, accurate modulation and demodulation cannot be performed unless the phase rotation changes linearly. Therefore, in order to perform accurate modulation / demodulation, the group delay time characteristic is used as an index representing the deviation from the linearity of the phase rotation by the time difference.

An example of a conventional piezoelectric filter will be described below with reference to the drawings.

FIG. 15 shows the structure of a conventional four-element ladder-type piezoelectric ceramic filter utilizing spread vibration of a rectangular plate, and FIG. 16 shows its equivalent circuit. In FIG. 15, reference numeral 21 is a rectangular plate type piezoelectric vibrator for series elements, and 22 is a rectangular plate type piezoelectric vibrator for parallel elements. Further, 23 is an input electrode plate, 24 is an output electrode plate, 25 is a ground electrode plate, and 27 is a wiring electrode plate. Embossing is applied to a portion for electrically connecting the electrode plate and the vibrator. 26 are provided. The rectangular plate type piezoelectric vibrators 21 and 22 are supported by an embossing 26 at the center of the plate surface corresponding to the node of mechanical vibration.
It is electrically configured as a ladder type circuit via 6. Two
Reference numeral 8 is a package case, and 29 is a leaf spring for applying pressure for supporting the vibrator. Reference numeral 30 denotes a sealing plate that is bonded to the package after inserting the vibrator, the electrode plate, and the leaf spring into the package.

A conventional method for flattening the group delay characteristic in the required band of the piezoelectric ceramic filter having the above basic structure will be described with reference to the drawings.

FIG. 17 shows a filter characteristic of a conventional piezoelectric filter, and FIG. 18 is a filter characteristic diagram for explaining a conventional design method for flattening a group delay characteristic within a required band. is there. The solid line in FIG. 17 shows the attenuation characteristic, and the broken line shows the group delay characteristic. In FIG.
The attenuation characteristic outside the pass band is extremely gradual, so that the selection characteristic in the portion where the amount of attenuation is large is insufficient. This is because in order to flatten the group delay characteristic in the required bandwidth, the bandwidth is designed to be large by using a vibration mode having a large electro-mechanical coupling coefficient or a material having a large coupling coefficient, and the group delay characteristic becomes flat in advance. This is because the frequency range is set large and the mechanical quality Qm is lowered by the damping resistance and the conductive rubber, so that the bandwidth is narrowed to the required bandwidth while keeping a large area where the group delay characteristic is flat. . Therefore, the insertion loss is also large.

The above design method will be described in detail with reference to FIG.

In FIG. 18, broken line 1 represents the attenuation characteristic in the first design, and broken line 2 represents the group delay characteristic at that time. Here, the initial design bandwidth is designed to be larger than the required bandwidth so that the part where the group delay time becomes large is located on both sides outside the required bandwidth as viewed from the center frequency. Become. In this state, the group delay characteristic within the required bandwidth becomes almost flat. By decreasing the Qm of the vibrator from this state, the attenuation characteristic of the solid line 1 is realized and the required bandwidth is obtained. At this time, the group delay characteristic becomes as shown by the solid line 2, the peak values on both sides of the group delay time become small, and the flat portion of the group delay characteristic is realized without narrowing. As a method of lowering the Qm of the vibrator, as shown in FIG. 19, a conductive rubber 31 is inserted between the electrode plate and the piezoelectric vibrator (for example, actual flat plate 04-114229), or as shown in FIG. There is a method of connecting a damping resistor R32 to each vibrator (for example, actual opening 04-121124) or a method of directly using a piezoelectric material having a small Qm.

As another design method, the Qm values of the piezoelectric vibrators in the straight and parallel branches of the input / output terminals forming the filter are made 1.5 times or more than the Qm values of the piezoelectric vibrators in the middle part. Setting (for example, Japanese Patent Laid-Open No. 01-314008), changing the difference Δf between the resonance frequency and the anti-resonance frequency of each piezoelectric vibrator,
You can change the way they are arranged (for example, Actual Kaihei 01-091328
Etc.) or changing the capacitance ratio of the vibrator (see, for example,
54-163649) etc., the attenuation characteristics and group delay characteristics have been improved by devising materials and processing processes.

[0010]

However, in the conventional method as described above, even if the group delay characteristic is improved by designing a large bandwidth of the filter, a design method for ensuring the symmetry of the filter characteristic is not available. There is a problem as a design method because it has not been established and selection characteristics outside the required band cannot be obtained. In addition, the number of parts is increased by using conductive rubber, damping resistance, etc. to obtain flat group delay characteristics.
There was a problem that assembly was difficult. Furthermore, in order to change the Qm of the vibrator and the frequency difference Δf between the resonance frequency and the anti-resonance frequency, it is necessary to use multiple piezoelectric materials, partial electrodes, or piezoelectric vibrators with unsaturated polarization. There are also problems that the number of points is increased, the process is complicated, and stable piezoelectric characteristics cannot be obtained. Further, in either case, there is a problem that the insertion loss of the filter increases due to the reduction of Qm (addition of resistance). Furthermore, in the conventional configuration, only adding the resistance lowers the peak value of the group delay ripple deviation,
A configuration in which the group delay characteristic is symmetrical with respect to the center frequency has not been studied.

In consideration of such problems of the conventional piezoelectric filter, the present invention requires a small number of parts, simplifies the steps of assembly and adjustment, reduces the insertion loss of the filter, and is symmetrical with respect to the center frequency. An object of the present invention is to provide a ladder type piezoelectric filter having good group delay characteristics and attenuation characteristics.

[0012]

[Means for Solving the Problems ] The above-mentioned problems are solved.
For this reason, the first aspect of the present invention provides an L-shaped structure using a piezoelectric vibrator.
Less than each including connected series and parallel elements
A ladder with more than two cascade-connected units
A child-type piezoelectric filter, comprising:
A singular point whose phase is a frequency of π / 2 rotation is defined as the ladder.
Symmetric on the frequency axis with respect to the center frequency of the piezoelectric filter
And each of the units is arranged so that they are evenly spaced.
Bandwidth of knit or matching impedance of each unit
This is a ladder-type piezoelectric filter designed for dance. Also,
The invention of No. 2 relates to the circuit of the ladder-type piezoelectric filter.
And the bands of the two units in electrically symmetrical positions
Bandwidth is practically equal, and input stage unit and output
Bandwidth of both middle stage units
A ladder according to the first aspect of the present invention that is narrower or wider than the bandwidth
Type piezoelectric filter. The third aspect of the present invention is the ladder
Electrically symmetrical position in the sub-piezoelectric filter circuit
The product of the matching impedances of the two said units in
2 units of the input stage and the output stage and 2 of the intermediate stage
The first or second unit is set to be substantially equal.
Alternatively, it is the ladder-type piezoelectric filter according to the second aspect of the present invention. Also,
A fourth aspect of the present invention provides a matching impeder for the input stage unit.
And the matching impedance of the output stage unit
Is substantially equal to the matching impedance of the entire circuit
The ladder-type piezoelectric filter according to the third aspect of the present invention. Also,
The present invention according to claim 5 provides at least one matching step of the intermediate stage.
Impedance and the matching impedance of the entire circuit
The third or fourth ladder-type piezoelectric filter according to the present invention is different.
It In the sixth aspect of the present invention, the number of units is an even number.
The ladder-type piezoelectric fill according to any one of the first to fifth aspects of the present invention
It is Further, in the 7th aspect of the present invention, the number of the units is
Any one of the first to fifth ladder-type piezoelectric elements according to the present invention
It is a filter.

[0013]

[0014]

According to the present invention, by making the bandwidth of the input stage unit and the bandwidth of the output stage unit substantially the same among a plurality of stages of units and making the bandwidth larger than the bandwidth of the intermediate stage unit, for example, The unit has a four-stage configuration, the maximum bandwidths of both the input stage unit and the output stage unit, and the bandwidths of the two units in the intermediate stage are both smaller than the bandwidth of the input stage unit, thereby reducing the design bandwidth. It is possible to control the group delay characteristic with almost no change.

Further, according to the present invention, the value of the matching impedance of the output stage unit is a value obtained by dividing the value obtained by squaring the matching impedance of the input side or the matching impedance of the output side by the matching impedance of the input stage unit, and the entire unit. By making the product of the matching impedances of the two units at symmetrical positions in the intermediate unit relative to the product of the matching impedance of the output stage unit and the matching impedance of the input stage unit substantially equal to that of the ladder type piezoelectric filter. A singular point whose phase is rotated by π / 2 can be arranged at a frequency point symmetrical with respect to the center frequency. Since the group delay characteristic is obtained by differentiating the phase with respect to the frequency, if the singular point is symmetrical, the attenuation characteristic and the group delay characteristic which are symmetrical with respect to the center frequency of the filter are realized.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing its embodiments. (Embodiment 1) FIG. 1 is a configuration diagram of a ladder type piezoelectric filter according to a first embodiment of the present invention. In the present embodiment and the following embodiments, a ladder type piezoelectric filter having a 4-unit structure as shown in FIG. 1 will be described as an example. Figure 1
In each unit, the series element 1 and the parallel element 2 are L
The L-shaped elements are connected to the mold, and the input stage unit 3, the intermediate stage unit 5, the intermediate stage unit 5, and the output stage unit 4 are connected in this order from the input side to the output side.
Here, for the sake of explanation, each unit is referred to as the first, second, third, and fourth units from the input side.

Df1 and df described below the equivalent circuit of this embodiment
Reference numerals 2, df3, and df4 represent design bandwidths of the respective units from the first unit to the fourth unit, which are L-type elements. Further, R1, R2, R3, and R4 represent matching impedances of each unit. Here, the design bandwidth of the entire filter is df0, and the matching impedance of the filter is R0.
However, df0 is the design bandwidth of each unit df1, df2, df3, df4
Is the average value of. In addition, k is the matching impedance of each unit, which is standardized by the matching impedance R0 of the filter.
1, k2, k3, k4 (that is, the matching impedance of the entire filter is 1). Further, the mechanical qualities Qm of the piezoelectric vibrators constituting each unit are respectively Q1, Q2, Q3 and Q4.

The operation of the ladder-type piezoelectric filter having the design parameters set as described above will be described below with reference to FIGS. The design parameters and the number of constituent units in the present embodiment are merely examples embodying the present invention, and the values of the design parameters and the number of constituent units do not limit the technical scope of the present invention.

FIG. 2 is a filter characteristic diagram of the ladder-type piezoelectric filter of this embodiment. In FIG. 2, the solid line in the figure shows the attenuation characteristic, and the broken line shows the group delay characteristic. Further, FIG. 3 shows a filter characteristic when the present embodiment is not applied, which is a filter characteristic in a comparative example for comparing the filter characteristics in the present embodiment, and the filter has a 4-unit (8-element) configuration. Here, FIGS. 2 and 3B are enlarged views of the pass band portion of the filter characteristic diagram of FIG.

The design parameters in the comparative example of FIG.
Using the symbols in FIG. 1, it is assumed that a material having df1 = df2 = df3 = df4 = 24kHz, k1 = k2 = k3 = k4 = 1, Q1 = Q2 = Q3 = Q4 = 150, and a coupling coefficient of 0.35 is used. That is, this is the case where the bandwidths of all the units are the same. On the other hand, FIG.
(B) maximizes the design bandwidth of the input stage unit 3 and the design bandwidth of the output stage unit 4, and gradually reduces the design bandwidth of each unit from the input stage unit 3 to the intermediate stage unit 5, This is an example of a ladder type piezoelectric filter designed to gradually increase the design bandwidth of each unit from the intermediate unit 5 to the output unit 4, and the design parameters are df1 = df4 = 28kHz, df2 = df3 = 20kHz, This is the case when k1 = k2 = k3 = k4 = 1. Here, comparing FIG. 2B and FIG. 3B, about ± 10 kHz from the center frequency of 450 kHz.
It can be seen that the shapes of the group delay characteristics at the portions separated by z are different.

This phenomenon will be described in detail with reference to FIG. FIG. 4 is an explanatory diagram showing changes in the singularity of phase rotation depending on the bandwidth configuration. Here, the singular point has a phase of π
The point is one-half rotation, the horizontal axis shows the frequency, and the vertical axis shows the constituent bandwidth of each unit. The group delay characteristic is flattened by linearly changing the change of the phase rotation, but the positions of the singular points must be equidistant for the linear change of the phase rotation. FIG. 4 shows that the position of the singular point changes by changing the constituent bandwidth between the units.

For example, in the case where the bandwidths are all constant (24-24-24-24) as in the configuration of FIG. 3B, the center frequency and the center frequency of 450 kHz extend outward to the first singular point. The frequency interval is larger than the interval from the first singular point to the second singular point. Therefore, between the 1st and 2nd singular points, it has shown that the phase rotates more rapidly than between the 1st singular points from a center frequency. Therefore, the group delay deviation becomes large between the first and second singular points. Similarly, this phenomenon also applies between the second and third singular points.

On the other hand, when the frequency configuration is as shown in FIG.
-20-20-28), the interval from the center frequency to the first singular point and the interval from the first to the second singular point are compared to the same bandwidth configuration (24-24-24-24). More evenly spaced. Therefore, the group delay deviation is smaller than that in FIG. That is, the position of the singular point can be moved symmetrically with respect to the center frequency without changing the design bandwidth depending on the constituent bandwidth of the unit.

As described above, according to this embodiment, the group delay characteristic can be controlled without changing the design bandwidth. Further, unlike the conventional method of configuring with a vibrator having different frequency difference Δf between resonance frequency and anti-resonance frequency and mechanical quality factor Qm, the filter characteristics are improved by changing the bandwidth of each unit. In addition, a filter with low insertion loss can be realized. Further, the piezoelectric materials can be made of the same material, and the manufacturing process can be simplified. (Embodiment 2) FIG. 5 is a filter characteristic diagram of a ladder type piezoelectric filter according to a second embodiment of the present invention. Figure 5
The solid line in the middle shows the attenuation characteristic, and the broken line shows the group delay characteristic.
In FIGS. 5A and 5B, the input side matching impedance and the output side matching impedance of the entire circuit are made substantially equal, the matching impedance of the input / output stage unit is made equal to the input / output matching impedance of the entire circuit, and 1 is an example of a ladder-type piezoelectric filter in which each unit is arranged such that a square value of a product of a matching impedance of a unit and a matching impedance of a third unit is substantially equal to a matching impedance of an input / output stage unit. The number of units is 4 units (8 elements), design parameters are as follows: df1 = df2 = df3 = df4 = 24kHz, k1 = k4 = 1, k2 = 2, k3 = 0.5, coupling coefficient 0.35. is there. The difference from the first embodiment is that the bandwidth of each unit is the same and the impedance configuration of each unit is changed. Comparing FIG. 3 (b) and FIG. 5 (b), both have symmetric filter characteristics with respect to the center frequency, but the flat part of the group delay characteristic in the vicinity of the center frequency is the same as in the configuration of FIG. 5 (b). Is slightly larger.
Further, FIG. 5B has a small ripple in the portion away from the center frequency, and therefore the group delay characteristic including the ripple portion can be wider than that in FIG. 3B. This phenomenon will be described with reference to FIG.

FIG. 7 is an explanatory diagram showing changes in the singular point of the phase rotation due to the impedance configuration between the units. The horizontal axis represents frequency, and the vertical axis represents impedance configuration between units.

The impedance configuration (1-2-0.
In 5-1), the second and the third are outward from the center frequency.
The interval of the singular point is smaller than that of the first and second intervals. That is, in the frequency between these, the inclination of the phase becomes large, and therefore the deviation of the group delay time becomes large.
Regarding the impedance configuration (1-1-1-1) in Fig. 3 (b), the second and third impedances for the interval between the first and second singular points
Since the interval between the second singular points is slightly larger than that in FIG. 5B, there is a small ripple in a portion apart from the center frequency by about ± 10 kHz.

FIG. 7 shows that the position of the singular point can be moved symmetrically with respect to the center frequency by changing the impedance configuration between the units under the conditions of this embodiment.

As described above, according to the present embodiment, the group delay characteristic can be controlled without impairing the symmetry of the filter characteristic. Further, unlike the conventional method of forming a vibrator having different Δf and Qm, the filter characteristics are improved by changing the matching impedance of the unit, so that the piezoelectric material can be made of the same material and the manufacturing process can be simplified. .

The design parameters and the number of constituent units in this embodiment are merely examples embodying the present invention.
The values of the design parameters and the number of constituent units do not limit the technical scope of the present invention. (Embodiment 3) FIGS. 6A and 6B are filter characteristic diagrams of a ladder-type piezoelectric filter according to a third embodiment of the present invention. Here, FIG. 6B is an enlarged view of the pass band portion of FIG. The solid line in FIG. 6 shows the attenuation characteristic, and the broken line shows the group delay characteristic. Also, the filter has a 4-unit (8-element) configuration, and the design parameters use the symbols in Fig. 1, df1 = df2 = df3 = df4 = 24kHz, k1 = k4 = 1, k2 = 0.5, k3 = 2, Q1 = Q2 = Q3 = Q4 = 150, and use a material with a coupling coefficient of 0.35. The difference from the second embodiment is that the second unit and the third unit are exchanged. That is, under the conditions of the second embodiment, the product of the matching impedance of the second unit and the matching impedance of the third unit having a larger value is almost the product of the matching impedance of the input stage unit and the matching impedance of the output stage unit. The point is that they are configured to be equal.

Comparing FIG. 5 (b) and FIG. 6 (b), it can be seen that the group delay characteristic of this embodiment has a small ripple in the portion closer to the center frequency. When this ripple is small enough to satisfy the specifications, the group delay characteristic including the ripple portion becomes flat in an extremely wide region. This phenomenon will be described with reference to FIG. 7 again.

As described above, FIG. 7 is an explanatory diagram showing changes in the singular point of the phase rotation due to the impedance configuration between the units.

The impedance configuration (1-2-0.
In 5-1), the second and the third are outward from the center frequency.
The interval between the second singular points is smaller. That is, at the frequency in this period, the phase rotation rapidly occurs, so that the deviation of the group delay becomes large. Impedance configuration (1-0.5-
For 2-1), since the interval between the first and second singular points is slightly smaller than the interval between the first singular point and the center frequency, ripples occur between frequencies corresponding to the interval between the first and second singular points. Occurs. Since the interval between the second and third singular points is larger than the interval between the first and second singular points, the phase rotation is gentle and the group delay time changes slowly during this period.

From FIG. 7, the product of the matching impedance of the second unit and the matching impedance of the third unit having a larger value is substantially equal to the product of the matching impedance of the input stage unit and the matching impedance of the output stage unit. With such a configuration, the distance between the singular points of the first and second phase rotations on the outer side from the center frequency is
Since it becomes smaller with respect to the interval between the singular points of the second and third phase rotations, ripples occur in the portion close to the center frequency. This ripple can be controlled by the Qm of the constituent materials,
The smaller the Qm of the material, the smaller the ripple. Further, with the above configuration, the positions of the singular points are located symmetrically with respect to the center frequency.

As described above, according to the present embodiment, the group delay characteristic can be controlled without impairing the symmetry of the filter characteristic. Furthermore, by designing the ripple of the group delay characteristic that occurs near the center frequency within a range that satisfies the specifications, a flat group delay characteristic can be obtained in a wider area than before.
Also, unlike the conventional design method that uses oscillators with different Δf and Qm, the filter characteristics are improved by changing the matching impedance of the unit, so the piezoelectric material can be made of the same material and the manufacturing process is simplified. Can be converted.

The design parameters and the number of constituent units in this embodiment are merely examples embodying the present invention.
The values of the design parameters and the number of constituent units do not limit the technical scope of the present invention. (Embodiment 4) FIGS. 8A and 8B are filter characteristic diagrams of a ladder-type piezoelectric filter according to a fourth embodiment of the present invention. Here, FIG. 8B is an enlarged view of the pass band portion of FIG. The solid line in FIG. 8 shows the attenuation characteristic, and the broken line shows the group delay characteristic. Also, the filter has a 4-unit (8-element) configuration, and the design parameters at this time use the symbols in Fig. 1, df1 = df2 = df3 = df4 = 24kHz, k1 = 0.667, k2 = k3 = 1, k4 = 1.5. , Q1 = Q2 = Q3 = Q4 = 150, and a material with a coupling coefficient of 0.35 is used. The difference from the first, second, and third embodiments is that the matching impedances of the input stage unit and the output stage unit have different values. That is, the matching impedance of the input side and the matching impedance of the output side are made substantially equal to each other, and this value is set as the matching impedance of the entire circuit, and the product of the matching impedances of the two units at symmetrical positions in the intermediate unit with respect to the whole unit is It is an example of a ladder-type piezoelectric filter in which each unit is arranged so as to be equal to the product of the matching impedances of the input stage unit and the output stage unit.

In FIG. 8, the attenuation characteristic of the filter characteristic is a good characteristic with no ripple in the pass band. Regarding the group delay characteristic, there is almost no ripple in the band and a characteristic symmetrical with respect to the center frequency is obtained.

In the above structure, the second unit and the third unit
The operation when the matching impedance of the unit is changed will be described in detail with reference to the drawings.

FIG. 9 shows the second embodiment of the fourth embodiment.
The product of the matching impedance of the unit and the third unit is
The position of the singular point of phase rotation is shown when the matching impedance of the second unit and the third unit is changed under the condition that the product of the matching impedances of the input stage unit and the output stage unit is equal. Here, the horizontal axis represents frequency and the vertical axis represents the impedance ratio configuration of each unit.

From FIG. 9, when the impedance of the input stage unit is smaller than the impedance of the output stage unit, the impedance of the second unit is set to be large so that the second and third singular points are seen from the center frequency to the outside. Position approaches the center frequency. At this time, the position of the first singular point approaches the center frequency slightly, but does not approach the center frequency as much as it does. Therefore, the interval between the second and third singular points becomes small, and a ripple of group delay occurs in the frequency region corresponding to this. The ripple approaches the center frequency as the impedance of the second unit increases.

On the contrary, when the impedance of the second unit is small, the third singular point is far from the center frequency, the interval between the second and third singular points is large, and the first and second singular points are large. Approach the same interval as. Therefore, the phase rotation changes linearly and the group delay characteristic becomes flat.

The characteristics obtained by these impedance configurations are shown in FIGS. 10 (a) and 10 (b) and FIG. 11 (a).
It shows in (b). Here, the solid line in FIGS. 10 and 11 indicates the attenuation characteristic, and the broken line indicates the group delay characteristic. Further, in each figure, (b) is an enlarged view of the pass band portion of (a). The design parameters of FIG. 10 (a) use the symbols of FIG. 1, df1 = df2 = df3 = df4 = 24kHz, k1 = 0.667, k2 = 2, k3 = 0.5, k4 = 1.5, Q1 = Q2 = Q3 = Q4 = 150, a material with a coupling coefficient of 0.35 is used, and in FIG. 11A, df1 = df2 = df3 = df4 = 24kHz, k1 = 0.667, k2 = 0.5, k3 = 2, k4 = 1.5, Q1 = Q2 = Q3 = Q4 = 150, using a material with a coupling coefficient of 0.35.

As described above, according to the present embodiment, the position of the singular point of phase rotation can be controlled without impairing the symmetry of the filter characteristics. Further, regarding the pass band, it is possible to realize good attenuation characteristics with a smaller ripple than when the impedances of the input / output units are made equal. Also, unlike the conventional design method that uses oscillators with different Δf and Qm, the filter characteristics are improved by changing the matching impedance of the unit, so the piezoelectric material can be made of the same material and the manufacturing process is simplified. Can be converted.

The design parameters and the number of constituent units in this embodiment are merely examples embodying the present invention.
The values of the design parameters and the number of constituent units do not limit the technical scope of the present invention. (Embodiment 5) FIGS. 12 (a), 12 (b) and 13 (a),
FIG. 6B is a filter characteristic diagram of the ladder type piezoelectric filter according to the fifth embodiment of the present invention. 12 and 13
The solid line in the middle shows the attenuation characteristic, and the broken line shows the group delay characteristic.
Here, in each drawing, (b) is an enlarged view of the pass band portion of (a). Also, the filter is 4 units (8 elements)
The design parameters at this time are represented by the symbols in FIG. 1, and in FIG. 12A, df1 = df4 = 28kHz, df2 = df3 = 20kHz, k1 = k4 = 1, k2 = 1.414, k3 = 0.707, Q1 = Q2 = Q3 = Q4 = 150, and for FIG. 13 (a), df1 = df4 = 28kHz, df2 = df3 = 20kHz, k1 = k4 = 1, k2 = 0.707, k3 = 1.414, Q1 = Q2 = Q3 = Q4 = 150, and both use materials with a coupling coefficient of 0.35. The difference from the other embodiments is that the bandwidth and impedance of the constituent units are changed at the same time. That is, it is a ladder-type piezoelectric filter designed by the image parameter method, in which a plurality of basic unit units in which a series element and a parallel element are connected in an L-shape are connected, and the design bandwidth of the input stage unit and the design band of the output stage unit are Designed to maximize the width, gradually reduce the design bandwidth of each unit from the input stage unit to the intermediate stage unit, and gradually increase the design bandwidth of each unit from the intermediate stage unit to the output stage unit. , And the matching impedance of the input side and the matching impedance of the output side of the entire circuit are almost equal, and the value obtained by squaring this value is divided by the matching impedance of the input stage unit to obtain the matching impedance of the output stage unit. Of the matching impedance of each two units in the symmetrical position in the middle unit to the unit There is an example of the ladder type piezoelectric filter placing the units to be equal to the product of the matching impedance of the input stage unit and output stage unit.

The ladder type piezoelectric filter constructed as described above will be described with reference to FIG.

FIG. 14 shows the phase when the impedance configuration of each unit is changed when the bandwidths of the constituent units are set to df1 = df4 = 28kHz and df2 = df3 = 20kHz in the ladder type piezoelectric filter of this embodiment. The position of the singular point of rotation is shown. Here, the horizontal axis represents frequency and the vertical axis represents the impedance ratio configuration of each unit.

Comparing FIG. 14 with the case shown in FIG. 7 of the third embodiment (where the bandwidths of each unit are all the same), it can be seen that the position of the singular point with respect to the impedance is significantly different. For example, focusing on the third singular point outward from the center frequency, the impedance of the second unit becomes the third
When the impedance is larger than that of the unit, the singular point is located at a frequency apart from the center frequency in the present embodiment, as compared with the same bandwidth configuration. For this reason, the interval between the second and third singular points becomes large and approaches the interval between the first and second singular points, and the group delay deviation between these frequencies becomes small (see FIG. 12 (b)).

On the contrary, the impedance of the second unit is the third
If it is smaller than the impedance of the unit, the first,
The second and third singular points are all located on the center frequency side,
Moreover, since the distance between the first singular point and the second singular point is smaller than the distance between the center frequency and the first singular point,
The group delay characteristic causes ripples closer to the center frequency (see FIG. 13B).

That is, the group delay characteristic can be improved more effectively by simultaneously operating the frequency configuration and the impedance configuration of each unit.

As described above, according to this embodiment, the group delay characteristic can be operated more effectively without impairing the symmetry of the filter characteristic. Also, unlike the conventional design method that uses oscillators with different Δf and Qm, the filter characteristics are improved by changing the unit configuration bandwidth and impedance. Therefore, the piezoelectric material can be made of the same material. The process can be simplified.

The design parameters and the number of constituent units in this embodiment are merely examples embodying the present invention.
The values of the design parameters and the number of constituent units do not limit the technical scope of the present invention.

As described above, according to the present invention, the singular point at which the phase of the ladder type piezoelectric filter rotates by π / 2 can be arranged at the frequency point symmetrical with respect to the center frequency. Since the group delay characteristic is obtained by differentiating the phase with respect to the frequency, if the singular point is symmetrical, the attenuation characteristic and the group delay characteristic which are symmetrical with respect to the center frequency of the filter are realized. Also,
By combining the bandwidth configuration between the units and the impedance configuration with the above configuration, it is possible to keep the position of the singular point symmetrical with respect to the center frequency and make the interval constant within the band. To have a flat group delay characteristic,
It suffices if the phase rotation changes linearly, and in this case, the singular points are equidistant in frequency. Therefore, it is possible to realize a group delay characteristic that is symmetrical and flat with respect to the center frequency within the band.

Accordingly, since the group delay characteristics of the filter can be controlled by using the same piezoelectric material without using conductive rubber or damping resistance, the number of parts can be reduced, the manufacturing can be facilitated, and the resonance frequency and the anti-resonance frequency can be reduced. In order to improve the group delay characteristics, it is possible to realize good group delay characteristics and attenuation characteristics that are symmetrical with respect to the center frequency without requiring complicated steps such as changing the difference Δf of Since it is not necessary to reduce the mechanical quality Qm of using a damping resistance or a conductive rubber, it is possible to realize a characteristic with a small insertion loss.

In each of the above embodiments, the number of units to be connected is four, but it goes without saying that the number of units is not limited to this. Further, the number of units may be an odd number.

In the first and fifth embodiments described above, only the case where the bandwidth of the input / output stage unit is larger than the bandwidth of the intermediate stage unit will be explained, and the case where it is smaller is not particularly explained. As is clear from the above (refer to the lower part of (22-26-26-22) for the bandwidth configuration shown in FIG. 4), there is almost the same effect as the first embodiment, and the fifth embodiment
The same applies to the bandwidth configuration of each unit in the embodiment.

[0055]

As is apparent from the above description, the present invention requires a small number of parts, simplifies the steps of assembly and adjustment, reduces the insertion loss of the filter, and is symmetric with respect to the center frequency. It has an advantage that group delay characteristics and attenuation characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a ladder-type piezoelectric filter according to a first embodiment of the present invention.

FIGS. 2A and 2B are filter characteristic diagrams of the ladder-type piezoelectric filter of the first embodiment.

3A and 3B are filter characteristic diagrams of a filter of a comparative example for comparison with the ladder type piezoelectric filter of the first embodiment.

FIG. 4 is an explanatory diagram showing a change in a singular point of phase rotation in the ladder-type piezoelectric filter of the first embodiment.

5A and 5B are filter characteristic diagrams of a ladder-type piezoelectric filter according to a second embodiment of the present invention.

6A and 6B are filter characteristic diagrams of a ladder-type piezoelectric filter according to a third embodiment of the present invention.

FIG. 7 is an explanatory diagram showing changes in a singular point of phase rotation in the ladder-type piezoelectric filters of the second and third embodiments.

FIGS. 8A and 8B are filter characteristic diagrams of a ladder-type piezoelectric filter according to a fourth embodiment of the present invention.

FIG. 9 is an explanatory diagram showing changes in singular points of phase rotation in the ladder-type piezoelectric filter according to the fourth embodiment.

FIGS. 10A and 10B are filter characteristic diagrams of an example of a ladder-type piezoelectric filter according to the fourth embodiment.

FIGS. 11A and 11B are filter characteristic diagrams of another example of a ladder-type piezoelectric filter in the fourth embodiment.

12 (a) and 12 (b) are views showing a fifth embodiment of the present invention.
FIG. 6 is a filter characteristic diagram of an example of a ladder-type piezoelectric filter in the example of FIG.

13A and 13B are filter characteristic diagrams of another example of a ladder-type piezoelectric filter in the fifth embodiment.

FIG. 14 is an explanatory diagram showing a change in singular point of phase rotation in the ladder-type piezoelectric filter of the fifth embodiment.

FIG. 15 is a structural diagram of a conventional basic ladder type piezoelectric filter.

FIG. 16 is an equivalent circuit diagram of a conventional ladder-type piezoelectric filter.

FIG. 17 is a filter characteristic diagram of a conventional ladder-type piezoelectric filter.

FIG. 18 is a filter characteristic diagram for explaining a conventional method for designing a ladder-type piezoelectric filter.

FIG. 19 is a structural diagram showing an example of a conventional ladder-type piezoelectric filter.

FIG. 20 is a structural diagram showing another example of a conventional ladder-type piezoelectric filter.

[Explanation of symbols]

1 series element 2 parallel elements 3 input stage unit 4 output stage unit 5 Intermediate unit 21 Square plate type piezoelectric vibrator (for series element) 22 Square plate type piezoelectric vibrator (for parallel element) 23 Input electrode plate 24 Output electrode plate 25 Grounding electrode plate 31 Conductive sheet 32 damping resistance

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Kawasaki 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Tomoki Ueno 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. In-house (56) Reference JP-A-5-22073 (JP, A) JP-A-55-127720 (JP, A) JP-A-54-163649 (JP, A) JP-A-53-105156 (JP, A) ) JP-A-62-261208 (JP, A) J. Kohei 5-13061 (JP, Y2) (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/00-9/215 H03H 9 / 54-9/60

Claims (7)

(57) [Claims] 1. An L-shaped connection using a piezoelectric vibrator
At least two, each containing a series element and a parallel element
Ladder piezos with units connected in more cascade
It ’s a filter, Frequency at which the phase of the ladder-type piezoelectric filter rotates by π / 2
Is the center frequency of the ladder-type piezoelectric filter.
Are arranged symmetrically on the frequency axis with respect to and at substantially equal intervals
So that each unit's bandwidth or each
Ladder Piezoelectric Designing Matching Impedance of Unit
filter. 2. A circuit of the ladder type piezoelectric filter.
And the bands of the two units in electrically symmetrical positions
Bandwidth is practically equal, and input stage unit and output
Bandwidth of both middle stage units
The ladder according to claim 1, wherein the ladder is narrower or wider than the width.
Child type piezoelectric filter. 3. The circuit of the ladder-type piezoelectric filter
To align the two said units in electrically symmetrical positions.
The product of the combined impedances is
The unit and the two units in the middle stage are substantially equal
The ladder-type piezoelectric flap according to claim 1 or 2, wherein
Ilta. 4. The matching impedance of the input stage unit
And the matching impedance of the output stage unit,
A contract that is substantially equal to the matching impedance of the entire circuit.
The ladder-type piezoelectric filter according to claim 3. 5. At least one matching stage of the intermediate stage.
Impedance and the matching impedance of the entire circuit
The ladder-type piezoelectric filter according to claim 3 or 4, which is different. 6. The number of units is an even number.
The ladder-type piezoelectric filter according to any one of 1 to 5. 7. The number of units is an odd number.
The ladder-type piezoelectric filter according to any one of 1 to 5.