JPH07154200A - Resonator surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To secure a sufficient attenuation value for an attenuation band without incurring increase in a loss of a pass band, increase in chip size and deterioration in the yield. CONSTITUTION:The resonator surface acoustic wave filter is formed by a 1st stage ladder circuit comprising surface acoustic wave resonators 24, 25 and a surface acoustic wave resonator 26 in cascade connection to the ladder circuit as a series element. The resonance frequency of the resonator 24 and the antiresonance frequency of the resonator 25 are set the same. Furthermore, the resonance frequency and the antiresonance frequency of the resonator 26 are set higher than the resonance frequency and the antiresonance frequency of the resonator 24. Thus, the attenuation characteristics are obtained, in which the attenuation value in the attenuation band on the high frequency side of the band pass characteristics obtained by the ladder circuit is emphasized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonator type surface acoustic wave filter used as a filter of a high frequency part of a radio communication device.

[0002]

2. Description of the Related Art FIG. 2 is a plan view showing the structure of a conventional resonator type surface acoustic wave filter (hereinafter referred to as "filter").

In the illustrated filter, a ladder-type circuit is formed by alternately connecting a plurality of surface acoustic wave resonators (hereinafter referred to as "resonators") in series and in parallel. . In addition, in the drawing, the case where a single-stage ladder circuit is formed is shown as a representative.

In the drawing, 11 is an input terminal to which a signal to be filtered is supplied, and 12 is an output terminal to which a filtered signal is supplied. Thirteen
Is a piezoelectric substrate, and 14 and 15 are piezoelectric substrates 13
It is a resonator formed above.

The resonator 14 is located on both sides of the interdigital electrode 141 which excites the surface acoustic wave, and the reflectors 142 which reflect the surface acoustic wave excited by the interdigital electrode 141. 143. Similarly, the resonator 15 includes the interdigital transducer 151 and the reflector 15 as well.
2,153.

The input terminal 11 is connected to one end of the interdigital electrode 141, and the other end of the interdigital electrode 141 is connected to the output terminal 12 and grounded via the interdigital electrode 151.

As a result, a one-stage ladder circuit as shown in the equivalent circuit of FIG. 3 is constructed. Where Z11 is
Z12 is the impedance of the resonator 14 that forms the series element of the ladder circuit, and Z12 is the resonator 1 that also forms the parallel element.
5 impedance.

The equivalent circuit of each of the resonators 14 and 15 can be expressed approximately as shown in FIG. 4 in the vicinity of the resonance frequency, "Surface acoustic wave engineering, published by Institute of Electronics and Communication Engineers, Showa 58".
Year ”, page 192, where L0 is
It is an equivalent inductor, C0 is an equivalent capacitance, and Cd is a damping capacitance. In FIG. 4, the loss resistance r is omitted for the sake of simplicity.

The equivalent circuit of each of the resonators 14 and 15 is expressed as shown in FIG.
The frequency characteristics of 11 and Z12 (hereinafter referred to as "impedance characteristics") are shown in FIG. Where f
r is the resonance frequency and fa is the anti-resonance frequency. These are expressed by the following equations (1) and (2), respectively.

[0010]

[Equation 1]

[Equation 2] FIG. 6 is a characteristic diagram showing the relationship between the impedance characteristics of the resonators 14 and 15. As shown in the figure, the resonance frequency fr1 of the resonator 14 forming a series element and the resonator 15 forming a parallel element
The anti-resonance frequency fa2 of is set to the same value f0. Thereby, as the frequency characteristic of the attenuation amount of the filter (hereinafter, referred to as “attenuation characteristic”), the band pass characteristic as shown in FIG. 7 is obtained.

Here, f0 is the center frequency of the pass band and is represented by the resonance frequency fr1 of the resonator 14 and the anti-resonance frequency fa2 of the resonator 15. Further, f1 is the frequency of the pole in the attenuation band on the high frequency side, and is the anti-resonance frequency f of the resonator 14.
It is represented by a1. Further, f2 is the frequency of the pole in the attenuation band on the low frequency side, and is represented by the resonance frequency fr2 of the resonator 15.

Conventionally, the electromechanical coupling coefficient k ² of each of the resonators 14 and 15 is set to the same value. This electromechanical coupling coefficient k ² is expressed by the following equation (3).

[0013]

[Equation 3] From this equation, it is understood that when either the resonance frequency fr or the anti-resonance frequency fa is determined, the other is uniquely determined. As a result, in the attenuation characteristic of FIG.
For example, when the center frequency f0 is determined, the pole frequencies f1 and f
2 will also be decided uniquely.

Although a detailed description is omitted, even when a ladder circuit having two or more stages is constructed, the resonance frequency fr of all series elements and the anti-resonance frequency fa of all parallel elements are fa.
Are set to the same value. Further, the electromechanical coupling coefficient k ^{2 of} all the constituent elements (series element and parallel element) is set to the same value.

As a result, the center frequencies f0 of all stages are
And the pole frequencies f1 and f2 match.
As a result, it is possible to obtain the characteristic that the attenuation amount is in the attenuation region according to the number of stages of the ladder circuit.

[0016]

As described above, in the conventional filter, the electromechanical coupling coefficient k ^{2 of} all the resonators is set to the same value.

However, in such a structure, if a large amount of attenuation is required in the attenuation range, it is difficult to meet this requirement.

That is, when the above-described filter is used in a wireless communication device, various specifications are given. Therefore, for example, as shown by the diagonal lines in FIG. 7, a large attenuation amount may be required in the attenuation region on the high frequency side. Also,
As shown by the diagonal lines in FIG. 8, a large attenuation amount may be required in the attenuation region on the low frequency side.

In such a case, the conventional resonator type surface acoustic wave filter cannot cope with the specifications unless the number of stages of the ladder type circuit is increased.

However, when the number of stages of the ladder type circuit is increased, the number of resonators is increased and the loss in the pass band is increased. Moreover, the chip size of the filter becomes large and the yield also deteriorates. Therefore, when a large amount of attenuation is required in the attenuation range, it is difficult for the conventional filter to meet the demand.

Therefore, the present invention provides a filter capable of ensuring a sufficient amount of attenuation in the attenuation region without increasing the loss in the pass band, expanding the chip size, and degrading the yield. To aim.

[0022]

In order to achieve the above object, the invention according to claim 1 provides a ladder circuit in which the resonance frequency of a series element and the antiresonance frequency of a parallel element are set to the same value. The resonator forming the ladder circuit is a resonator in which the attenuation increasing resonators having different resonance frequencies and anti-resonance frequencies are connected in series.

According to a second aspect of the invention, in the first aspect of the invention, the attenuation increasing resonators are connected in series to a ladder circuit as a series element.

According to a third aspect of the invention, in the first aspect of the invention, the attenuation increasing resonators are connected in cascade to a ladder circuit as a parallel element.

Furthermore, the invention according to claim 4 is the invention according to claim 1.
In the invention according to (3), the number of stages of the ladder circuit is plural.

[0026]

In the invention according to claim 1, the attenuation characteristic of the filter is a combination of the attenuation characteristic of the ladder circuit and the attenuation characteristic of the resonator for increasing the attenuation amount.

Here, the damping characteristic of the ladder type circuit is such that the resonance frequency of the series element and the anti-resonance frequency of the parallel element of this circuit are set to the same value, and thus the basic characteristic of the filter is obtained. .

The damping characteristic of the resonator for increasing the damping amount is
Because this resonator is cascaded in a ladder circuit,
At the anti-resonance frequency or resonance frequency, the amount of attenuation is maximized.

That is, when this resonator is cascade-connected as a series element to a ladder-type circuit as in the invention according to claim 2, the damping characteristic of this resonator is that its anti-resonance frequency is equal to the amount of attenuation. Will be the largest.

On the other hand, when the resonator is cascade-connected as a parallel element to the ladder-type circuit as in the invention according to claim 3, the damping characteristic of the resonator is that its resonance frequency is such that the amount of attenuation is It will be the largest.

Therefore, in the invention according to claim 2, the anti-resonance frequency of the resonator for increasing the attenuation amount is set to the attenuation region on the high frequency side of the ladder circuit to increase the attenuation amount in this attenuation region. be able to.

On the other hand, according to the third aspect of the present invention, by setting the resonance frequency of the resonator in the attenuation region on the low frequency side of the ladder circuit, the attenuation amount in this attenuation region can be increased.

As a result, the amount of attenuation in the attenuation band can be increased without increasing the number of stages in the ladder circuit, so that there is no loss in the pass band, enlargement of the chip size, and deterioration of the yield. Can solve the problem.

In the invention according to claim 1, the number of stages of the ladder circuit may be one, or as in the invention according to claim 4, there may be a plurality of stages.

[0035]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a plan view showing the configuration of the first embodiment of the present invention.

In the figure, 21 is an input terminal to which a signal to be filtered is supplied, and 22 is an output terminal to which a filtered signal is supplied. 23
Is a piezoelectric substrate, and 24, 25 and 26 are resonators formed on the piezoelectric substrate 23.

The respective resonators 24, 25, 26 are respectively
Interdigital electrodes 241, 251, 2 which excite surface acoustic waves
61 and reflectors 242, 24 for reflecting this surface acoustic wave
3,252,253,262,263.

The input terminal 21 is connected to one end of the interdigital electrode 241, the other end of the interdigital electrode 241 is grounded via the interdigital electrode 251, and the output terminal 22 is connected via the interdigital electrode 261. It is connected to the.

As a result, as shown in the equivalent circuit of FIG. 9, a one-stage ladder circuit 27 including the resonators 24 and 25 is provided.
And a resonator 26 cascade-connected as a series element to the ladder circuit 27 is obtained. In addition,
In the figure, Z11, Z12, and Z21 are resonators 2, respectively.
The impedances of 4, 25 and 26 are shown.

The equivalent circuit of each resonator 24, 25, 26 is
As in the conventional case, it is represented as shown in FIG. As a result, the impedance characteristics of the resonators 24, 25 and 26 are also shown in FIG.
It is expressed as.

The resonance frequency fr1 of the resonator 24 forming the series element of the ladder circuit 27 and the resonator 25 forming the parallel element of the ladder circuit 27.
The anti-resonance frequency fa2 of is set to the same value f0. As a result, the damping characteristic of the ladder circuit 27 is
As shown in FIG. 10A, the attenuation characteristic is similar to that of the conventional filter. In addition, in FIG.
1 is the anti-resonance frequency of the resonator 24, and fr2 is the resonance frequency of the resonator 25.

The resonator 26 is connected in series to the ladder circuit 27 as a series element. As a result, the attenuation characteristic of the resonator 26 is such that the amount of attenuation becomes maximum at the anti-resonance frequency fa3, as shown in FIG. 10 (b).

Here, the resonance frequency fr3 and the anti-resonance frequency fa3 of the resonator 26 are set to values higher than the resonance frequency fr1 and the anti-resonance frequency fa1 of the resonator 24, respectively. Accordingly, the attenuation characteristic of the filter is shown in FIG.
As shown in (c), the attenuation characteristic of the ladder circuit has a characteristic in which one pole is added to the attenuation region on the high frequency side.

The frequency of this pole is represented by the anti-resonance frequency fa3 of the resonator 26. Therefore, when the anti-resonance frequency fa3, the attenuation amount at the frequency fa3, and the attenuation amount at the frequency fa3 or lower are appropriately set according to the specifications of the filter, a case where a large attenuation amount is requested is given. But you can work around this specification.

The amount of attenuation of the resonator 26 at the anti-resonance frequency fa3 and the amount of attenuation at the frequency fa3 and below are set, for example, by appropriately setting the length, pitch, and number of electrode fingers of the interdigital transducer 261 of the resonator 26. Therefore, it can be adjusted.

In this embodiment, the above adjustment is made by the number of electrode fingers. In this case, in this embodiment, as shown in FIG. 1, the number of interdigital electrodes 261 of the resonator 26 is larger than the number of interdigital electrodes 241 and 251 of the resonators 24 and 25.

As a result, the damping capacitance Cd of the resonator 26 becomes large, and the damping characteristic of the resonator 26 is such that the amount of damping increases rapidly from the resonance frequency fr3 to the anti-resonance frequency fa3. As a result, the amount of attenuation in the attenuation band on the high frequency side of the filter can be increased more than ever before.

In this embodiment, the ladder type circuit 27 is used.
The electromechanical coupling coefficient k of the resonators 24 and 25 constituting the
² is set to the same value. The electromechanical coupling coefficient k ^{2 of the} resonator 26 cascade-connected to the ladder circuit 27 is also set to the same value as the electromechanical coupling coefficient k ^{2 of the} resonators 24 and 25.

According to this embodiment described in detail above, the following effects can be obtained.

(1) First, a ladder circuit 27 is connected in series with a resonator 26 having a resonance frequency fr3 different from the resonators 24 and 25 and an antiresonance frequency fa3.
Since the attenuation amount is increased, the ladder circuit 27
The amount of attenuation can be increased without increasing the number of steps. As a result, a large amount of attenuation can be secured without increasing the loss in the pass band, increasing the chip size, and reducing the yield.

(2) Further, for the same reason as (1), the electromechanical coupling coefficient k ² of the resonator 26 is set to the resonators 24 and 25.
Can be set to the same value as the electromechanical coupling coefficient k ² . As a result, the number of manufacturing steps of the filter can be reduced as compared with the case of setting different electromechanical coupling coefficients k ² .

FIG. 11 is a plan view showing the structure of the second embodiment of the present invention. In FIG. 11, the same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the above embodiment, the resonator 2 for increasing the attenuation amount is used.
The case where 6 is connected in series to the ladder circuit 27 as a series element has been described. On the other hand, in this embodiment, the resonator for increasing the attenuation is connected in cascade to the ladder circuit as a parallel element.

That is, in FIG. 11, 31 is a resonator for increasing the attenuation amount of this embodiment. This resonator 31
Also, like the resonators 24 and 25, it is also composed of the interdigital transducer 311 and the reflectors 312 and 313.

One end of the interdigital transducer 311 has an output terminal 22.
, And the other end is grounded. As a result,
As shown in FIG. 2, an equivalent circuit is obtained in which the resonator 31 is cascade-connected as a parallel element to the ladder circuit 27.

Since the resonator 31 is cascade-connected as a parallel element to the ladder circuit 27, the damping characteristic of the resonator 31 has a resonance frequency fr as shown in FIG. 13 (b).
A characteristic of 3 is that the amount of attenuation becomes maximum.

Here, the resonance frequency fr3 and the anti-resonance frequency fa3 of the resonator 31 are set to values smaller than the resonance frequency fr1 and the anti-resonance frequency fa1 of the resonator 24, which is a parallel element of the ladder circuit 27, respectively. . As a result, as shown in FIG. 13C, as a damping characteristic of the filter, one pole is added to the lower damping region of the damping characteristic of the ladder circuit (see FIG. 13A). The characteristics are obtained. As a result, the amount of attenuation in this attenuation range can be increased more than ever before.

The frequency of the newly added pole is the resonator 3
It is represented by a resonance frequency fr3 of 1. Therefore, by appropriately setting the resonance frequency fr3, the attenuation amount at the frequency fr3, and the attenuation amount at the frequency fr3 or higher, it is possible to adjust the attenuation amount in the attenuation region on the high frequency side.

The amount of attenuation at the resonance frequency fr3 and the amount of attenuation at the frequency fr3 and above can be adjusted by appropriately setting the length, pitch, and number of the electrode fingers of the interdigital transducer 311 of the resonator 31.

In this embodiment, the number of interdigital electrodes 311 is set to be smaller than the number of interdigital electrodes 241 and 251. As a result, the damping capacitance Cd of the resonator 31 is reduced, and the amount of attenuation from the antiresonance frequency fa3 to the resonance frequency fr3 can be rapidly increased. As a result, the amount of attenuation in the attenuation region on the low frequency side of the filter can be greatly increased as compared with the conventional one.

[0062] Also in this embodiment, the electromechanical coupling coefficient k ² of the resonator 31 is set to the same value as the electromechanical coefficient k ² of the resonator 24.

Also in this embodiment described in detail above, the same effect as the previous embodiment can be obtained.

Although the two embodiments of the present invention have been described in detail above, the present invention is not limited to the above-mentioned embodiments.

(1) First, in the above embodiment, the case where the resonator 26 (or 31) for increasing the attenuation amount is provided on the output terminal 22 side has been described. However, in the present invention, this may be provided on the input terminal 21 side.

(2) Further, in the above embodiment, the case where only one resonator 26 (or 31) for increasing the attenuation amount is provided has been described. However, in the present invention, a plurality of them may be provided.

In this case, the plurality of resonators 26 (or 3
If the attenuation characteristics of 1) are set to the same characteristics, one pole with a large attenuation amount can be added. On the other hand, if different characteristics are set, a plurality of poles can be added although the respective attenuation amounts are small.

(3) In the above embodiment, the case where the resonator for increasing the attenuation is provided only as the series element (26) or as the parallel element (31) has been described. However, the present invention may be provided as a series element and a parallel element.

With this structure, it is possible to increase the attenuation amount in both the high frequency band and the low frequency band.

(4) In the above embodiment, the resonator 2
The case where the resonator composed of the interdigital electrode and the reflector is used as 4, 25 and 26 has been described. However, in the present invention, a resonator including only the interdigital transducer may be used.

(5) In addition, in the previous embodiment, as the ladder circuit 27, the electromechanical coupling coefficient k of the series element (24) is set.
^The case where the circuit in which the electromechanical coupling coefficient k ^{2 of 2} and the parallel element (25) are the same is used is described. However, in the present invention, circuits different from each other may be used.

(6) Further, in the above embodiment, the electromechanical coupling coefficient k of the resonator 26 (or 31) for increasing the attenuation amount.
^{The case where 2} and the electromechanical coupling coefficient k ^{2 of} the resonators 24 and 25 forming the ladder circuit 27 are set to the same value has been described. However, in the present invention, these may be set to different values.

(7) In the above embodiment, the ladder circuit 27 is a single-stage ladder circuit. However, the present invention may use a ladder circuit having two or more stages.

In this case, the electromechanical coupling coefficient k ² of the plurality of series elements (24) forming the ladder circuit 27 may be the same or different. Similarly, the electromechanical coupling coefficient k ² of the plurality of parallel elements (25) may be the same or different.

If they are different, a plurality of poles are set as the poles of the attenuation band on the high band side or the low band side. Therefore,
In this case, the pole by the resonator for increasing the attenuation is set between the plurality of poles.

(8) In addition to this, it goes without saying that the present invention can be variously modified and implemented without departing from the scope of the invention.

[0077]

As described above in detail, according to the present invention,
It is possible to provide a filter capable of ensuring a sufficient amount of attenuation in the attenuation region without increasing the loss in the pass band, increasing the chip size, and reducing the yield.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of a first embodiment of the present invention.

FIG. 2 is a plan view showing the configuration of a conventional filter.

FIG. 3 is a circuit diagram showing an equivalent circuit of a conventional filter.

FIG. 4 is a circuit diagram showing an equivalent circuit of a resonator.

FIG. 5 is a characteristic diagram showing an impedance characteristic of a resonator.

FIG. 6 is a characteristic diagram showing impedance characteristics of a ladder circuit.

FIG. 7 is a characteristic diagram for explaining attenuation characteristics and problems of a conventional filter.

FIG. 8 is a characteristic diagram for explaining a damping characteristic of a conventional filter and a problem.

FIG. 9 is a circuit diagram showing an equivalent circuit of the first embodiment.

FIG. 10 is a characteristic diagram showing the attenuation characteristic of the first embodiment.

FIG. 11 is a plan view showing the configuration of the second embodiment of the present invention.

FIG. 12 is a circuit diagram showing an equivalent circuit of the second embodiment.

FIG. 13 is a characteristic diagram showing the attenuation characteristic of the second embodiment.

[Explanation of symbols]

21 ... Input terminal 22 ... Output terminal 23 ... Piezoelectric substrate 24, 25, 26, 31 ... Resonator 27 ... Ladder circuit 241, 251, 261, 311 ... Interdigital electrode 242, 243, 252, 253, 262, 263, Three
12, 313 ... Reflector.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Woo Hook Hoa 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (4)

[Claims] 1. A resonator type surface acoustic wave filter comprising a plurality of surface acoustic wave resonators, wherein a series element and a parallel element are respectively constituted by a surface acoustic wave resonator, and the resonance frequency of the series element and the parallel element are parallel to each other. The ladder-type circuit in which the anti-resonance frequency of the element is set to the same value and the surface acoustic wave resonator that constitutes this ladder-type circuit have different resonance frequencies and anti-resonance frequencies, and are connected in cascade to the ladder-type circuit. And a surface acoustic wave resonator for increasing the amount of attenuation described above. 2. The surface acoustic wave resonator for increasing the attenuation,
The resonator type surface acoustic wave filter according to claim 1, wherein the ladder type circuit is connected in series as a series element. 3. The surface acoustic wave resonator for increasing the attenuation,
The resonator type surface acoustic wave filter according to claim 1, wherein the ladder type circuit is connected in cascade as a parallel element. 4. The resonator type surface acoustic wave filter according to claim 1, wherein the ladder circuit has a plurality of stages.