JP3274009B2 - Resonator type surface acoustic wave 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 resonator type surface acoustic wave filter used, for example, as a filter for a high frequency section of a radio communication device.

[0002]

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

In the illustrated filter, a plurality of surface acoustic wave resonators (hereinafter referred to as "resonators") are alternately cascaded in series and parallel to form a ladder circuit. . The figure shows a typical case where a two-stage ladder-type circuit is formed by four resonators.

In FIG. 2, reference numeral 11 denotes an input terminal to which a signal to be filtered is supplied, and reference numeral 12 denotes an output terminal to which a filtered signal is supplied. 13
Is a piezoelectric substrate, and 14, 15, 16, and 17 are resonators formed on the piezoelectric substrate 13.

The resonator 14 has interdigital electrodes 141 for exciting surface acoustic waves, and reflectors 142 located on both sides of the interdigital electrodes 141 for reflecting the surface acoustic waves excited by the interdigital electrodes 141. 143. Similarly, the resonators 15, 16, 17 also have IDTs 151, 161, 171 and reflectors 152, 153, respectively.
162, 163, 172, 173.

The input terminal 11 is connected to one end of an interdigital electrode 141, and the other end of the interdigital electrode 141 is grounded via the interdigital electrode 151 and connected to one end of an interdigital electrode 161. I have. The other end of the IDT 161 is connected to the output terminal 12,
It is grounded via an interdigital electrode 171.

As a result, a two-stage ladder circuit as shown in the equivalent circuit of FIG. 3 is formed. Here, Z11 is
Z12 indicates the impedance of the resonators 15 and 17 which form the parallel elements of the ladder type circuit, and Z12 indicates the impedance of the resonators 14 and 16 which form the serial elements of the ladder type circuit.

The equivalent circuit of each of the resonators 14, 15, 16 and 17 is approximately represented as shown in FIG. 4 in the vicinity of the resonance frequency. 1983, pp. 192. Here, L0 is an equivalent inductor, C0 is an equivalent capacitance, and Cd is a braking capacitance. In FIG. 4, the loss resistance r is omitted for the sake of simplicity.

The equivalent circuit of each of the resonators 14, 15, 16, 17 is represented as shown in FIG. 4 so that the frequency characteristics of these impedances (hereinafter referred to as "impedance characteristics")
That. ) Is represented as shown in FIG. Here, fr is a resonance frequency, and fa is an anti-resonance frequency. These are represented 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, 15, 16, and 17.

As shown in the figure, the resonance frequency fr1 of the resonator 14 forming the first series element of the ladder circuit and the anti-resonance frequency fa2 of the resonator 15 forming the parallel element are set to the same value f0. As a result, the frequency characteristic of the first-stage attenuation (hereinafter, referred to as “attenuation characteristic”) becomes a band-pass characteristic with f0 as the center frequency.

Similarly, a resonator 1 forming a second-stage series element
The anti-resonance frequency fa4 of the resonator 17 forming a parallel element with the resonance frequency fr3 of No. 6 is also set to f0. As a result, the second-stage attenuation characteristic also becomes a band-pass characteristic having f0 as the center frequency.

The anti-resonance frequencies fa1 and fa3 of the resonators 14 and 16 forming the series elements are also set to the same value f1. This is because the electromechanical coupling coefficient k of the resonators 14 and 16 is
^{This is because 2} is set to the same value.

That is, the electromechanical coupling coefficient k ² is represented by a constant π / 4, a resonance frequency fr and an anti-resonance frequency fa as shown in the following equation.

[0015]

(Equation 3) From this equation, the resonance frequency fr or the anti-resonance frequency fa
It can be seen that when one of the two is determined, the other is uniquely determined. Thus, when the resonance frequencies fr1 and fr3 of the resonators 14 and 16 are set to the same value, the anti-resonance frequencies fa1 and fa3 are set to the same value.

Similarly, resonators 15 and 17 forming parallel elements
The same value is set for the electromechanical coupling coefficient k2. Thereby, the anti-resonance frequency fa of the resonators 15 and 17 is
2 and fa4 are also set to the same value f2.

From the above, the attenuation characteristics of each stage of the ladder-type circuit are band-pass characteristics in which the center frequency is f0 and the pole frequencies in the attenuation region are f1 and f2. Accordingly, as shown in FIG. 7, the ladder-type circuit obtained by synthesizing them has band-pass characteristics in which the center frequency is f0 and the pole frequencies in the attenuation region are f1 and f2.

[0018] In the conventional filter, as well as the electromechanical coupling coefficient k ² between the electromechanical coupling factor k ² and parallel elements between series element, the electrical coupling coefficient k ² between the series element parallel elements They are set to the same value. That is,
In the conventional filter, all the resonators 14, 1
Electromechanical coupling coefficient k ² of 5,16,17 are set to the same value.

[0019]

As described above [0006] In the conventional filter, the electromechanical coupling coefficient k ² of all the resonators are set to the same value.

However, such a configuration has a problem in that when a large amount of attenuation is required in the attenuation region, it is difficult to satisfy the requirement.

That is, when the filter is used in a wireless communication device, various specifications are given. Therefore, for example, as shown by hatching in FIG. 7, a large amount of attenuation may be required in the attenuation region. In such a case, the conventional filter cannot cope with the specifications without increasing the number of stages of the ladder circuit.

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. Further, the chip size of the surface acoustic wave filter increases, and the yield also deteriorates. Therefore, in a conventional filter, if a large amount of attenuation is required in an attenuation region, it is difficult to meet this requirement.

Accordingly, an object of the present invention is to provide a filter capable of securing a large amount of attenuation in an attenuation region without increasing a loss in a pass band, increasing a chip size, and deteriorating a yield. And

[0024]

According to one aspect of the present invention, there is provided a resonator type surface acoustic wave filter configured to form a ladder circuit by a plurality of surface acoustic wave resonators. , Two are set as the electromechanical coupling coefficients of at least one of the series element and the parallel element, and one of the two electromechanical coupling coefficients is used.
Surface acoustic wave resonator is a piezoelectric surface shear wave type quasi-elastic
Surface acoustic wave resonator having a second surface acoustic wave resonator
Is a Love-wave type pseudo-surface acoustic wave resonator.
I do .

[0025]

[0026]

[0027]

According to the first aspect of the invention, the resonance frequencies of all the series elements and the anti-resonance frequencies of all the parallel elements are set to the same value. This is because the ladder circuit forms the filter.

On the other hand, at least one of the anti-resonance frequency of the series element and the resonance frequency of the parallel element is provided two . This is because two coefficients are set as the electromechanical coupling coefficients of at least one of the series element and the parallel element.

Therefore, as the attenuation characteristic of the filter, the pole of at least one of the high-frequency side and the low-frequency side is in the attenuation range.
As a result, two characteristics are obtained. As a result, the amount of attenuation in at least one of the high-frequency side and the low-frequency side is increased.

[0030] Also, in this manner, when setting the two electromechanical coupling coefficient, as a resonator, the use of the pressure conductive surface sliding wave type leaky surface wave resonator and the Love wave type leaky surface wave resonator The objective can be achieved by one filter without using two filters having different attenuation characteristics.

[0031]

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

FIG. 1 is a plan view showing the structure of one 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, 26, and 27 are resonators formed on the piezoelectric substrate 23.

Each of the resonators 24, 25, 26, and 27 has an IDT 241, 25 for exciting a surface acoustic wave, respectively.
1,261,271 and reflectors 242,243,252,253,262,263, which reflect this surface acoustic wave
272, 273.

The input terminal 21 is connected to one end of the interdigital transducer 241, and the other end of the interdigital transducer 241 is grounded via the interdigital transducer 251 and connected to one end of the interdigital transducer 261. I have. The other end of the interdigital electrode 261 is connected to the output terminal 22,
It is grounded via the interdigital electrode 271.

As a result, a two-stage ladder circuit as shown in the equivalent circuit of FIG. 8 is formed. Here, Z11, Z
12, Z21, Z22 are resonators 24, 25,
26 and 27 show impedances.

The equivalent circuit of each of the resonators 24, 25, 26 and 27 is represented as shown in FIG. Thereby, the attenuation characteristics of the resonators 24, 25, 26, and 27 are also represented as shown in FIG.

The above is the same as the configuration of the conventional filter. Next, a configuration which is a feature of the filter of this embodiment will be described.

In the conventional filter, all the resonators 1
Electromechanical coupling coefficient k ² of 4,15,16,17 is set to the same value.

On the other hand, in the filter of this embodiment,
The electromechanical coupling coefficient k ² of the first-stage resonator 24 and 25 are set to the same value, the electromechanical coupling coefficient k ² of the second-stage resonators 26, 27 is also set to the same value, 1 Stage 2
Electromechanical coupling coefficient k ² of the stage is set to a different value.

In this case, in this embodiment, for example, the electromechanical coupling coefficient k ² of the second-stage resonators 26 and 27 is
It is set so as to be larger than the electromechanical coupling coefficient k ² of the first-stage resonators 24 and 25.

In order to satisfy this condition, in this embodiment, as the first-stage resonators 24 and 25, for example, a resonator using a piezoelectric surface shear wave type pseudo-surface acoustic wave is used.
As the second-stage resonators 26 and 27, for example, a resonator using a Love-wave-type pseudo-surface acoustic wave is used.

The piezoelectric surface shear wave type quasi-surface acoustic wave is 3
It is known that excitation is performed by a 6 ° Y-cut X-propagation lithium tantalate substrate, and as an electrode metal, generally,
Aluminum (Al) is used. It is also well known electromechanical coupling coefficient k ² at that time is about 5%.

It is well known that a Love-wave type pseudo-surface acoustic wave propagates on a layered substrate having different propagation speeds. The use of gold or silver, which has a low propagation speed, as the IDTs to excite the surface acoustic wave, can be used to excite a Love-wave type pseudo-surface acoustic wave, and the electromechanical coupling coefficient k ² at that time can reach about 30%. Is disclosed in the IEICE technical report (document number US-37).

Therefore, the piezoelectric substrate 23 of this embodiment is
For example, the first-stage interdigital electrodes 241 and 251 are formed of, for example, aluminum, and the second-stage interdigital electrodes 261 and 271 are formed of, for example, aluminum.
Is formed of, for example, gold or silver.

FIG. 9 shows each of the resonators 24, 25, 26, 27.
FIG. 6 is a diagram showing a relationship between impedance characteristics of the first and second embodiments. In FIG. 9, in order to prevent the figure from becoming complicated, the characteristic curve is omitted, and only the resonance frequency fr and the anti-resonance frequency fa are shown.

In the figure, fr11, fr12, fr1
3, fr14 are resonators 24, 25, 26, 2 respectively.
7 and the resonance frequencies of fa11, fa12, fa1
3, fa14 are resonators 24, 25, 26, 27, respectively.
Shows the anti-resonance frequency of.

As shown, the resonator 2 forming a series element
4 and 26 and anti-resonance frequencies fa12 and fa of resonators 25 and 27 forming parallel elements.
14 is set to the same value f0 as in the prior art.

On the other hand, the anti-resonance frequency f of the resonator 26
a13 is set to a value higher than the anti-resonance frequency fa11 of the resonator 24. This is because the electromechanical coupling coefficient k ² of the resonator 26 is larger than the electromechanical coupling coefficient k of the resonator 24.
^This is because the value is set to a value larger than ² .

That is, the anti-resonance frequency fa of the resonator is
By transforming the above equation (3), it is expressed as the following equation (4). Therefore, if the resonance frequency fr is the same, as the electromechanical coupling coefficient k ² is large, the anti-resonance frequency fa is greater. As a result, fa13 becomes fa1
That is, it is larger than 1.

[0051]

(Equation 4) The resonance frequency fr14 of the resonator 27 is
Is set to a value smaller than the resonance frequency fr12.
This is because towards the electromechanical coupling coefficient k ² of the resonator 27 is set to a value greater than the electromechanical coupling coefficient k ² of the resonator 25.

That is, the resonance frequency fr of the resonator is expressed by the following equation (5) by modifying the equation (3). Therefore, if the anti-resonance frequency fa is the same, the larger the electromechanical coupling coefficient k ² , the higher the resonance frequency f
r becomes smaller. As a result, fr14 becomes fr12
It is smaller.

[0053]

(Equation 5) From the above, as shown in FIG. 10, the attenuation polarity of the filter is such that the center frequency is f0 and the high frequency side and the low frequency side are 2 respectively.
The characteristic is such that there are two poles. As a result, the attenuation amount in each of the high-frequency side and the low-frequency side becomes larger than before.

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

(1) First, the electromechanical coupling coefficients k ² of the first-stage resonators 24 and 25 and the second-stage resonators 26 and 27 are set to different values. One pole can be set.

As a result, a large amount of attenuation can be ensured in the attenuation region without increasing the number of stages of the ladder-type circuit, thereby increasing the loss in the pass band, increasing the chip size, and deteriorating the yield. Instead, the conventional problem can be solved.

(2) Since two electromechanical coupling coefficients k ² are set for each of the series element and the parallel element, the attenuation on the high frequency side and the low frequency side can be increased.

(3) As the resonators 24 and 25 having a small electromechanical coupling coefficient k ² , piezoelectric surface shear wave type quasi-surface acoustic wave resonators are used, and the resonators 26 and 27 having a large electromechanical coupling coefficient k ² are used. Since the Love-wave type pseudo-surface acoustic wave resonator is used, the purpose can be achieved with one filter without using two filters having different attenuation characteristics.

As described above, one embodiment of the present invention has been described in detail. However, the present invention is not limited to the above-described embodiment.

(1) For example, in the above embodiment, a piezoelectric surface shear wave type pseudo surface acoustic wave resonator is used as the first-stage resonators 24 and 25, and the second-stage resonators 26 and 27 are used as the first-stage resonators 26 and 27. The case where the Love wave type pseudo SAW resonator is used has been described. However, the present invention may have a reverse configuration.

[0061] (2) Further, in the above embodiment, as a small resonator electromechanical coupling coefficient k ^2, using a piezoelectric surface sliding wave type leaky surface wave resonators, the electromechanical coupling coefficient k ² large resonator In the above, the case where the Love wave type pseudo-surface acoustic wave resonator is used has been described. However, the present invention may use other resonators.

(3) In the above embodiment, the case where the number of stages of the ladder circuit is set to two has been described. However, in the present invention, three or more stages may be set.

(4) In the above embodiment, the case where two coefficients are set as the electromechanical coupling coefficient k ² has been described. However, in the present invention, three or more coefficients may be set. In this case, this may be set for the number of stages of the ladder circuit, or may be set for a smaller number. In the latter case, steps having the same electromechanical coupling coefficient k ² is the presence of a plurality.

(5) In the above embodiment, the case where a plurality of electromechanical coupling coefficients k ² are set for each of the series element and the parallel element has been described. However, according to the present invention, only one of them may be set, and only the amount of attenuation in the high frequency band or the low frequency band may be increased.

(6) In the above embodiment, the case where the electromechanical coupling coefficient k ² of the series element and the parallel element of each stage are set to the same value has been described. However, the present invention may set these to different values. Therefore, for example, in the above-described embodiment, the resonators 24 and 27 may be Love wave-type pseudo-surface acoustic wave resonators, and the resonators 25 and 26 may be piezoelectric surface shear wave-type pseudo-surface acoustic wave resonators.

(7) In the above embodiment, the case where a resonator including an interdigital transducer and a reflector is used as the resonator has been described. However, the present invention may use a resonator consisting only of interdigital electrodes.

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

[0068]

As described in detail above, according to the present invention,
It is possible to provide a resonator-type surface acoustic wave filter capable of securing a sufficient attenuation in an attenuation region without increasing a loss in a pass band, increasing a chip size, and deteriorating a yield.

[Brief description of the drawings]

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

FIG. 2 is a plan view showing a 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 the resonator.

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

FIG. 6 is a characteristic diagram showing a relationship of impedance characteristics between resonators of a conventional filter.

FIG. 7 is a characteristic diagram showing an attenuation characteristic of a conventional filter.

FIG. 8 is a circuit diagram showing an equivalent circuit of the filter of one embodiment.

FIG. 9 is a characteristic diagram showing a relationship of impedance characteristics between resonators of the filter according to one embodiment.

FIG. 10 is a characteristic diagram illustrating an attenuation characteristic of the filter according to the embodiment;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 21 ... Input terminal 22 ... Output terminal 23 ... Piezoelectric substrate 24,25,26,27 ... Resonator 241,251,261,271 ... Interdigital electrode 242,243,252,253,262,263,2
72,273 ... Reflector.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Woo Hook Hoa 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (56) References JP-A-3-205908 (JP, A (58) Fields surveyed (Int. Cl. ⁷ , DB name) H03H 9/64

Claims (1)

(57) [Claims] 1. A resonator type surface acoustic wave filter configured to form a ladder type circuit by a plurality of surface acoustic wave resonators, wherein the ladder type circuit has at least one of a series element and a parallel element. Two are set as coupling coefficients .
And has one of the two electromechanical coupling coefficients
The surface acoustic wave resonator is a piezoelectric surface shear wave type pseudo elastic surface.
Surface acoustic wave resonator having the other
A resonator-type surface acoustic wave filter , which is a pseudo-surface acoustic wave resonator .