JPH11266138A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a balanced surface acoustic wave filter which is low in loss and superior in out-of-pass band attenuation amount. SOLUTION: A first filter track 12, consisting of an input inter-digital transducer(IDT) electrode pair 14 and an output IDT electrode pair 15 and a second filter track 13 consisting of an input IDT electrode pair 16 and an output IDT electrode pair 17, are provided on a piezoelectric substrate 11. The first filter track 12 and the second filter track 13 are made into a symmetric structure, with a symmetrical shaft as a propagation direction of an elastic surface wave. A terminal is formed by connecting a lower electrode 14b of the input IDT electrode pair 14 of the first filter track 12 to an upper electrode 16a of the input IDT electrode pair 16 of the second filter track 13 and is connected to a ground. A terminal is formed by connecting a lower electrode 15b of the output IDT electrode pair 15 of the first filter track 12 to an upper electrode 17a of the input IDT electrode pair 17 of the second filter track 13 and is connected to the ground.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter used for, for example, a high-frequency circuit in a radio communication device.

[0002]

2. Description of the Related Art An electromechanical mechanism component using a surface acoustic wave has a property that the sound speed of a wave is several km / s and the energy of the wave is concentrated on the surface of a propagation medium. Attention is being paid to this trend. Electromechanical components using surface acoustic waves are interdigital transducers (hereinafter referred to as “I
DT ”. ) With the development of electrodes and the development of thin film fabrication technology and surface processing technology that made it possible to develop and deform the electrodes, they have been put to practical use in delay lines for radar and bandpass filters for television receivers. Widely used as RF and IF stage filters in circuits.

[0003] In recent years, with the digitization of mobile communication equipment, the development of digital portable telephones, digital cordless telephones, and the like has been actively pursued, and the occupied bandwidth per channel has been increasing. In particular, an IF filter used in a CDMA system (code division multiplex system), which has attracted attention in recent years, has a very wide band and very flat group delay time deviation and a selectivity for distinguishing a signal of an adjacent channel from a desired signal. Excellent characteristics are required.

Conventionally, as a surface acoustic wave filter suitable for the IF stage, a resonator type surface acoustic wave filter and a transversal type surface acoustic wave filter are well known. The resonator-type surface acoustic wave filter has a narrow band and a steep cutoff characteristic, has a small insertion loss and a small element size, but has a poor group delay time deviation characteristic. On the other hand, the transversal surface acoustic wave filter has a large insertion loss and a large element size, but has a wide band and flat group delay time deviation characteristic. From the above characteristics, a transversal surface acoustic wave filter is suitable as an IF filter in the CDMA system.

Hereinafter, a conventional transversal surface acoustic wave filter will be described.

FIG. 7 is a configuration diagram showing an electrode pattern of a transversal surface acoustic wave filter according to the prior art. In FIG. 7, reference numeral 71 denotes a single crystal piezoelectric substrate. By forming an electrode pattern on the piezoelectric substrate 71, surface acoustic waves can be excited. Piezoelectric substrate 7
1, an input IDT electrode pair 72 including an upper electrode 72a and a lower electrode 72b and an output IDT electrode pair 73 including an upper electrode 73a and a lower electrode 73b are provided at a predetermined distance. Thus, a transversal type surface acoustic wave filter is formed.

In the surface acoustic wave filter having the above configuration, various weights are applied to the input / output IDT electrode pairs, or at least one of the input / output IDT electrode pairs is used as a unidirectional IDT electrode pair. As a result, the frequency characteristic of the filter is determined, and a steep filter characteristic over a wide band is realized.

[0008]

In recent years, the use of balanced input / output ICs before and after the IF filter has been progressing, and there has been a strong demand for the appearance of a balanced input / output type IF filter. Further, in order to distinguish between a signal of an adjacent channel and a desired signal, an improvement in attenuation outside the pass band is demanded.

However, in the conventional surface acoustic wave filter having the configuration shown in FIG. 7, the weight of the input / output IDT electrode pair causes the configuration of the upper electrode and the lower electrode of the IDT electrode pair to be different. However, there is a problem that the degree of balance is deteriorated. Further, when at least one of the input / output IDT electrode pairs is a unidirectional IDT electrode pair, a problem arises in that the side lobe level near the pass band is deteriorated.

The present invention has been made to solve the above-mentioned problems in the prior art, and has as its object to provide a balanced surface acoustic wave filter having low loss and excellent attenuation outside the pass band.

[0011]

In order to achieve the above object, a first configuration of a surface acoustic wave filter according to the present invention comprises:
A first filter track formed on the piezoelectric substrate, the first filter track including a first input interdigital transducer electrode pair and a first output interdigital transducer electrode pair disposed at a predetermined distance; A second input interdigital transducer electrode pair and a second output interdigital transducer electrode pair formed on the piezoelectric substrate in parallel with the first filter track and arranged at a predetermined distance. A second filter track, wherein the first filter track and the second filter track have a symmetric structure in which a propagation direction of the surface acoustic wave is a symmetric axis, and wherein the first input interdigital transducer electrode The lower electrode of the pair and the upper electrode of the second input interdigital transducer electrode pair are both connected. And the lower electrode of the first output interdigital transducer electrode pair and the upper electrode of the second output interdigital transducer electrode pair are both grounded, and the upper electrode of the first input interdigital transducer electrode pair is grounded. An electrode and a lower electrode of the second input interdigital transducer electrode pair form a balanced input terminal, and an upper electrode of the first output interdigital transducer electrode pair and the second output interdigital transducer electrode. A balanced output terminal is formed by the pair of lower electrodes. According to the first configuration of the surface acoustic wave filter, the configuration of the electrodes connected to the input / output terminal pair becomes the same, and the degree of balance between the terminals when operated as a balanced type is greatly improved. As a result, a balanced surface acoustic wave filter with low loss and excellent attenuation outside the passband can be realized.

Further, in the first configuration of the surface acoustic wave filter according to the present invention, a dummy electrode is provided on a lower electrode of the first input interdigital transducer electrode pair and an upper electrode of the second input interdigital transducer electrode pair. The dummy electrodes are formed on the lower electrode of the first output interdigital transducer electrode pair and the upper electrode of the second output interdigital transducer electrode pair and are weighted by the thinning. Is preferred. According to this preferred example, when the frequency characteristics of the filter are determined by performing the thinning-out weighting and the steep filter characteristics are realized in a wide band, the influence of the stray capacitance due to the dummy electrode can be suppressed. As a result, the insertion loss can be improved.

In the first configuration of the surface acoustic wave filter according to the present invention, both the first input / output interdigital transducer electrode pair and the second input / output interdigital transducer electrode pair are single-phase unidirectional. A converter (SPUDT), comprising a reflector electrode on a lower electrode of said first input interdigital transducer electrode pair and a reflector electrode on an upper electrode of said second input interdigital transducer electrode pair, and said first output interface. Preferably, reflector electrodes are provided on the lower electrode of the digital transducer electrode pair and the upper electrode of the second output interdigital transducer electrode pair. According to this preferred example, when the first and second input / output interdigital transducer electrode pairs are made of SPUDT, the frequency characteristics of the filter are determined. The effect of stray capacitance can be suppressed. As a result, the insertion loss and the attenuation outside the pass band can be improved.

Also, in the first configuration of the surface acoustic wave filter according to the present invention, the first filter track and the second
Are preferably provided at a distance of 10λ (λ: wavelength of a surface acoustic wave propagating on a piezoelectric substrate) or more. According to this preferred example, the first filter track and the second filter track do not affect each other acoustically, and excellent filter characteristics can be obtained.

Further, a second configuration of the surface acoustic wave filter according to the present invention is a piezoelectric substrate, and a first input interdigital transducer electrode pair formed on the piezoelectric substrate and arranged at a predetermined distance. A first filter track comprising: a first output interdigital transducer electrode pair; and a second filter track formed on the piezoelectric substrate in parallel with the first filter track and disposed at a predetermined distance from the piezoelectric substrate. A second filter track comprising a pair of input interdigital transducer electrodes and a second pair of output interdigital transducer electrodes;
In the propagation direction of the surface acoustic wave with respect to the second filter track in the direction of (n + ／) λ (n:
An integer greater than or equal to 0, λ: the wavelength of the surface acoustic wave propagating on the piezoelectric substrate), the first filter track and the second filter track are symmetrical with the propagation direction of the surface acoustic wave as the axis of symmetry. The lower electrode of the first pair of input interdigital transducer electrodes and the upper electrode of the second pair of input interdigital transducer electrodes are both grounded, and the first output interface The lower electrode of the digital transducer electrode pair and the upper electrode of the second output interdigital transducer electrode pair are both grounded, and the upper electrode of the first input interdigital transducer electrode pair and the second input interdigital transducer. A balanced input terminal is formed by the lower electrode of the electrode pair, Wherein the force balance type output terminal and interdigital transducer electrode pair of the upper electrode by a lower electrode of the second output interdigital transducer electrode pair is formed. According to the second configuration of the surface acoustic wave filter, the first filter track and the second filter track do not affect each other acoustically, and the first filter track and the second filter track do not affect each other. It is not necessary to take enough distance from
The element size can be significantly reduced.

According to a third aspect of the surface acoustic wave filter according to the present invention, there is provided a piezoelectric substrate, a pair of input interdigital transducer electrodes formed on the piezoelectric substrate, and an input interdigital transducer formed on the piezoelectric substrate. A surface acoustic wave filter comprising an interdigital transducer electrode pair and an output interdigital transducer electrode pair arranged at a predetermined distance, wherein the input interdigital transducer electrode pair or the output interdigital transducer electrode pair One of them is a single-phase unidirectional converter (SPUDT). According to the second configuration of the surface acoustic wave filter, it is possible to suppress the TTE (Triple Transit Echo) level in a state of impedance matching. A surface acoustic wave filter having an excellent attenuation near the band can be realized.

Further, in the third configuration of the surface acoustic wave filter according to the present invention, in the impulse response characteristic of the input interdigital transducer electrode pair, the frequency f _1h of the pole on the high frequency side closest to the center frequency and the center frequency are set to the center frequency. Δf ₁ is the difference from the frequency f ₁₁ of the nearest lower pole, and the frequency f of the higher pole closest to the center frequency in the impulse response characteristic of the output interdigital transducer electrode pair.
_When the difference between _2h and the frequency f _2l of the lower pole closest to the center frequency is Δf ₂ , it is preferable that the interdigital transducer electrode pair having the smaller Δf is SPUDT. According to this preferred example, since the side lobe level of the entire filter can be suppressed, an excellent attenuation can be obtained in the vicinity of the pass band.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described more specifically with reference to embodiments.

<First Embodiment> FIG. 1 shows a first embodiment of the present invention.
FIG. 4 is a configuration diagram showing an electrode pattern of a surface acoustic wave filter according to the embodiment.

In FIG. 1, reference numeral 11 denotes a single crystal piezoelectric substrate, and a surface acoustic wave can be excited by forming a strip line electrode pattern having a periodic structure on the piezoelectric substrate 11. A first filter track 12 and a second filter track 1 are placed on a piezoelectric substrate 11.
3 are provided, and the first filter track 12 and the second filter track 13 have a symmetrical structure with the propagation direction of the surface acoustic wave as the axis of symmetry.

The first filter track 12 has an input ID
It comprises a T electrode pair 14 and an output IDT electrode pair 15. In addition, the second filter track 13 has the input I
It comprises a DT electrode pair 16 and an output IDT electrode pair 17.

Input IDT of the first filter track 12
The lower electrode 14b of the electrode pair 14 and the upper electrode 16a of the input IDT electrode pair 16 of the second filter track 13 are connected to form a terminal, and are connected to ground. Also,
The lower electrode 15b of the output IDT electrode pair 15 of the first filter track 12 and the upper electrode 17a of the input IDT electrode pair 17 of the second track 13 are connected to form a terminal,
Connected to earth.

The difference in the electrode pattern configuration between the surface acoustic wave filter of this embodiment shown in FIG. 1 and the conventional surface acoustic wave filter shown in FIG. 7 is that the conventional surface acoustic wave filter shown in FIG. While the configuration of the electrode 72a and the electrode 72b connected to the terminal pair is different, the surface acoustic wave filter of the present embodiment shown in FIG. 1 has the configuration of the electrode 14a and the electrode 16b connected to the input terminal. The configuration is the same. Further, in the surface acoustic wave filter of the present embodiment, the configuration of the electrode 15a and the electrode 17b connected to the output terminal pair is the same. That is, by adopting the two-track configuration as described above, the configuration of the electrodes connected to the input / output terminal pair becomes the same, and compared to the conventional surface acoustic wave filter shown in FIG. In this case, the degree of balance between terminals is greatly improved.
As a result, a balanced surface acoustic wave filter with low loss and excellent attenuation outside the passband can be realized. When attention is paid to the attenuation outside the pass band, the filter bandwidth 1.3 MHz, attenuation outside the pass band as f ₀ the center frequency of the filter is f ₀ ± 5 MHz, and the conventional surface acoustic wave filter shown in FIG. 7 It is improved by about 10 dB as compared with the case.

In the surface acoustic wave filter according to the present embodiment shown in FIG. 1, the input / output IDT electrode pair 1 of the first filter track 12 and the second filter track 13 is used.
4, 15, 16 and 17 have “thinning weighting”, one of the input / output IDT electrode pairs has an ID
A dummy electrode for adjusting the sound speed of the surface acoustic wave is provided in a portion where the T electrodes are thinned out. In this case, the lower electrodes 14b and 15b of the input / output IDT electrode pairs 14 and 15 of the first filter track 12 and the second filter track 13
Or the upper electrodes 16a and 17a of the input / output IDT electrode pairs 16 and 17 have dummy electrodes, or the upper electrodes 14a and 15a of the input / output IDT electrode pairs 14 and 15 of the first filter track 12 An electrode configuration in which dummy electrodes are provided for the upper electrodes 16b and 17b of the input / output IDT electrode pairs 16 and 17 of the second filter track 13 can be considered.
If the connection between the electrode 14b and the IDT electrode 16a and the connection between the IDT electrode 15b and the IDT electrode 17a are connected to ground, it is possible to suppress the influence of the dummy electrode such as stray capacitance. As a result, the insertion loss can be improved.

FIG. 2 shows that the input / output IDT electrode pairs 14, 15, 16 and 17 of the first filter track 12 and the second filter track 13 in the surface acoustic wave filter of FIG. FIG. 4 is a configuration diagram showing an electrode pattern in the case of a device (SPUDT). SPUDT is a unidirectional electrode, and has a configuration having a reflector electrode inside the IDT electrode. In this case, the lower electrodes 24b and 25b of the input / output IDT electrode pairs 24 and 25 of the first filter track 22 and the upper electrodes 26a and 27 of the input / output IDT electrode pairs 26 and 27 of the second filter track 23.
a, or the upper electrodes 24a and 25a of the input / output IDT electrode pairs 24 and 25 of the first filter track 22 and the input / output IDT electrode pairs 26 and 27 of the second filter track 23. Either of the configurations having the reflector electrodes on the lower electrodes 26b and 27b is possible, but in the former configuration, the connection between the IDT electrode 24b and the IDT electrode 26a and the IDT electrode 25b and the IDT electrode 2
If the connection with 7a is connected to ground, the influence of the stray capacitance and the like by the dummy electrode and the reflector electrode can be suppressed as compared with the case where the latter configuration is selected. as a result,
Excellent characteristics in terms of insertion loss and attenuation outside the pass band are obtained.

<Second Embodiment> FIG. 3 shows a second embodiment of the present invention.
FIG. 4 is a configuration diagram showing an electrode pattern of a surface acoustic wave filter according to the embodiment.

In FIG. 3, reference numeral 31 denotes a single-crystal piezoelectric substrate, and a surface acoustic wave can be excited by forming a strip line electrode pattern having a periodic structure on the piezoelectric substrate 31. A first filter track 32 and a second filter track 3 are placed on the piezoelectric substrate 31.
The first filter track 32 and the second filter track 33 have the following structure. That is, the first filter track 32 includes the input IDT electrode pairs 34 and the output IDT electrode pairs 35. The second filter track 33 is
It comprises an input IDT electrode pair 36 and an output IDT electrode pair 37. Here, the first filter track 32
With respect to the second filter track 33 in the direction of propagation of the surface acoustic wave to (n + ／) λ (n: an integer of 0 or more, λ:
When the track (wavelength of the surface acoustic wave propagating on the piezoelectric substrate) is shifted, both tracks 32 and 33 have a symmetrical structure with the propagation direction of the surface acoustic wave as the axis of symmetry. Conversely, two tracks having a symmetrical structure with the propagation direction of the surface acoustic wave as the axis of symmetry are mutually (n + 1/2) λ in the propagation direction of the surface acoustic wave.
By shifting, the structure shown in FIG. 3 is obtained. Furthermore, the input IDT electrode pair 3 of the first filter track 32
The fourth lower electrode 34b and the upper electrode 36a of the input IDT electrode pair 36 of the second filter track 33 are connected to form a terminal, and are connected to ground. Further, the lower electrode 35b of the output IDT electrode pair 35 of the first filter track 32 and the upper electrode 37a of the input IDT electrode pair 37 of the second track 33 are connected to form a terminal and are connected to the ground. .

Here, the first filter track 32 and the second filter track 33 are displaced by (n + 1/2) λ in the direction of propagation of the surface acoustic wave, so that the first filter track 32 and the second filter track 33 are shifted from each other. Since the two filter tracks 33 do not affect each other acoustically and it is not necessary to provide a sufficient distance between the first filter track 32 and the second filter track 33, the element size is significantly reduced. Can be achieved.

Also, the first filter track 32 and the second
The filter track 33 is preferably provided at a distance of 10λ or more. According to this configuration, the first filter track 32 and the second filter track 33 do not affect each other acoustically, and excellent filter characteristics can be obtained.

<Third Embodiment> FIG. 4 shows a third embodiment of the present invention.
FIG. 4 is a configuration diagram showing an electrode pattern of a surface acoustic wave filter according to the embodiment.

In FIG. 4, reference numeral 41 denotes a single-crystal piezoelectric substrate. By forming a strip line electrode pattern having a periodic structure on the piezoelectric substrate 41, surface acoustic waves can be excited. On the piezoelectric substrate 41, a first filter track 42 and a second filter track 4
The first filter track 42 and the second filter track 43 have the following structure. That is, the first filter track 42 is constituted by the input IDT electrode pair 44 which is a bidirectional electrode and the output IDT electrode pair 45 which is a SPUDT electrode. The second filter track 43 includes an input IDT electrode pair 46 which is a bidirectional electrode and an output IDT electrode pair 47 which is a SPUDT electrode. Here, the first filter track 42 is moved with respect to the second filter track 43 in the propagation direction of the surface acoustic wave by (n + 1 /
2) When λ (n: an integer of 0 or more, λ: the wavelength of the surface acoustic wave propagating on the piezoelectric substrate) is shifted, both tracks 42, 4
3 has a symmetrical structure with the propagation direction of the surface acoustic wave as the axis of symmetry. Conversely, by shifting two tracks having a symmetrical structure with the propagation direction of the surface acoustic wave as the axis of symmetry, by (n + 1/2) λ in the propagation direction of the surface acoustic wave, the structure shown in FIG. 4 is obtained. Can be Further, the lower electrode 44b of the input IDT electrode pair 44 of the first filter track 42 and the upper electrode 46a of the input IDT electrode pair 46 of the second filter track 43 are connected to form a terminal, and are connected to ground. I have. Further, the lower electrode 45b of the output IDT electrode pair 45 of the first filter track 42 and the upper electrode 47a of the input IDT electrode pair 47 of the second track 43 are connected to form a terminal and are connected to the ground. .

In a conventional surface acoustic wave filter, a transversal filter using a bidirectional electrode as an input / output IDT electrode pair can obtain a large amount of attenuation near the pass band. Due to the directional electrode, a large ripple is generated in the pass band due to the influence of TTE (Triple Transit Echo).

Since the filter used in the IF stage of the digital mobile communication device is required to have a flat characteristic in a pass band, the filter must be used in an impedance mismatch state in order to suppress the TTE. The insertion loss becomes very large. On the other hand, when the SPUDT electrode is used for the input / output IDT electrode pair, the TTE
Since the level is sufficiently low, a filter having a small insertion loss with a flat characteristic in the pass band can be obtained. However, since the electrode is a unidirectional electrode, the attenuation in the vicinity of the pass band deteriorates.

However, in the surface acoustic wave filter according to the present embodiment shown in FIG. 4, one electrode pair is a bidirectional electrode and the other electrode pair is a SPUDT electrode. , And the insertion loss with a flat passband characteristic is smaller than that of a transversal filter using a bidirectional electrode for the input / output IDT electrode pair. Further, a surface acoustic wave filter having an excellent attenuation in the vicinity of the pass band can be realized as compared with a surface acoustic wave filter using a SPUDT electrode for an input / output IDT electrode pair.

The elastic surface of the present embodiment shown in FIG.
In the wave filter, the input IDT electrode pair shown in FIG.
Center frequency f in impulse response characteristic 51₀Closest to
Higher pole frequency f_1hAnd center frequency f₀Closest to
Low frequency pole frequency f_1lΔf₁, Output IDT
Center frequency f in impulse response characteristic 52 of pole pair ₀
Frequency f of the high-frequency pole closest to_2hAnd center frequency f₀To
Frequency f of the nearest lower pole_2lΔf_TwoMade
At this time, the IDT electrode pair having the smaller Δf (here, the output
When IDT electrode pairs 45 and 47) are used as SPUDT electrodes
In this case, the characteristics of the output IDT electrode pairs 45 and 47 are
The input level of the other input IDT electrode pair 44 and
Of the filter
As a body, it can suppress the side lobe level,
Excellent attenuation near the pass band
You.

As described above, according to the present embodiment, it is possible to realize a surface acoustic wave filter that is excellent in the degree of balance and that can improve the attenuation outside the pass band and the insertion loss.

In the present embodiment, the case where two filter tracks as shown in FIG. 4 are used has been described as an example. However, as shown in FIG. , .DELTA.f (here, the output IDT electrode pair 63) can be used as SPUDT electrodes to obtain excellent attenuation in the vicinity of the pass band.

Further, the above-mentioned one-track filter has the following structure.
Although FIG. 6 shows a balanced filter, the configuration is not necessarily limited to this configuration. An unbalanced input / output terminal configuration, or one is a balanced type and the other is an unbalanced type input / output terminal configuration However, the same effect can be obtained.

The piezoelectric substrate according to the present invention includes:
It is preferable to use ST-cut quartz having excellent temperature characteristics, but LiTaO ₃ , LiNbO ₃ , Li ₂ B ₄ O ₇ , L
a ₃ Ga ₅ SiO ₁₄ or the like can be used as the piezoelectric substrate.

As the electrode material, it is preferable to use aluminum having a relatively small density for which the film thickness can be easily controlled, but it is also possible to use a gold electrode.

[0041]

As described above, according to the present invention,
It is possible to realize a surface acoustic wave filter which is excellent in the degree of balance and has a large attenuation outside the pass band as compared with the related art.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an electrode pattern of a surface acoustic wave filter according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing another example of the electrode pattern of the surface acoustic wave filter according to the first embodiment of the present invention.

FIG. 3 is a configuration diagram illustrating an electrode pattern of a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing an electrode pattern of a surface acoustic wave filter according to a third embodiment of the present invention.

5 is a diagram showing an impulse response characteristic of an input / output IDT electrode pair of the surface acoustic wave filter of FIG. 4;

FIG. 6 is a configuration diagram showing another example of the electrode pattern of the surface acoustic wave filter according to the third embodiment of the present invention.

FIG. 7 is a configuration diagram showing an electrode pattern of a transversal surface acoustic wave filter according to a conventional technique.

[Explanation of symbols]

11, 21, 31, 41, 61 Piezoelectric substrate 12, 13, 22, 23, 32, 33, 42, 43 Filter track 14, 16, 24, 26, 34, 36, 44, 46, 6
2 input IDT electrode pairs 15, 17, 25, 27, 35, 37, 45, 47, 6
3 Output IDT electrode pairs 14a, 16a, 24a, 26a, 34a, 36a, 4
4a, 46a Upper electrode of input IDT 14b, 16b, 24b, 26b, 34b, 36b, 4
4b, 46b Input IDT lower electrode 15a, 17a, 25a, 27a, 35a, 37a, 4
5a, 47a Upper electrode of output IDT 15b, 17b, 25b, 27b, 35b, 37b, 4
5b, 47b Lower electrode of output IDT

Claims (7)

[Claims] A first input interdigital transducer electrode pair and a first output interdigital transducer electrode pair formed on the piezoelectric substrate and arranged at a predetermined distance from each other; A second input interdigital transducer electrode pair and a second output interdigital transducer formed on the piezoelectric substrate in parallel with the first filter track and arranged at a predetermined distance from each other. A second filter track consisting of an electrode pair;
And the second filter track has a symmetric structure with the propagation direction of the surface acoustic wave as the axis of symmetry,
The lower electrode of the first input interdigital transducer electrode pair and the upper electrode of the second input interdigital transducer electrode pair are both grounded, and the lower electrode of the first output interdigital transducer electrode pair is connected to the lower electrode. The upper electrodes of the two output interdigital transducer electrode pairs are grounded together, and the balanced input is provided by the upper electrode of the first input interdigital transducer electrode pair and the lower electrode of the second input interdigital transducer electrode pair. A surface acoustic wave filter having a terminal formed and a balanced output terminal formed by an upper electrode of the first output interdigital transducer electrode pair and a lower electrode of the second output interdigital transducer electrode pair. 2. A dummy electrode is formed on a lower electrode of a first input interdigital transducer electrode pair and an upper electrode of a second input interdigital transducer electrode pair to perform thinning-out weighting and to output a first output interface. 2. The surface acoustic wave filter according to claim 1, wherein dummy electrodes are formed on a lower electrode of the digital transducer electrode pair and an upper electrode of the second output interdigital transducer electrode pair, and weighting is performed by thinning. 3. An electrode pair of a first input / output interdigital transducer and a pair of second input / output interdigital transducer electrodes are each a single-phase unidirectional converter (SPUD).
T) wherein a lower electrode of the first input interdigital transducer electrode pair and a reflector electrode on an upper electrode of the second input interdigital transducer electrode pair, and wherein the first output interdigital transducer electrode is provided. 3. The surface acoustic wave filter according to claim 1, wherein a reflector electrode is provided on a lower electrode of the pair and an upper electrode of the second output interdigital transducer electrode pair. 4. The first filter track and the second filter track are provided at a distance of 10λ (λ: wavelength of a surface acoustic wave propagating on a piezoelectric substrate) or more. The surface acoustic wave filter according to any one of the above. 5. A first substrate comprising a piezoelectric substrate, a first input interdigital transducer electrode pair and a first output interdigital transducer electrode pair formed on the piezoelectric substrate and arranged at a predetermined distance. A second input interdigital transducer electrode pair and a second output interdigital transducer formed on the piezoelectric substrate in parallel with the first filter track and arranged at a predetermined distance from each other. A second filter track consisting of an electrode pair;
In the propagation direction of the surface acoustic wave with respect to the second filter track in the direction of (n + ／) λ (n:
An integer greater than or equal to 0, λ: the wavelength of the surface acoustic wave propagating on the piezoelectric substrate), the first filter track and the second filter track are symmetrical with the propagation direction of the surface acoustic wave as the axis of symmetry. The lower electrode of the first pair of input interdigital transducer electrodes and the upper electrode of the second pair of input interdigital transducer electrodes are both grounded, and the first output interface The lower electrode of the digital transducer electrode pair and the upper electrode of the second output interdigital transducer electrode pair are both grounded, and the upper electrode of the first input interdigital transducer electrode pair and the second input interdigital transducer. A balanced input terminal is formed by the lower electrode of the electrode pair, Power interdigital transducer electrode wherein a pair of the upper electrode and the second output interdigital transducer electrode pair surface acoustic wave filter balanced output terminal is formed by a lower electrode. 6. A piezoelectric substrate, a pair of input interdigital transducer electrodes formed on the piezoelectric substrate, and a predetermined distance formed from the pair of input interdigital transducer electrodes formed on the piezoelectric substrate. A surface acoustic wave filter having an output interdigital transducer electrode pair, wherein one of the input interdigital transducer electrode pair and the output interdigital transducer electrode pair is a single-phase unidirectional converter (SPUDT). A surface acoustic wave filter, comprising: 7. The difference between the frequency f _1h of the pole on the high frequency side closest to the center frequency and the frequency f ₁₁ of the pole on the low frequency side closest to the center frequency in the impulse response characteristics of the input interdigital transducer electrode pair is Δf. _1. The difference between the frequency f _2h of the high-frequency pole closest to the center frequency and the frequency f _2l of the low-frequency pole closest to the center frequency in the impulse response characteristics of the output interdigital transducer electrode pair is Δf.
7. The surface acoustic wave filter according to claim 6, wherein when ₂ , the interdigital transducer electrode pair having the smaller Δf is SPUDT.