JPH10209806A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce ripple in a pass band by providing a surface acoustic wave device with a ladder-connected filter, constituting a serial arm resonator and a parallel arm resonator, of a cavity type surface acoustic wave resonator, and specifying the ratio of the electrode finger pitch of a surface acoustic wave converter to the electrode finger pitch of a reflector. SOLUTION: In a ladder type surface acoustic wave filter in which serial arm resonators 10a-10c and parallel arm resonators 20a-20c are formed so that the propagation direction of a surface acoustic wave is a single crystal X-axial direction on a lithium tantalate substrate 1, the serial arm resonators 10a-10c and the parallel arm resonators 20a-20c constitute a cavity type surface acoustic wave resonator. In this case, the rate of the electrode finger pitches of surface acoustic wave converters 11a-11c to the electrode finger pitches of reflectors 12a-12c and 13a-13c in the serial arm resonator 10a-10c is set 0.967-1, and the rate of the electrode finger pitches of surface acoustic wave converters 21a-21c to the electrode finger pitches of reflectors 22a-22c and 23a-23c in the parallel arm resonators 20a-20c is set almost 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device. More specifically, the present invention relates to a surface acoustic wave device including a ladder-connected filter including a series arm resonator and a parallel arm resonator.

[0002]

2. Description of the Related Art In recent years, mobile communication, which began with a car phone, has been transferred to a mobile phone and is rapidly spreading. With the spread of portable telephones, there has been a great demand for smaller, lighter weight and lower loss, and similar demands have been increasing for individual components used. By the way, a surface acoustic wave filter is being adopted as a high frequency filter which is a main component in a mobile communication device such as a portable telephone so as to meet this demand.

A surface acoustic wave filter uses a surface acoustic wave propagating on a solid surface, and a number of methods for forming the filter have been reported. One suitable configuration for a surface acoustic wave filter used as a high frequency filter for a mobile communication device is a ladder type surface acoustic wave filter. For example, Japanese Patent Application Laid-Open No. 52-19044 discloses a cavity-type surface acoustic wave resonator in which reflectors are arranged on both outer sides of a surface acoustic wave converter (IDT) as a serial arm and a resonant cavity is formed, and a parallel arm is used. A ladder-type surface acoustic wave filter constituted by a multi-pair surface acoustic wave converter type resonator composed of a large number of electrode finger pairs is disclosed. Important factors in designing this ladder-type surface acoustic wave filter are that the resonance frequency Frs of the series arm resonator should be matched with the anti-resonance frequency Fap of the parallel arm resonator. This indicates that the equivalent parallel capacitance Cop of the parallel arm resonator should be set to a larger value than the equivalent parallel capacitance Cos of the resonator.

However, the configuration using the cavity surface acoustic wave resonator as the series arm resonator and the many-pair surface acoustic wave resonator as the parallel arm resonator has the following problems. Generally, resonance of a surface acoustic wave resonator is caused by the reflection of surface acoustic waves from an electrode strip formed on a piezoelectric substrate. Therefore, as the thickness of the electrode strip is larger, the reflection performance is enhanced, and the Q value representing the resonance performance is also improved. However, as the thickness of the electrode increases, side effects such as an increase in loss due to mode conversion into a bulk wave at the electrode strip edge and the growth of a subresonance that becomes spurious appear, deteriorating the main resonance. Therefore, there is an appropriate electrode film thickness to enhance the resonance performance. The appropriate electrode film thickness differs between the cavity surface acoustic wave resonator and the many-pair surface acoustic wave resonator. For example, Toyo Communication Equipment Technical Report No. In 33 pp1-101083, when the upper limit of the appropriate film thickness is expressed by normalizing the film thickness with the surface acoustic wave wavelength, the cavity surface acoustic wave resonator is 3%, and the many-pair surface acoustic wave resonator is 2%. Is disclosed. Incidentally, it is preferable that the ladder-type surface acoustic wave filter is formed on the same piezoelectric substrate at the same time for both the series arm resonator and the parallel arm resonator in the same process, in order to reduce the size of the surface acoustic wave device and simplify the manufacturing process. . However, if the appropriate values of the electrode thicknesses of the series arm and the parallel arm resonator are different as described above, it becomes difficult to improve the performance of the entire surface acoustic wave device. Also, when using a many-pair resonator as a resonator,
The surface acoustic wave device becomes large.

[0005] As another prior art of a ladder type surface acoustic wave filter, see IEICE Transactions A, Vol. J76
-A No. 2, pp 245-252, February 1993, "Low Loss Bandpass Filter Using SAW Resonator".
In this document, a cavity type surface acoustic wave resonator is used for both the serial arm and the parallel arm. As in Japanese Patent Application Laid-Open No. 52-19044, the resonance frequency Frs of the series arm resonator and the anti-resonance of the parallel arm resonator are used. That the frequency Fap should be substantially matched;
It is shown that the attenuation level outside the band can be controlled by changing the ratio between the equivalent parallel capacitance Cos of the series arm resonator and the equivalent parallel capacitance Cop of the parallel arm resonator. Since both the series arm resonator and the parallel arm resonator are cavity type surface acoustic wave resonators, it is preferable in that both characteristics can be simultaneously improved.

By the way, there are many disclosed examples of the structure of the cavity surface acoustic wave resonator constituting the ladder type surface acoustic wave filter. For example, the above-mentioned Toyo Telecommunications Technical Report No. 33 pp1-10, 1983,
A structure using quartz as the piezoelectric substrate has been studied. Assuming that the electrode finger pitch of the surface acoustic wave converter is Lt and the electrode finger pitch of the reflector is Lr, the ratio of these is Lt /
It is shown that Lr = 0.994. That is, it is disclosed that the electrode finger pitch Lt of the surface acoustic wave converter is set smaller than the electrode finger pitch Lr of the reflector in order to improve the performance of the cavity surface acoustic wave resonator. Further, an equation is given as to what value the electrode finger pitch ratio Lt / Lr should be in accordance with the structure of the piezoelectric substrate and the surface acoustic wave resonator to be used.

[0007]

However, the above-mentioned setting of the electrode finger pitch ratio of the cavity surface acoustic wave resonator, the resonance frequency of the series arm resonator and the antiresonance frequency of the parallel arm resonator only match. When the ladder-type surface acoustic wave filter is configured by the above, there arises a problem that a ripple appears in a pass band of the filter and sufficient performance cannot be obtained. The in-band ripple needs to be 1 dB or less for practical use, but this is not achieved by the conventional technology.

[0008] Further, although how to set the parallel capacitance value, resonance and anti-resonance frequency of the series arm resonator and the parallel arm resonator is disclosed, it is a main design parameter of the cavity surface acoustic wave resonator. It is unknown how to determine the electrode finger pitch ratio between a certain surface acoustic wave converter and a reflector. For both the series arm resonator and the parallel arm resonator, the electrode finger pitch ratio Lt / Lr is the same value smaller than 1. Is set.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave device having a ladder type filter in which ripples in a pass band are reduced by using a cavity surface acoustic wave resonator for both a series arm and a parallel arm. .

[0010]

In order to solve the above-mentioned problems, the present invention improves a surface acoustic wave device provided with a ladder-connected filter composed of a series arm resonator and a parallel arm resonator. That is, in the surface acoustic wave device according to the present invention, the series arm resonator and the parallel arm resonator are formed of a cavity type surface acoustic wave resonator, and the electrode finger pitch Lt of the surface acoustic wave converter in the series arm resonator. The ratio (Lt / Lr) between the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the parallel arm resonator is 0.967 to 1. The ratio (Lt / Lr) is substantially equal to 1.

In a surface acoustic wave device according to another aspect of the present invention, according to a preferred aspect of the present invention, the series arm resonator and the parallel arm resonator each include a cavity surface acoustic wave resonator, and The ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter in the arm resonator to the electrode finger pitch Lr of the reflector is 1, and the electrode finger pitch of the surface acoustic wave converter in the parallel arm resonator is 1. The ratio (Lt / Lr) of Lt and the electrode finger pitch Lr of the reflector is substantially equal to 1.

Further, in a surface acoustic wave device according to another aspect of the present invention, the series arm resonator and the parallel arm resonator are formed of a cavity type surface acoustic wave resonator, and the surface acoustic wave in the series arm resonator is provided. Transducer electrode finger pitch Lt
And the electrode finger pitch Lr of the reflector (Lt / Lr) is
0.984 to 1, and the ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector in the parallel arm resonator is substantially 1. .

In the surface acoustic wave device according to a preferred embodiment of the present invention, the series arm resonator and the parallel arm resonator are provided with at least 50 reflectors, and the surface acoustic wave converter and the electrode film forming the reflectors are provided. The value h / λ obtained by normalizing the thickness h with the surface acoustic wave wavelength λ at the center frequency is 0.063 (6.
3%) or more.

Further, in the surface acoustic wave device according to a preferred embodiment of the present invention, the piezoelectric substrate of the surface acoustic wave device has 36 piezoelectric substrates.
It is characterized by being a rotation Y cut X propagation lithium tantalate substrate.

With the above arrangement, no ripple occurs near the resonance frequency for the resonator connected to the series arm, and near the anti-resonance frequency for the resonator connected to the parallel arm. , And a configuration in which ripple does not occur on the high frequency side. As a result, in the case of a ladder connection configuration, ripples in the filter pass band can be reduced.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a basic configuration of a ladder type surface acoustic wave filter device according to an embodiment of the present invention. First, as shown in FIG. 1, a 36 ° rotated Y-cut lithium tantalate substrate 1 is prepared as a piezoelectric substrate. This substrate 1
It can be formed by cutting from a lithium tantalate single crystal. The series arm resonators 10a, 10b, and 10c and the parallel arm resonators 20a, 20b, and 20c are formed on the 36 ° rotated Y-cut lithium tantalate substrate 1 such that the propagation direction of the surface acoustic wave is in the crystal X-axis direction. I do.
These series arm resonators 10a, 10b, and 10c and parallel arm resonators 20a, 20b, and 20c have three stages when a set of a series arm resonator and a parallel arm resonator connected in a ladder form is one stage. They are arranged so as to constitute a ladder type surface acoustic wave filter having the above configuration. The series arm resonator includes a surface acoustic wave converter 11a and reflectors 12a and 13a disposed on both outer sides thereof, a surface acoustic wave converter 11b and reflectors 12b and 13b disposed on both outer sides thereof, and a surface acoustic wave converter. Reflector 11c and reflectors 12c arranged on both outer sides thereof
And 13c, the parallel arm resonator is composed of a surface acoustic wave converter 21a and reflectors 22a and 23a disposed on both outer sides thereof, and a surface acoustic wave converter 21b and reflectors 22b and 23b disposed on both outer sides thereof. , Surface acoustic wave converter 2
1c and reflectors 22c and 23 arranged on both outer sides thereof
c. Here, each of the series arm resonator and the parallel arm resonator constitutes a cavity surface acoustic wave resonator in which a resonance cavity is formed using reflectors on both outer sides of the surface acoustic wave converter. . The surface acoustic wave filter thus obtained has terminals located on both outer sides of the three-stage series arm resonator, that is, the surface acoustic wave converter 11.
a and one terminal of the surface acoustic wave converter 11c are used as input / output terminals, and the surface acoustic wave converter 21a in the parallel arm resonators 20a, 20b, 20c is connected to the other terminal of the three-stage series arm resonator. One terminal of each of 21b and 21c is connected, and the other terminal is grounded. In this way, a ladder-type surface acoustic wave filter having a three-stage configuration in which the set of the series arm resonator and the parallel arm resonator is one stage is formed.

FIG. 2 shows a cavity type surface acoustic wave resonator of the series arm resonator and the parallel arm resonator shown in FIG.
Represents the pitch of the electrode fingers of the surface acoustic wave converter, and Lr represents the pitch of the electrode fingers of the reflector. The electrode film thickness of the three-stage ladder type surface acoustic wave filter in this embodiment is Al-0.
The composition of the 5 wt% Cu alloy 0.4 μm and the series arm resonator is as follows.
The logarithm of the surface acoustic wave converter is 85.5 pairs, the electrode finger pitch of the surface acoustic wave converter is 2.2 μm (0.5λ, λ: surface acoustic wave wavelength), the intersection width is 60 μm, and the surface acoustic wave converter The gap between the reflectors is 2.2 μm, and the number of electrode fingers of the reflector is 75
Book. The configuration of the parallel arm resonator is such that the logarithm of the surface acoustic wave converter is 85.5 pairs, and the electrode finger pitch of the surface acoustic wave converter is 2.298 μm (this corresponds to the resonance frequency of the series arm resonator and the parallel arm resonator). , The intersection width is 116 μm, the gap between the surface acoustic wave converter and the reflector is 2.298 μm, and the number of electrode fingers of the reflector is 75.
Book. Since the resonance frequency of the series arm resonator becomes the center frequency of the filter, the surface acoustic wave wavelength at this time is λ =
2.2 × 2 = 4.4 μm, and when the electrode film thickness is represented by a normalized film thickness, h / λ = 9.0%.

FIG. 3 shows the transmission characteristics of the three-stage ladder type surface acoustic wave filter of the above embodiment (the horizontal axis is frequency, the center is 825 MHz, and the span is 100 MHz, ie, 10 M
Hz / Div. The vertical axis represents the voltage transmission characteristic S21 from the input to the output, the upper end is 0 dB, and the span is 5 dB, that is, 0.5.
dB / Div. ). FIG. 3A shows a ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the serial arm resonator (Lt / Lr) = 1.000,
Ratio (Lt / Lr) of electrode finger pitch Lt of surface acoustic wave converter and electrode finger pitch Lr of reflector in parallel arm resonator =
This is an example in the case of 1.000. The ripple in the pass band (the difference between the maximum value and the minimum value in the pass band) is 0.
It is suppressed as small as 7 dB. FIG. 3B shows a ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the serial arm resonator (Lt / Lr) = 0.9.
84, the ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the parallel arm resonator
This is an example when r) = 1.000. The in-band ripple is suppressed as low as 0.6 dB. FIG.
(C) is the ratio (Lt) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the series arm resonator.
/Lr)=0.967, the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch L of the reflector in the parallel arm resonator
This is an example when the ratio of r (Lt / Lr) = 1.000. The in-band ripple is kept as low as 0.9 dB.

FIG. 3D shows a comparative example, in which the ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector in the serial arm resonator (Lt / Lr) = 0.967, and the parallel connection. Electrode finger pitch L of surface acoustic wave converter in arm resonator
Ratio of t to electrode finger pitch Lr of reflector (Lt / Lr) =
Other conditions are the same as above when 0.967 is set. (The ratio of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector in the parallel arm resonator is out of the range of the present invention) A large ripple exceeding 1.0 dB appears in the center of the band.

FIG. 4 shows that the electrode finger pitch Lt of the surface acoustic wave converter is 2.2 μm, and the ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector.
r) is 1.000, and the dimensions and structure such as the crossing width show the resonator performance of the example in the same cavity type surface acoustic wave resonator as that described above. FIG. 4A shows the impedance and phase frequency characteristics of the cavity surface acoustic wave resonator, and FIG. 4B shows the reflection coefficient of the reflector and the conductance peak of the surface acoustic wave converter at the resonance point.
From these examples, when the ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector is 1.000, the resonance point is located at the lower frequency side end of the reflector stop band. To position. Therefore, a wide stop band is obtained from the resonance point to the anti-resonance point, the resonance frequency is at the lower end of the filter band, and the anti-resonance frequency is at the filter center frequency.

FIG. 5 shows a comparative example, in which the ratio (Lt / Lt) of the electrode finger pitch Lt of the surface acoustic wave transducer to the electrode finger pitch Lr of the reflector is shown.
The resonator performance when Lr) is set to 0.957 is shown. 5A shows the frequency characteristics of the impedance and phase of the resonator, and FIG. 5B shows the reflection coefficient of the reflector and the conductance peak of the surface acoustic wave converter at the resonance point (position of the resonance frequency). Is shown. As is clear from this comparative example, the resonance point is represented by the ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector.
Although it is preferable to be located substantially at the center of the reflector stop band, the anti-resonance point is shifted to the high frequency side from the reflector stop band (the main lobe portion where the reflection coefficient is almost 1), and thus ripples appear near the anti-resonance point. I understand. Therefore, when the resonator having this pitch ratio (Lt / Lr) is used as a series arm resonator of a ladder type surface acoustic wave filter,
Ripple appears at the upper end of the band, while when used as a parallel arm resonator, ripple appears near the center frequency of the band.

FIG. 6 shows a comparative example in which the ratio (Lt / Lt) of the electrode finger pitch Lt of the surface acoustic wave transducer to the electrode finger pitch Lr of the reflector is shown.
The resonance performance when Lr) is set to 1.024 is shown. FIG.
FIG. 6A shows the impedance and phase frequency characteristics of the resonator, and FIG. 6B shows the reflection coefficient of the reflector and the conductance peak of the surface acoustic wave converter at the resonance point. As is clear from the comparative example, this pitch ratio (Lt / Lr)
In the above resonator, the resonance point deviates from the reflector stop band, and a ripple is generated between the resonance frequency and the antiresonance frequency, which is not preferable. Therefore, this configuration is not suitable as a parallel arm resonator or a series arm resonator.

FIG. 7 shows the resonance / anti-resonance peak-to-valley ratio q (anti-resonance impedance Za and resonance resistance R0, defined by q = 20 log (Za / R0)) representing the performance of the resonator, and the surface acoustic wave. This is an example showing the relationship between the electrode finger pitch Lt of the converter and the electrode finger pitch Lr of the reflector (Lt / Lr) (the conditions other than those exemplified are the same as those described above).
According to this embodiment, the ratio (Lt / Lr) of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector is 0.967 or more, and the resonance / anti-resonance peak-to-valley ratio q increases. When q approaches 50 dB and the pitch ratio (Lt / Lr) between the surface acoustic wave converter and the reflector is 0.98 or more, the peak / valley ratio q of resonance / anti-resonance is almost saturated. This pitch ratio (Lt /
When Lr) is less than 0.967, the anti-resonance frequency shifts to a higher frequency beyond the reflector stop bandwidth, so that resonance
The anti-resonance peak-to-valley ratio q becomes noticeably smaller. A preferred ratio of the electrode finger pitch Lt of the surface acoustic wave converter to the electrode finger pitch Lr of the reflector (Lt / Lt /) is a cavity surface acoustic wave resonator used as a series arm resonator of a ladder type surface acoustic wave filter.
The range of Lr) is 0.967 ≦ Lt / Lr ≦ 1.000
Was confirmed to be preferable.

From the above examples and comparative examples, the optimum electrode finger pitch L of the surface acoustic wave converter as a parallel arm resonator is obtained.
The ratio (Lt / Lr) between t and the electrode finger pitch Lr of the reflector is:
It is approximately 1, and the series arm resonator may be 0.967 to 1.000.

FIG. 8 shows that the electrode finger pitch Lt of the surface acoustic wave converter of the cavity type surface acoustic wave resonator is 2.2 μm (=
0.5λ, λ: surface acoustic wave wavelength), intersection width W is 60μ
m, the gap Gtr between the surface acoustic wave transducer and the reflector is 0.5
[lambda], the electrode finger pitch Lr of the reflector is 0.5 [lambda], the electrode thickness of the surface acoustic wave converter and the reflector is 0.35 [mu] m, and the material is an Al-0.5 wt% Cu alloy formed by sputtering. Other conditions are the same as those described above.) Examples of frequency characteristics of impedance when the number Nref of electrode fingers of the reflector is 0, 5, and 50 are shown. The peak-to-valley ratio q of the resonance is improved as the anti-resonance impedance particularly increases as the number Nref of electrode fingers of the reflector increases.

In FIG. 9, the other conditions are the same as above, and the number Nref of electrode fingers of the reflector is set to 0, 5, 10, 20, 50, 1
Examples of the resonance peak-to-valley ratio q and resonance resistance of the resonator when the number is changed to 00, 150, and 200 lines will be described. FIG. 4A shows the q value, and FIG. 4B shows the resonance resistance. From these figures,
It has been clarified that the resonator performance can be improved by setting the number of electrode fingers Nref of the reflector to 50 or more. In this experiment, each q value and resonance resistance value were measured for seven samples, and the average value was obtained.

Next, an example of the thickness of the electrode finger constituting the surface acoustic wave converter and the reflector of the resonator will be described.
The number of electrode fingers Nref of the reflector is set to 50, and the pitch of the surface acoustic wave converter of each resonator is adjusted so that the anti-resonance frequency of the parallel arm resonator and the resonance frequency of the series arm resonator substantially match. FIG. 10 shows the measurement results of the frequency characteristics of the three-stage ladder type surface acoustic wave filter in the case where the structure and dimensions are the same as those of the resonator described above. FIG. 10 (a)
The electrode film thickness is 0.25 μm, that is, the normalized film thickness h / λ =
In the case of 5.7%, FIG.
5 μm, that is, the normalized film thickness h / λ = 8.0%. In the figure, occurrence of ripple due to a spurious signal is seen at a position indicated by an arrow. At the normalized film thickness of 5.7% in FIG. 10 (a), it can be seen that this spurious also exists in the filter band. However, in FIG. 10 (b), the spurious in the band disappears, and it is good. Characteristics are obtained. Therefore, in this example, it was clarified that in-band spurious can be removed by increasing the electrode film thickness.

Further, the film thickness was changed up to h / λ = 10.5%. In this range, no remarkable increase in loss was observed and good characteristics were obtained. Specifically, in a cavity surface acoustic wave resonator having a normalized film thickness h / λ = 10.5%, a sufficiently low resonance resistance of 1.7Ω was obtained. When the electrode film thickness was further increased, fine processing could not be performed with high precision due to an increase in the film thickness with respect to the electrode width, so-called aspect ratio.

The cause of the in-band spurious and the method of removing it will be further described with reference to FIGS. FIG.
In FIG. 1, a series arm resonator and a parallel arm resonator are shown together with the frequency characteristics of their impedances. The vertical axis represents the absolute value of the impedance, and the horizontal axis represents the frequency. Each peak position of the impedance curve is given the reference sign as follows. Frp: resonance frequency of the parallel arm resonator, Fap: anti-resonance frequency of the parallel arm resonator, Fsp: frequency at which spurious is generated by the parallel arm resonator, Frs: resonance frequency of the series arm resonator, Fas: resonance frequency of the series arm resonator Anti-resonance frequency, Fss: frequency at which spurious emission occurs in the series arm resonator. In the ladder type surface acoustic wave filter, the resonance frequency Frp of the parallel arm resonator forms a pole on the low frequency side of the band,
The anti-resonance frequency Fas of the series arm resonator forms a pole on the band high frequency side. Therefore, Frp to Fas are called bands. The anti-resonance frequency Fap of the parallel arm resonator and the resonance frequency Frs of the series arm resonator are set to be substantially equal, and this point is the center frequency of the filter. From FIG. 11, spurious signals appear inevitably on the high frequency side of both the parallel arm resonator and the series arm resonator. Among them, the spurious position Fsp of the parallel arm resonator exists near the band of the filter, and the spurious in the band It turns out that it becomes. The distance between the anti-resonance frequency Fap and the spurious frequency Fsp of the parallel arm resonator is D
It can be easily understood from FIG. 11 that if the interval D is equal to or more than half the bandwidth of the filter, the in-band spurious can be removed. More practically, the interval D may be at least half of the required pass bandwidth (may be inside Fas).

Consider the value of the interval D described above. In mobile communications such as mobile phones, the maximum required fractional bandwidth (the value obtained by dividing the bandwidth by the center frequency) is 4.
3%. Specifically, in the European PCN system, the pass band is 1710 to 1785 MHz and the center frequency is 1747.5 MHz. In consideration of the fluctuation due to temperature, a specific bandwidth of 4.7% or more is required (the temperature coefficient of lithium tantalate is set to 40 ppm / ° C.). Accordingly, the required value of the interval D is 2.35% or more. This is 22.3 when read again in the frequency band 950 MHz band of the present embodiment.
MHz or higher.

The distance D can be controlled by the thickness of the electrode. FIG. 12 shows a change in the interval D with respect to the normalized film thickness of the electrode. As is clear from FIG. 12, in this embodiment,
It became clear that the normalized film thickness had to be 6.3% or more.

In the above embodiment, the case where a 36 ° rotated Y-cut lithium tantalate is used as the piezoelectric substrate has been described as a preferred example. However, a cut which can use the same surface acoustic wave mode in the lithium tantalate substrate can be used.
For example, the effects of the present invention can be exerted on a 40 ° rotation Y-cut substrate and a 42 ° rotation Y-cut substrate.

[0033]

As described above, according to the embodiment,
In the ladder type surface acoustic wave filter, if the cavity type surface acoustic wave resonators used in the series arm and the parallel arm are configured according to the present invention, good filter characteristics with reduced in-band spurious can be obtained.

[Brief description of the drawings]

FIG. 1 shows a basic configuration of a surface acoustic wave device provided with a ladder-type filter of the present invention.

FIG. 2 shows a cavity surface acoustic wave resonator used in the surface acoustic wave device of the present invention.

FIGS. 3A, 3B and 3C show transmission characteristics of a three-stage ladder type surface acoustic wave filter according to the present invention. (D)
Shows transmission characteristics of the three-stage ladder type surface acoustic wave filter of the comparative example.

FIG. 4 shows (a) impedance characteristics and (b) a resonance position with respect to a reflection coefficient in the cavity surface acoustic wave resonator of the present invention.

FIG. 5 shows (a) impedance characteristics and (b) a resonance position with respect to a reflection coefficient in the cavity surface acoustic wave resonator of the present invention.

FIG. 6 shows (a) impedance characteristics and (b) a resonance position with respect to a reflection coefficient in the cavity surface acoustic wave resonator of the present invention.

FIG. 7 shows a pitch ratio (Lt / L) between the surface acoustic wave converter and the reflector in the cavity surface acoustic wave resonator of the present invention.
The relationship between r) and the peak / valley ratio q of resonance / antiresonance is shown.

FIG. 8 shows a change in impedance characteristic with respect to the number of electrode fingers of the reflector in the cavity surface acoustic wave resonator of the present invention.

FIG. 9 shows (a) a peak-to-valley ratio q of resonance with respect to the number of electrode fingers of the reflector in the cavity surface acoustic wave resonator of the present invention.
(B) shows a change in resonance resistance.

FIG. 10 shows frequency characteristics of a ladder type surface acoustic wave filter.

FIG. 11 shows a positional relationship among a resonance frequency, an anti-resonance frequency, and a spurious frequency of a series arm and a parallel arm resonator.

FIG. 12 shows a relationship between an interval between an anti-resonance frequency and a spurious frequency and a normalized film thickness in a cavity surface acoustic wave resonator.

[Description of Signs] 1 Piezoelectric substrate 10a, 10b, 10c, 20a, 20b, 20c Cavity type surface acoustic wave resonator 11a, 11b, 11c, 21a, 21b, 21c Surface acoustic wave converter 12a, 13a, 12b, 13b , 12c, 13c, 2
2a, 23a, 22b, 23b, 22c, 23c reflector

Claims (5)

[Claims] 1. A surface acoustic wave device comprising a ladder-connected filter consisting of a series arm resonator and a parallel arm resonator, wherein the series arm resonator and the parallel arm resonator have a cavity surface acoustic wave. And a ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the series arm resonator.
r) is 0.967 to 1, and the ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave transducer to the electrode finger pitch Lr of the reflector in the parallel arm resonator.
r) is approximately 1, wherein the surface acoustic wave device is characterized in that: 2. A surface acoustic wave device comprising a ladder-connected filter consisting of a series arm resonator and a parallel arm resonator, wherein the series arm resonator and the parallel arm resonator have a cavity surface acoustic wave. And a ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the series arm resonator.
r) is 1, and the ratio (Lt / L) between the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the parallel arm resonator is 1.
r) is approximately 1, wherein the surface acoustic wave device is characterized in that: 3. A surface acoustic wave device comprising a ladder-connected filter consisting of a series arm resonator and a parallel arm resonator, wherein the series arm resonator and the parallel arm resonator have a cavity surface acoustic wave. And a ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave converter and the electrode finger pitch Lr of the reflector in the series arm resonator.
r) is 0.984 to 1, and the ratio (Lt / L) of the electrode finger pitch Lt of the surface acoustic wave transducer to the electrode finger pitch Lr of the reflector in the parallel arm resonator.
r) is approximately 1, wherein the surface acoustic wave device is characterized in that: 4. The surface acoustic wave device according to claim 1, wherein the series arm resonator and the parallel arm resonator are provided with at least 50 reflectors. The value h / λ obtained by normalizing the thickness h of the electrode forming the surface acoustic wave converter and the reflector with the surface acoustic wave wavelength λ at the center frequency is 0.063 (6.3%) or more. A surface acoustic wave device characterized by the above-mentioned. 5. The surface acoustic wave device according to claim 4, wherein the piezoelectric substrate of the surface acoustic wave device is a 36 ° rotation Y-cut X-propagation lithium tantalate substrate. Surface acoustic wave device.