JP3327433B2 - 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 surface acoustic wave filter which excites a surface acoustic wave on a substrate made of a piezoelectric material and selectively extracts a desired frequency band. The present invention relates to a ladder type surface acoustic wave filter suitable for a filter.

[0002]

2. Description of the Related Art In recent years, the demand for a surface acoustic wave filter has been increasing in accordance with the reduction in size and weight of mobile communication devices, and the surface acoustic wave filter has excellent characteristics of not only low loss but also large out-of-band attenuation. Is required. In particular, ladder-type surface acoustic wave filters have attracted recent attention because they are more advantageous in reducing loss compared to multi-electrode surface acoustic wave filters in which a plurality of input and output electrodes are alternately arranged in the direction of surface acoustic wave propagation. Have been.

A conventional ladder type surface acoustic wave filter is disclosed in, for example, Japanese Patent Laid-Open Publication No.
No. 183380. JP-A-52-1
The ladder type surface acoustic wave filter described in Japanese Patent No. 9044 realizes a narrow-band filter characteristic having a sufficiently narrow pass band width and a sufficiently large out-of-band attenuation. For this purpose, the ladder-type surface acoustic wave filter described in the above-mentioned document has a surface acoustic wave resonance filter in which each of a series arm and a parallel arm is provided on a substrate made of a piezoelectric material (hereinafter referred to as a piezoelectric substrate). And a configuration in which the resonance frequency of the series arm resonator and the antiresonance frequency of the parallel arm resonator are equal, and the frequency is the center frequency of the pass band. Further, by making the equivalent parallel capacitance of the parallel arm resonator larger than the equivalent parallel capacitance of the series arm resonator and making the equivalent parallel capacitance ratio larger than 1, the bandwidth is sufficiently narrow and the attenuation outside the band is sufficiently large. Narrow band filter characteristics are realized.

In a ladder type surface acoustic wave filter described in Japanese Patent Application Laid-Open No. 5-183380, an inductance is additionally connected in series to a parallel arm resonator to reduce the anti-resonance frequency of the parallel arm resonator to a low frequency. To the side. The bandwidth of the filter characteristic can be controlled by the frequency difference between the resonance frequency and the antiresonance frequency of the parallel arm resonator.
The frequency difference between the anti-resonance frequencies is widened to widen the band.

[0005]

However, in the configuration described in Japanese Patent Application Laid-Open No. 52-19044 for realizing a narrow band filter, the out-of-band attenuation is reduced by increasing the equivalent parallel capacitance ratio. While it can be increased, the bandwidth becomes narrower as the equivalent parallel capacitance ratio increases.

In the configuration described in Japanese Patent Laid-Open No. 5-183380, in order to ensure a large amount of out-of-band attenuation, it is necessary to increase the equivalent parallel capacitance ratio.
Alternatively, it is necessary to increase the number of ladder-type filters connected in cascade. However, since an inductance is added in series to the parallel arm resonator, the connection wiring between the surface acoustic wave resonators on the piezoelectric substrate and the electrical resistance and inductance of the wire bonding significantly affect the characteristics of the parallel arm resonator. I do. As a result, the resonance frequency of the parallel arm resonator shifts to a lower frequency side, so that a steep rise to a pass band in the filter characteristics cannot be obtained, and a satisfactory out-of-band attenuation cannot be obtained.

As described above, it is difficult for the conventional ladder type surface acoustic wave filter to secure both a wide bandwidth and a sufficient out-of-band attenuation.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems, and an object of the present invention is to provide a ladder type surface acoustic wave filter having a large out-of-band attenuation and a wide bandwidth. It is in.

[0009]

A surface acoustic wave filter according to the present invention comprises a substrate made of a piezoelectric material, an input terminal and an output terminal provided on the substrate, and the input terminal and the output terminal on the substrate. A surface acoustic wave filter comprising three basic resonator units connected in cascade between terminals, wherein each of the three basic resonator units is connected in series between the input terminal and the output terminal. A connected first and second surface acoustic wave resonator; and a third surface acoustic wave resonator connected between the first and second surface acoustic wave resonators and further grounded. And thereby achieves the objectives set forth above.

[0010]

A surface acoustic wave filter according to the present invention comprises a substrate made of a piezoelectric material, an input terminal and an output terminal provided on the substrate, and the input terminal and the output terminal on the substrate. A surface acoustic wave filter comprising three basic resonator units connected in cascade between terminals, wherein each of the three basic resonator units is connected in series between the input terminal and the output terminal. comprising a first and second surface acoustic wave resonators connected, is connected between the first and second surface acoustic wave resonator, further a third surface acoustic wave resonators being grounded And two further ground terminals.
One of the two ground terminals is the three fundamental resonances
Third surface acoustic wave included in two of the detector units
It is commonly used to ground resonators, thereby achieving the above objectives.

[0011] In the surface acoustic wave filter of the present invention, the width of the wiring connecting the said <br/> first and second surface acoustic wave resonator and said third surface acoustic wave resonator is 50μm or more .

[0012] In the surface acoustic wave filter of the present invention, the area of the <br/> input terminal and said output terminal, is 0.14 square mm or less, respectively.

In one embodiment, the substrate is a 36 ° Y-cut.
X-propagating lithium tantalate substrate;
3. The IDT included in the surface acoustic wave resonator of No. 3 is made of aluminum
It is composed of a metal film containing, as a main component, nickel.

In another embodiment, the interdigital transducer comprises
The thickness of the metal film to be formed is equal to the thickness of the third surface acoustic wave resonator.
8% or more and 10% or less of the electrode period of the interdigital electrode
Is the value within

In still another embodiment, the first and the second
The capacitance of the interdigital transducer of the surface acoustic wave resonator is Cs,
The capacitance of the interdigital transducer of the third surface acoustic wave resonator
Is set to Cp, a set of values of the Cs and Cp (C
s, Cp) is plane coordinates using the Cs and Cp as coordinate axes.
In the area (including the boundary) surrounded by the coordinates of the following formula in the system
Yes, A: (Cs, Cp) = (2.00,8.00) B: (Cs, Cp) = (2.75,5.09) C: (Cs, Cp) = (4.32,8 .00) The center frequency value is in the range of 800 MHz to 1000 MHz.
The value in the box.

[0016]

[0017]

According to the above configuration, the ripple in the pass band is small, the attenuation outside the band is large, and the frequency characteristic in a wide band is realized.

The 36 ° Y-cut X-propagating lithium tantalate substrate has physical characteristics suitable for an RF filter for mobile communication in terms of the speed of elastic waves propagating through the substrate, the electromechanical coupling coefficient, the temperature characteristics, and the like. ing. A metal film containing aluminum as a main component has a low electric resistance and a low density, and is effective in obtaining desired good filter characteristics.

By controlling the thickness of the metal film constituting the IDT included in the surface acoustic wave resonator, the bandwidth and the amount of out-of-band attenuation can be controlled, and the ripple in the pass band can be reduced. . Further, by setting the capacitance of the IDT to an appropriate value, the R for mobile communication can be improved.
The frequency characteristics required for the F filter are satisfied.

By appropriately setting the arrangement of the ground terminals and the width of the connection wiring, the influence of the resistance component and the inductance component due to the wiring on the frequency characteristics is reduced. Further, by setting the area of the input / output terminal to an appropriate value, it is possible to reduce the influence of the ground capacitance generated at that portion.

[0021]

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

FIG. 1 schematically shows the configuration of a surface acoustic wave resonator 10 used in the present invention. Surface acoustic wave resonator 10
Is a single interdigital electrode (hereinafter referred to as IDT) 2 provided on the piezoelectric substrate 1 and two reflectors provided on the piezoelectric substrate 1 so as to sandwich the IDT 2 from both sides. 3 is provided. The IDT 2 includes an input electrode and an output electrode, and the input / output terminal 4 is connected to each of them. When the electrode period of the IDT 2 is L (see FIG. 1), a high-frequency signal is applied from the terminal 4 to the IDT 2, whereby a surface acoustic wave having a wavelength of approximately L is excited by the IDT 2. The excited surface acoustic wave is confined between the reflectors 3 and becomes a standing wave, causing a resonance phenomenon. The surface acoustic wave resonator 10 utilizes the elastic wave resonance phenomenon.

FIG. 2 is a configuration diagram of the ladder type surface acoustic wave filter 20 according to the embodiment of the present invention. In the ladder type surface acoustic wave filter 20, the series arm resonators 22a to 22f and the parallel arm resonators 23a to 23f are formed on the piezoelectric substrate 21.
23c is provided. Individual resonators 22a to 22
f, 23a to 23c are the surface acoustic wave resonators 10 having the configuration in which the IDT 2 is sandwiched between the two reflectors 3 as described above with reference to FIG. In the following description, when generically referring to the series arm resonators 22a to 22f, the reference numeral 22 is used.
The same applies to other components.

The series arm resonators 22 are connected in series, of which the first and last resonators 22a and 22f are connected.
Are connected to the input terminal 24a and the output terminal 24b, respectively. Each of the parallel arm resonators 23 is connected between the series arm resonators 22 paired by two by a wiring 26 and further connected to a ground terminal 25 to be grounded. Although not shown, the input / output terminal 24 and the ground terminal 25 are connected to wires by wire bonding.

In the above configuration, two series arm resonators 22 and one parallel arm resonator 23 are connected in a T-shape to form a basic unit 27, and a total of three stages of basic units 27a to 27a are provided. 27c are cascaded. That is, the series arm resonators 22a and 22b and the parallel arm resonator 23a form a first basic unit 27a, and similarly, the series arm resonators 22c and 22d and the parallel arm resonator 23b form the second basic unit 27b. The series arm resonators 22e and 22f and the parallel arm resonator 23c constitute a third basic unit 27c.

In the configuration shown in FIG. 2, among the ground terminals 25 included in each basic unit 27, the ground terminal 25a of the first basic unit 27a is provided independently, and the second and third basic units 27a are provided independently. One common ground terminal 25b is provided for 27b and 27c. This is for the following reason.

As described above, the wire bonding wiring is connected to the ground terminal 25. Such a wiring necessarily has an inductance, and the inductance affects filter characteristics. If the ground terminals 25 are provided independently for each of the basic units 27, the wire bonding is also performed for each of the ground terminals 2.
5 are performed individually, and as a result, the value of the additionally connected inductance is different for each basic unit 27. Therefore, in order to obtain desired filter characteristics, it is necessary to make all the ground terminals 25 of the respective basic units 27 common, and to make the influence of the inductance of the wire bonding wiring on the filter characteristics uniform for each of the basic units 27. desirable.

However, on the other hand, when all the ground terminals 25 are shared as described above, the wiring pattern on the piezoelectric substrate 21 becomes complicated and the wiring becomes long. The complexity of the wiring pattern is disadvantageous in facilitating the manufacturing process and reducing costs. Further, an increase in the wiring length means an increase in the resistance component, which is a problem in obtaining desired filter characteristics.

Therefore, considering the factors having a trade-off relationship as described above, in the ladder type surface acoustic wave filter 20 of this embodiment in which the three basic units 27 are cascaded as shown in FIG. A common ground terminal 25 for two of the three basic units 27,
It is desirable to place the other one independently. In the configuration of FIG. 2, the ground terminal 25 for the first basic unit 27a is provided.
a are independently arranged, and the second and third basic units 27
Although the ground terminal 25b for b and 27c is common, the configuration is not limited to the above, and other combinations can be made according to the design of the entire filter. For example, the ground terminals for the third basic unit 27c are independently arranged, and the first and second basic units 27c are provided.
A common ground terminal for a and 27b may be used.

In the ladder type surface acoustic wave filter 20 using the surface acoustic wave resonator 10, the out-of-band attenuation is determined by the capacitance Cs of the IDT of the series arm resonator 22 and the I C of the parallel arm resonator 23.
The ratio is determined by the ratio Cr (= Cp / Cs) of the DT to the capacitance Cp and the number of stages of the cascade-connected basic units 27.

In the configuration of the ladder type surface acoustic wave filter 20 of this embodiment shown in FIG.
周波 数 Frequency characteristics (frequency and attenuation) when using a Y-cut X-propagating lithium tantalate substrate and fabricating the respective resonators 22 and 23 thereon so that the capacitance ratio becomes Cr = 2.5 3) is shown in FIG. As is clear from FIG. 3, in this case, an out-of-band attenuation of -50 dB can be obtained.

On the other hand, in the configuration of the conventional ladder-type surface acoustic wave filter in which the series arm resonators and the parallel arm resonators are alternately connected, a band of −50 dB is set in the same manner as described above with the capacitance ratio Cr = 2.5. FIG. 4 shows frequency characteristics when the external attenuation is secured. In the conventional configuration, in order to obtain the characteristic shown in FIG. 4, a combination of a series arm resonator and a parallel arm resonator is required in a total of four stages (a total of four series arm resonators and four parallel arm resonators). Resonator) must be connected.
However, in such a connection, since the number of parallel arm resonators is larger than in the configuration of the present embodiment, it is equivalently connected to each parallel arm resonator by connection wiring between the resonators and wire bonding or the like. The effect of the inductance on the filter characteristics appears significantly. As a result, as shown in FIG. 4, a steep rise to the pass band is not obtained in a frequency range lower than the pass band, and spurs also appear.

In the configuration of the surface acoustic wave filter 20 according to the present embodiment as shown in FIG. 2, a sufficient out-of-band attenuation is ensured without the above-mentioned problems of the prior art.
In the above description, a 36 ° Y-cut X-propagating lithium tantalate substrate is used as the piezoelectric substrate. However, a similar effect can be obtained by using a lithium niobate substrate or a lithium borate substrate. it can. However,
In order to obtain the best characteristics in an RF filter for mobile communication, a 36 ゜ Y-cut X-propagating lithium tantalate substrate is most suitable in consideration of an elastic wave velocity propagating through the substrate, an electromechanical coupling coefficient, and a temperature characteristic. Are suitable.

As a material of the IDT 2 of the surface acoustic wave resonator 10, a metal film (hereinafter, referred to as an Al film) mainly containing aluminum having a low electric resistance and a low density is used. Here, since the reflection coefficient of a surface acoustic wave at the electrode end and the substantial electromechanical coupling coefficient depend on the thickness of the Al film, the thickness between the resonance and antiresonance frequencies of the resonator depends on the thickness of the Al film. The frequency difference changes. As the Al film becomes thicker, the substantial electromechanical coupling coefficient becomes larger, and the above frequency difference becomes larger. Therefore, as the thickness of the Al film increases, the bandwidth of the filter can be increased.

In general, in mobile communication, there is a reception band and a transmission band, so that an RF filter having a good cutoff characteristic is required. For example, there is a domestic analog specification in Japan as an example of the 800 MHz band mobile communication specification. This specification specifies that the receiving band is 860MHz to 885M
Hz, the transmission band is 915 MHz to 940 MHz, and the interval between the reception band and the transmission band is 30 MHz.
Hereinafter, the characteristics of the surface acoustic wave filter 20 according to the present embodiment will be further described by taking a case where the filter is used as a reception filter of the domestic analog specification as an example.

FIG. 5 shows a ladder type surface acoustic wave filter 20 according to the present embodiment in which the structure shown in FIG. 2 is provided on a 36 ° Y-cut X-propagating lithium tantalate substrate. 5, the normalized Al film thickness (Al film thickness / IDT electrode period of the parallel arm resonator 23) and the specific bandwidth (bandwidth / center frequency), and the normalized Al film thickness and out-of-band in the transmission band. It is a relation figure with attenuation. Note that the bandwidth is 1d from the minimum insertion loss in the pass band.
B is defined in the point that it has fallen.

FIG. 5 shows that the bandwidth increases as the Al film thickness increases. The reception filter of the domestic analog specification has a center frequency of 872.5 MHz and a bandwidth of 25.
MHz, and the specification is satisfied if the relative bandwidth is 3% or more. Therefore, it is sufficient that the Al film thickness is 8% or more in terms of the normalized Al film thickness.

FIG. 5 shows that, as the Al film thickness increases, the out-of-band attenuation in the transmission band decreases. The cause will be described with reference to FIG. FIG.
4 shows the effect of the Al film thickness on the frequency characteristics of the ladder type surface acoustic wave filter 20 according to the embodiment of the present invention. However, the Al film thickness is shown as a normalized Al film thickness, and 8
The characteristics for%, 9% and 10% are depicted respectively. As a result, when the thickness of the Al film is increased, the pass band is widened and the band is widened. However, since the frequency difference between the stop bands on both sides of the pass band is increased, the cutoff characteristic is deteriorated and the out-of-band attenuation is rapidly reduced. It turns out that it becomes.
As shown in FIG. 6, the transmission band corresponds to a slightly higher frequency region than the pass band. However, since the side lobe from the pass band to the high frequency region enters the transmission band due to the above-described reason, this portion is not sufficient. Attenuation cannot be secured, which may cause noise. In order to solve the above problem, it is desirable that the normalized Al film thickness be 10% or less.

As described above, the optimum Al film thickness is 8% or more and 10% or less in terms of the normalized film thickness. In addition, other 800MHz
Band specifications, eg AMPS in North America, GSM in Europe
In such a case as well, although the value of the center frequency is different, the same filter characteristics as those of the above-mentioned domestic analog are required. Also in this case, by setting the normalized Al film thickness to a value within the above range, similarly good filter characteristics can be obtained.

FIG. 7 shows a ladder type surface acoustic wave filter 20 of this embodiment in which the structure shown in FIG. 2 is provided on a 36 ° Y-cut X-propagating lithium tantalate substrate 21.
The relationship between the normalized Al film thickness and the ripple in the pass band when the fractional bandwidth required for the RF filter for mobile band communication in the Hz band is 3.7% is shown. From this, standardized Al
It is clear that the ripple in the pass band takes a minimum value when the film thickness is about 9%. Furthermore, if the normalized Al film thickness is in the range of 8% or more and 10% or less, the ripple in the pass band can be reduced to 1.5 dB or less. Therefore, the range of the normalized Al film thickness described above is also effective in reducing the ripple in the pass band.

FIG. 8 shows a case where the configuration of the ladder type surface acoustic wave filter 20 of this embodiment is manufactured on a 36 ° Y-cut X-propagation lithium tantalate substrate 21 so that the normalized Al film thickness becomes 9%. , The IDT capacitance Cs of the series arm resonator 22 and the IDT capacitance Cp of the parallel arm resonator 23
And the relationship between the attenuation and the out-of-band attenuation. This is described in IEE Transactions, MT20 (7) (1972), 458-471 (IEEE Trans. MTT20 (7) (197)).
2) Simulation based on an equivalent circuit obtained by adding a slight correction to the Smith second model based on the description in pp. 458-471).

In general, parameters affecting the filter characteristics include the in-band VSWR (voltage standing wave ratio) in addition to the above-mentioned out-of-band attenuation and the specific bandwidth.
Here, the in-band VSWR is a parameter representing the reflection characteristic of the filter, and when this value is 1, it means that there is no reflection. Therefore, the closer the in-band VSWR is to one,
Good characteristics with little reflection.

The specification of the characteristics required for an actual filter depends on the design of the device in which each filter is used. In general, the out-of-band attenuation is -45 dB or more, and the specific bandwidth is 3% or more. If the in-band VSWR (voltage standing wave ratio) is 2.5 or less, required specifications can be realized. The above range in FIG. 8 corresponds to a region (including a boundary) surrounded by the coordinates of the three points in the following equation.

A: (Cs, Cp) = (2.00, 8.00) B: (Cs, Cp) = (2.75, 5.09) C: (Cs, Cp) = (4.32, 8.00) From this, if the ladder-type surface acoustic wave filter 20 is configured such that Cs and Cp fall within the above ranges, 80
Satisfies the frequency characteristics required for a 0 MHz band mobile communication RF filter.

FIG. 9 shows that the IDT capacitance of each of the resonators 22 and 23 is the value at the point T in FIG. 8, that is, (Cs, Cp).
= Frequency characteristics of a ladder type surface acoustic wave filter 20 manufactured to satisfy (3.1, 6.5). Specifically, the electrode period of the IDT of the series arm resonator 22 is set to 4.408.
μm, the intersection width is 55.10 μm, the logarithm is 120 pairs, and the electrode period of the IDT of the parallel arm resonator 23 is 4.612.
μm, the intersection width is 138.36 μm, and the logarithm is 100 pairs. At this time, the center frequency of the pass band is 872.5
MHz, the bandwidth is 31 MHz, and the in-band VSWR is 2.2.
, And the out-of-band attenuation in the transmission band is -50 dB, and a frequency characteristic in which the out-of-band attenuation is large and the ripple in the pass band is small is obtained.

The surface acoustic wave filter 20 includes a resonator 22,
The resonance characteristics vary depending on the electrical resistance and inductance of the connection wiring 26 between the wirings 23 and the wire bonding wiring. In particular, when the inductance equivalently connected to the resonator increases, the resonance frequency of the resonator shifts to a lower frequency side. However, since the resonance frequency of the parallel arm resonator 23 is a stop band on the lower frequency side of the pass band, if the inductance is large, the cutoff characteristics of the ladder type surface acoustic wave filter 20 deteriorate, and the pass band in the frequency characteristics is reduced. No steep rise to is obtained. Also, when the electric resistance existing in relation to the parallel arm resonator 23 increases, the resonance resistance at the resonance frequency increases, and the stop band disappears.

In particular, the connection wiring 26 from the series arm resonator 22 to the parallel arm resonator 23 shown in FIG.
0.4 μm in thickness to produce an Al film at the same time as the electrodes
m, while a length of 500 μm or more is required to connect to the parallel arm resonator 23. Therefore, in order to reduce the electric resistance and inductance of the connection wiring 26, it is necessary to increase the width.

FIG. 10 shows the configuration of FIG. 2 in which the width of the connection wiring 26 and the frequency of 30 MHz
It is a relation diagram with the amount of attenuation in a distant frequency value. The length of the connection wiring 26 is 500 μm. The width of the connection wiring 26 is 5
If the width is larger than 0 μm, the attenuation converges to 35 dB, but if the width of the connection wiring 26 is smaller than 50 μm, the inductor becomes large and the attenuation rapidly decreases. Therefore,
The width of the connection wiring 26 needs to be at least 50 μm.

On the other hand, at the input / output terminal 24, a ground capacitance is generated across the substrate 21. As the capacitance to the ground at the input / output terminal 24 increases, the ripple in the pass band increases. In an RF filter for mobile communication, the ripple in the pass band is preferably as small as possible, and the allowable ripple in the pass band is usually 1.5 dB or less.

FIG. 11 shows the relationship between the area per input / output terminal 24 and the ripple in the pass band. Than this,
When the area of the input / output terminal 24 is equal to or less than 0.14 square mm, the value of the in-passband ripple is equal to or less than 1.5 dB. However, the input / output terminal 24 has a side length of 150 μm for connecting wiring by wire bonding.
m or more.

As described above, the ladder type surface acoustic wave filter 20 according to the configuration of the present embodiment has a small ripple in the pass band and a sufficient amount of attenuation outside the band as compared with the conventional ladder type surface acoustic wave filter. Excellent frequency characteristics can be realized.

In the description of the present embodiment, the basic units 27 of the ladder-type surface acoustic wave filter 20 are cascade-connected in a total of three stages, but this is not limited to this value.
For example, when a ladder-type surface acoustic wave filter is formed by cascading two basic units 27, the three-stage basic unit described in this embodiment can be used in the range of the electrode film thickness and the capacitance of the IDT in this embodiment. Although the out-of-band attenuation is inferior to the case where the units 27 are connected in cascade, there is no particular disadvantage in terms of bandwidth and in-band ripple.

Alternatively, the last unit of the cascade-connected basic units 27 does not have the configuration of the T-type basic unit described above, but includes a series arm resonator 22 and a parallel arm resonator 23. A configuration in which each one is connected, that is, a configuration in which one final series arm resonator 22 is omitted may be employed. In the configuration of the ladder type surface acoustic wave filter 20 of the present embodiment shown in FIG.
4 and the ground terminal 25 are both depicted as having a square shape (rectangle). However, these terminals 24,
The shape of 25 is not limited to this, and may be, for example, a circle.

[0054]

As described above, the surface acoustic wave filter of the present invention comprises two surface acoustic wave resonators connected in series,
A basic resonator unit comprising another surface acoustic wave resonator connected in parallel to them and further grounded has a configuration in which a total of three stages are connected in cascade, whereby A wideband filter having excellent frequency characteristics with small in-band ripple and sufficient out-of-band attenuation is realized.

As a substrate made of a piezoelectric material on which the surface acoustic wave filter is formed, a 36 ° Y-cut X-propagating lithium tantalate substrate is used, and the interdigital electrodes included in the surface acoustic wave resonator are made mainly of aluminum. Particularly good characteristics can be obtained as an RF filter for mobile communication by using a metal film.

Further, the thickness of the metal film forming the interdigital transducer is set to a value within a range of 8% or more and 10% or less of the electrode cycle of the interdigital transducer of the surface acoustic wave resonator connected in parallel. By doing so, it is possible to obtain good filter characteristics in which the fractional bandwidth is 3% or more, the out-of-band attenuation is -50 dB or more, and the ripple in the pass band is 1.5 dB or less.

Furthermore, by setting the capacitance of the interdigital transducer of each surface acoustic wave filter to a value within the range described in claim 4, it is generally required for an RF filter for mobile communication in the 800 MHz band. Characteristics can be satisfied.

Further, by providing a common ground terminal to two of the three basic resonator units and providing the other one independently, the manufacturing process can be simplified and the manufacturing cost can be reduced. The trade-off between realization of good desired frequency characteristics can be best satisfied.

When the width of the connection wiring between the resonators is 50 μm or more, it is effective to secure a sufficient out-of-band attenuation.

The area of the input / output terminal is 0.14 square m.
m, the magnitude of the ripple in the pass band is 1.5
dB or less.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a surface acoustic wave resonator used in an embodiment of the present invention.

FIG. 2 is a diagram schematically illustrating a configuration of a ladder type surface acoustic wave filter according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating frequency characteristics of a ladder-type surface acoustic wave filter according to an embodiment of the present invention.

FIG. 4 is a diagram showing frequency characteristics of a conventional ladder type surface acoustic wave filter.

FIG. 5 is a diagram showing the relationship between the thickness of the Al film and the relative bandwidth and the relationship between the thickness of the Al film and the out-of-band attenuation in the transmission band of the ladder-type surface acoustic wave filter according to the embodiment of the present invention. .

FIG. 6 is a diagram showing the influence of the Al film thickness on the frequency characteristics of the ladder type surface acoustic wave filter according to the embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between an Al film thickness and a ripple in a pass band of the ladder type surface acoustic wave filter according to the embodiment of the present invention.

FIG. 8 is a diagram showing the relationship between the IDT capacitance of the series arm resonator and the IDT capacitance of the parallel arm resonator of the ladder type surface acoustic wave filter and the out-of-band attenuation in the embodiment of the present invention. .

FIG. 9 is a diagram showing a frequency characteristic of the ladder type surface acoustic wave filter according to the embodiment of the present invention.

FIG. 10 is a diagram showing a relationship between a connection wiring width between resonators of the ladder type surface acoustic wave filter and an out-of-band attenuation amount in the example of the present invention.

FIG. 11 is a diagram illustrating the relationship between the area of the input / output terminal of the ladder-type surface acoustic wave filter and the ripple in the pass band according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 21 Piezoelectric substrate 2 IDT 3 Reflector 4 Input / output terminal 10 Surface acoustic wave resonator 20 Ladder type surface acoustic wave filter 22a-22f Series arm resonator 23a-23c Parallel arm resonator 24a Input terminal 24b output terminal 25a, 25b ground terminal 26 connection wiring 27a-27c basic resonator unit

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Keiji Onishi 1006 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-183380 (JP, A) JP-A-1- 290308 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 9/64 H03H 9/145 H03H 9/25

Claims (3)

(57) [Claims] 1. A substrate made of a piezoelectric material, an input terminal and an output terminal provided on the substrate, and three basic resonances connected in cascade between the input terminal and the output terminal on the substrate. A surface acoustic wave filter comprising a first unit and a first unit connected in series between the input terminal and the output terminal.
And a second surface acoustic wave resonator, and a third surface acoustic wave resonator connected between the first and second surface acoustic wave resonators and further grounded. And one of the two ground terminals is commonly used to ground the third surface acoustic wave resonator included in two of the three basic resonator units. Surface acoustic wave filter. Wherein the width of the wiring for connecting the said first and second surface acoustic wave resonator and said <br/> third surface acoustic wave resonator is 50μ
The surface acoustic wave filter according to claim 1, wherein m is not less than m. Wherein the area of said input terminals and said output terminals, bullet <br/> surface wave filter according to claim 1 respectively is less than 0.14 square mm.