JPH07176979A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To provide the ladder type surface acoustic wave filter with a large out-band attenuation and a wide band width. CONSTITUTION:Series arm surface acoustic wave resonators 22a-22f connected in series between input output terminals 24a, 24b and parallel surface acoustic wave resonators 23a-23c connected between the resonators two in pairs by a wire, connected to earth terminals 25a, 25b and connecting to ground are provided on a 36 deg. Y-cut X propagation lithium tantalate substrate 21 to form the surface acoustic wave filter 20. The two series arm resonators and one parallel arm resonator are connected in T-shape to form basic units 27a-27c and the basic units 27a-27c in three stages in total are connected in cascade. Interdigital electrodes included in each resonator are made of metallic film using aluminium as a major component and the thickness of the metallic film is selected to be 8% or over and 10% or less with respect to an electrode pitch of interdigital electrodes of the parallel arm surface acoustic wave resonators 23a-23c.

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 for exciting a surface acoustic wave on a substrate made of a piezoelectric material to selectively extract a desired frequency band, and more particularly to an RF of mobile communication equipment. 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 surface acoustic wave filters has increased along with the downsizing and weight reduction of mobile communication devices, and the surface acoustic wave filters have excellent characteristics such as low loss and large out-of-band attenuation. Is required. Among them, the ladder-type surface acoustic wave filter has recently attracted attention because it is advantageous in reducing loss as compared with a multi-electrode surface acoustic wave filter in which a plurality of input electrodes and output electrodes are alternately arranged in the surface acoustic wave propagation direction. Has been done.

A conventional ladder type surface acoustic wave filter is disclosed in, for example, Japanese Patent Laid-Open No. 52-19044 or Japanese Patent Laid-Open No. 5-19044.
No. 183380.

The ladder-type surface acoustic wave filter described in Japanese Patent Laid-Open No. 52-19044 realizes a narrow band filter characteristic having a sufficiently narrow pass band width and a sufficiently large out-of-band attenuation. Therefore, the ladder-type surface acoustic wave filter described in the above document has one surface acoustic wave resonance on each of the series arm and the parallel arm on a substrate made of a piezoelectric material (hereinafter referred to as a piezoelectric substrate). The resonator is arranged so that the resonance frequency of the series arm resonator and the anti-resonance frequency of the parallel arm resonator are equal, and the frequency is the center frequency of the pass band. Furthermore, the equivalent parallel capacitance of the parallel arm resonator is made larger than the equivalent parallel capacitance of the series arm resonator,
By setting the equivalent parallel capacitance ratio to be larger than 1, a narrow band filter characteristic having a sufficiently narrow bandwidth and a sufficiently large out-of-band attenuation is realized.

In the ladder type surface acoustic wave filter described in Japanese Patent Laid-Open No. 5-183380, an inductance is additionally connected in series to the parallel arm resonator so that the anti-resonance frequency of the parallel arm resonator is low. It is shifting to the side. Since the bandwidth of the filter characteristic can be controlled by the frequency difference between the resonance frequency of the parallel arm resonator and the anti-resonance frequency, the resonance
Broadening the band by widening the frequency difference between the anti-resonance frequencies.

[0006]

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

Further, in the above-mentioned configuration disclosed in Japanese Patent Laid-Open No. 5-183380, in order to secure a large out-of-band attenuation amount, the equivalent parallel capacitance ratio should be increased, or
Alternatively, it is necessary to increase the number of stages of ladder filters connected in cascade. However, since an inductance is added in series to the parallel arm resonator, the electrical resistance and inductance of the connection wiring and wire bonding between the surface acoustic wave resonators on the piezoelectric substrate significantly affect the characteristics of the parallel arm resonator. To do. As a result, the resonance frequency of the parallel arm resonator shifts to the low frequency side, a steep rise to the pass band in the filter characteristic 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 amount.

An object of the present invention is to solve the above problems. 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.

[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, the input terminal and the output on the substrate. A surface acoustic wave filter comprising: three basic resonator units connected in series between terminals, wherein each of the three basic resonator units is connected in series between the input terminal and the output terminal. A first and second surface acoustic wave resonators connected to each other, and a third surface acoustic wave resonator connected between the first and second surface acoustic wave resonators and further grounded. It is provided, and thereby the above-mentioned object is achieved.

In one embodiment, the substrate is a 36 ° Y-cut X-propagation lithium tantalate substrate, and the interdigital electrode included in each of the first to third surface acoustic wave resonators is a metal whose main component is aluminum. Composed of a membrane.

In another embodiment, the thickness of the metal film forming the interdigital transducer is within a range of 8% to 10% of the electrode period of the interdigital electrode of the third surface acoustic wave resonator. Is the value of.

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

In yet another embodiment, there are further two ground terminals, one of the two ground terminals being
Commonly used for grounding the third surface acoustic wave resonator included in two of the three basic resonator units, and another ground terminal is included in the remaining basic resonator units. It is used to ground the third surface acoustic wave resonator.

In yet another embodiment, the first and second
The width of the wiring connecting the surface acoustic wave resonator and the third surface acoustic wave resonator is 50 μm or more.

In still another embodiment, the areas of the input terminal and the output terminal are each 0.14 square mm or less.

[0017]

With the above structure, the ripple in the pass band is small, the amount of attenuation outside the band is large, and the wide band frequency characteristic is realized.

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

By controlling the thickness of the metal film forming the interdigital transducer included in the surface acoustic wave resonator, the bandwidth and the out-of-band attenuation amount can be controlled, and the ripple in the pass band can be reduced. . Furthermore, by setting the electrostatic capacitance of the interdigital transducer to an appropriate value, it is possible to use R for mobile communication.
The frequency characteristics required for the F filter are satisfied.

By arranging the ground terminal and appropriately setting the width of the connection wiring, the influence of the resistance component and the inductance component due to the wiring on the frequency characteristic can be 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 structure of a surface acoustic wave resonator 10 used in the present invention. Surface acoustic wave resonator 10
Is one 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. Equipped with 3. The IDT 2 is composed of an input electrode and an output electrode, to which input / output terminals 4 are connected. When the electrode period of the IDT 2 is L (see FIG. 1), by applying a high frequency signal from the terminal 4 to the IDT 2, the IDT 2 excites a surface acoustic wave having a wavelength of approximately L. The excited surface acoustic wave is confined between the reflectors 3 and becomes a standing wave, which causes a resonance phenomenon. The surface acoustic wave resonator 10 utilizes the resonance phenomenon of this elastic wave.

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

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

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

In the configuration shown in FIG. 2, among the ground terminals 25 included in each of the basic units 27, the ground terminal 25a of the first basic unit 27a is independently provided, and the second and third basic units are provided. 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 wiring necessarily has an inductance, which affects the filter characteristics. When the ground terminal 25 is independently provided for each basic unit 27, wire bonding is also performed for each ground terminal 2.
5 individually, so that the value of the additionally connected inductance is different for each basic unit 27. Therefore, in order to obtain a desired filter characteristic, all the ground terminals 25 of the respective basic units 27 should be made common to make the influence of the inductance of the wire bonding wiring on the filter characteristic uniform for each basic unit 27. desirable.

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

Therefore, in consideration of the above trade-off factors, the ladder-type surface acoustic wave filter 20 of this embodiment in which a total of three basic units 27 are connected in cascade as shown in FIG. Make the ground terminal 25 common to 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
a is independently arranged, and the second and third basic units 27 are arranged.
Although the ground terminal 25b is commonly used for b and 27c, the configuration is not limited to the above, and other combinations may be possible depending on the design of the entire filter. For example, the ground terminal for the third basic unit 27c is independently arranged, and the first and second basic units 27 are
The ground terminals for a and 27b may be common.

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

In the structure of the ladder type surface acoustic wave filter 20 of this embodiment shown in FIG.
The frequency characteristics (resonance between the frequency and the attenuation amount) when the resonators 22 and 23 were manufactured so that the capacitance ratio Cr = 2.5 was used on the Y-cut X-propagation lithium tantalate substrate. (Relationship) is shown in FIG. As is apparent from FIG. 3, in this case, an out-of-band attenuation amount of −50 dB can be obtained.

On the other hand, in the structure of the conventional ladder type surface acoustic wave filter in which the series arm resonators and the parallel arm resonators are alternately connected, the capacitance ratio Cr = 2.5 and the band of -50 dB are set in the same manner as described above. FIG. 4 shows frequency characteristics when the external attenuation is secured. In the conventional configuration, in order to obtain the characteristics shown in FIG. 4, a total of four stages including a combination of a series arm resonator and a parallel arm resonator (as a whole, four series arm resonators and four parallel arm resonators are used). Resonator) need to be connected.
However, in such a connection, since the number of parallel arm resonators is larger than that of the configuration of the present embodiment, it is equivalently connected to each parallel arm resonator by connecting wiring between the resonators and wire bonding. The influence of the existing inductance on the filter characteristics is significant. As a result, as shown in FIG. 4, a steep rise to the pass band cannot be obtained in a frequency region lower than the pass band, and spurious also appears.

In the structure of the surface acoustic wave filter 20 of the present embodiment as shown in FIG. 2, a sufficient out-of-band attenuation amount is secured without the problems of the above-mentioned conventional technique.
Although the 36 ° Y-cut X-propagation lithium tantalate substrate is used as the piezoelectric substrate in the above description, the same 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 the RF filter for mobile communication, the 36 ° Y-cut X-propagation lithium tantalate substrate is the most suitable in consideration of the elastic wave velocity propagating through the substrate, the electromechanical coupling coefficient, and the temperature characteristics. 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) whose main component is aluminum having a low electric resistance and a low density is used. Here, since the reflection coefficient of the surface acoustic wave at the electrode end and the substantial electromechanical coupling coefficient depend on the thickness of the Al film, the thickness of the Al film causes a difference between the resonance and antiresonance frequencies of the resonator. The frequency difference changes. As the Al film becomes thicker, the substantial electromechanical coupling coefficient becomes larger, and the frequency difference becomes larger. Therefore, the thicker the Al film, the wider the bandwidth of the filter.

Generally, in mobile communication, since there are a reception band and a transmission band, an RF filter having a good cutoff characteristic is required. For example, there is Japanese domestic analog specification as an example of specifications of 800 MHz band mobile communication. This specification has a reception band of 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.
The features of the surface acoustic wave filter 20 of the present embodiment will be further described below by taking a case used as a reception filter of this domestic analog specification as an example.

FIG. 5 shows a ladder-type surface acoustic wave filter 20 of the present embodiment in which the structure shown in FIG. 2 is provided on a 36 ° Y-cut X-propagation lithium tantalate substrate. 5, the normalized Al film thickness (Al film thickness / IDT electrode period of the parallel arm resonator 23) and the relative bandwidth (bandwidth / center frequency), and the normalized Al film thickness and out-of-band in the transmission band It is a relationship diagram with an attenuation amount. The bandwidth is 1d from the minimum insertion loss in the pass band.
It is specified at the point where B is lowered.

It can be seen from FIG. 5 that the bandwidth increases as the Al film thickness increases. The reception filter of domestic analog specifications has a center frequency of 872.5 MHz and a bandwidth of 25.
If the bandwidth is 3 MHz and the specific bandwidth is 3% or more, the specifications are satisfied. Therefore, the Al film thickness should be 8% or more in terms of the standardized Al film thickness.

On the other hand, 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. Figure 6
[Fig. 4] shows the influence of the Al film thickness on the frequency characteristics of the ladder-type surface acoustic wave filter 20 in the example of the present invention. However, the Al film thickness is shown as a standardized Al film thickness, and
The characteristics for%, 9% and 10% are plotted respectively. As a result, when the Al film becomes thicker, the pass band widens to become a wide band, but since the frequency difference between the stop bands on both sides of the pass band becomes large, the cut-off characteristic deteriorates and the out-of-band attenuation amount sharply decreases. You can see.
As shown in FIG. 6, the transmission band corresponds to a slightly higher frequency region than the pass band. However, because the side lobe from the pass band to the high frequency region side enters the transmission band due to the above-mentioned cause, this portion is sufficient. The amount of attenuation cannot be secured, which may cause noise. In order to solve the above problems, it is desirable that the standardized Al film thickness be 10% or less.

From the above, the optimum Al film thickness is a normalized film thickness of 8% or more and 10% or less. In addition, other 800MHz
Band specifications, eg AMPS in North America, GSM in Europe
In such cases, the center frequency value is different, but the same filter characteristics as the domestic analog are required. Even in that case, similarly good filter characteristics can be obtained by setting the normalized Al film thickness to a value within the above range.

FIG. 7 shows a ladder type surface acoustic wave filter 20 of the present embodiment in which the structure shown in FIG. 2 is provided on a 36 ° Y-cut X-propagation lithium tantalate substrate 21 of 800M.
The relationship between the normalized Al film thickness and the ripple in the pass band when the specific 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 has a minimum value when the film thickness is about 9%. Furthermore, if the normalized Al film thickness is within the above range of 8% or more and 10% or less, the ripple in the pass band can be set to 1.5 dB or less, which is allowable. Therefore, the range of the standardized Al film thickness described above is also effective in reducing the ripple in the pass band.

FIG. 8 shows a case where 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 standardized Al film thickness is 9%. , The IDT capacitance Cs of the series arm resonator 22 and the IDT capacitance Cp of the parallel arm resonator 23 in
And the amount of out-of-band attenuation. This is the IEE transaction, MTT 20 (7) (1972), pp. 458 to 471 (IEEE Trans.MTT20 (7) (197).
2) This is a simulation by an equivalent circuit in which slight correction is added to Smith's second model based on the description of pp458-471).

Generally, in addition to the out-of-band attenuation amount and the ratio bandwidth, which have already been described, the parameters that affect the filter characteristics include the in-band VSWR (voltage standing wave ratio).
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 1, the more
It has good characteristics with little reflection.

The characteristic specifications required for an actual filter depend on the design of the equipment in which each filter is used, but generally, 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, the required specifications can be realized. FIG. 8 shows the above range, which corresponds to an area (including a boundary) surrounded by the coordinates of 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 so that Cs and Cp are values within the above range, 80
It satisfies the frequency characteristics required for an RF filter for mobile communication in the 0 MHz band.

FIG. 9 shows that the IDT capacitances of the resonators 22 and 23 are values at the point T in FIG. 8, that is, (Cs, Cp).
Is the frequency characteristic of the ladder-type surface acoustic wave filter 20 manufactured so as 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 crossing 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 cross 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, bandwidth 31 MHz, in-band VSWR 2.2
Thus, the out-of-band attenuation amount in the transmission band is -50 dB, and the out-of-band attenuation amount is large and the frequency characteristic with small ripple in the pass band is obtained.

The surface acoustic wave filter 20 includes a resonator 22,
The resonance characteristics change due to the electrical resistance and inductance of the connection wiring 26 between 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 the low frequency side. However, since the resonance frequency of the parallel arm resonator 23 is the stop band on the low frequency side of the pass band, if the inductance is large, the cut-off characteristic of the ladder-type surface acoustic wave filter 20 deteriorates, and the pass band in the frequency characteristic is deteriorated. It is not possible to obtain a sharp rise to When the electric resistance existing in association with 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.
Since it is made of Al film at the same time as the electrode of
While it is only about m, 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 the inductance of the connection wiring 26, it is necessary to widen the width.

FIG. 10 shows the configuration of FIG. 2 in which the width of the connection wiring 26 and 30 MHz from the pass band to the low frequency region side.
It is a relationship diagram with the amount of attenuation in the frequency value which left | separated. The length of the connection wiring 26 is 500 μm. The width of the connection wiring 26 is 5
If it is larger than 0 μm, the amount of 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 amount of attenuation sharply 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 ground capacitance 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 should be 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 of each input / output terminal 24 and the ripple in the pass band. Than this,
When the area of the input / output terminal 24 is 0.14 mm 2 or less, the ripple in the pass band becomes a value of 1.5 dB or less, which is allowable. However, the input / output terminal 24 has a side length of 150 μm in order to connect wiring by wire bonding.
It must be m or more.

As described above, the ladder-type surface acoustic wave filter 20 having the structure of this embodiment has a smaller ripple in the pass band and a sufficient amount of out-of-band attenuation as compared with the conventional ladder-type surface acoustic wave filter. Excellent frequency characteristics can be realized.

In the description of this embodiment, the basic units 27 of the ladder-type surface acoustic wave filter 20 are connected in cascade in a total of three stages, but this is not a limitation.
For example, if a ladder type surface acoustic wave filter is constructed by connecting two stages of the basic units 27 in cascade connection, within the range of the electrode film thickness and the capacitance of the IDT in this example, the three stages of the basic units described in this example will be described. Compared with the case where the units 27 are connected in cascade, the out-of-band attenuation amount is inferior, but there is no particular disadvantage in terms of bandwidth and in-band ripple.

Alternatively, regarding the final stage unit of the basic units 27 connected in cascade, the series arm resonator 22 and the parallel arm resonator 23 do not have the configuration of the T-type basic unit described above. It is also possible to adopt a configuration in which one each is connected, that is, one final series arm resonator 22 is omitted.

Further, in the structure of the ladder type surface acoustic wave filter 20 of this embodiment shown in FIG. 2, the input / output terminal 24 and the ground terminal 25 are both drawn to have a rectangular shape (rectangle). However, the shape of these terminals 24 and 25 is not limited to this, and may be circular, for example.

[0055]

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 consisting of another surface acoustic wave resonator that is connected in parallel with the above and is further grounded has a structure in which a total of three basic cascade units are connected in series, whereby A wideband filter having excellent frequency characteristics with small in-band ripple and sufficient out-of-band attenuation is realized.

A 36 ° Y-cut X-propagation lithium tantalate substrate is used as a substrate made of a piezoelectric material for forming a surface acoustic wave filter, and the interdigital electrode included in the surface acoustic wave resonator is made of aluminum as a main component. By using a metal film having the following characteristics, it is possible to obtain particularly good characteristics as an RF filter for mobile communication.

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

Further, by setting the electrostatic capacitance of the interdigital transducer of each surface acoustic wave filter to a value within the range defined in claim 4, it is generally required for an RF filter for mobile communication of 800 MHz band. It is possible to satisfy the characteristics.

Further, by making the ground terminal connected to two of the three basic resonator units common and independently providing the other one, it is possible to simplify the manufacturing process and reduce the manufacturing cost. The trade-off relationship between achieving a good desired frequency characteristic can be best satisfied.

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

The area of the input / output terminal is 0.14 square meters.
If m or less, the magnitude of ripple in the pass band is 1.5.
It can be less than or equal to dB.

[Brief description of drawings]

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

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

FIG. 3 is a diagram showing frequency characteristics of a ladder-type surface acoustic wave filter according to an example 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 specific bandwidth of the ladder-type surface acoustic wave filter in the example of the present invention, and the relationship between the thickness of the Al film and the out-of-band attenuation in the transmission band. .

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 in the example 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 a ladder-type surface acoustic wave filter according to an example of the present invention.

FIG. 8 is a diagram showing a 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 amount in the example of the present invention. .

FIG. 9 is a diagram showing frequency characteristics of a ladder-type surface acoustic wave filter according to an example of the present invention.

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

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

[Explanation of symbols]

 1, 21 Piezoelectric substrate 2 Interdigital electrode (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 to 27c Basic resonator unit

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Keiji Onishi Keiji Onishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (7)

[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 including: a first unit in which each of the three basic resonator units is 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, . 2. The substrate is a 36 ° Y-cut X-propagation lithium tantalate substrate, and the interdigital electrodes included in the first to third surface acoustic wave resonators are composed of a metal film containing aluminum as a main component. The surface acoustic wave filter according to claim 1. 3. The thickness of the metal film forming the interdigital transducer is a value within the range of 8% or more and 10% or less of the electrode period of the interdigital electrode of the third surface acoustic wave resonator. The surface acoustic wave filter according to claim 2. 4. The capacitance of the interdigital transducers of the first and second surface acoustic wave resonators is Cs, and the capacitance of the interdigital transducers of the third surface acoustic wave resonator is Cp. To
The set of Cs and Cp values (Cs, Cp) is the Cs
, And Cp in a plane coordinate system having coordinate axes as the axes (including boundaries) enclosed by the following formulas: A: (Cs, Cp) = (2.00, 8.00) B: (Cs, Cp ) = (2.75,5.09) C: (Cs, Cp) = (4.32,8.00) The elasticity according to claim 3, wherein the value of the center frequency is a value within the range of 800 MHz to 1000 MHz. Surface wave filter. 5. The apparatus further comprises two ground terminals, one of the two ground terminals grounding the third surface acoustic wave resonator included in two of the three basic resonator units. 5. The other grounding terminal is commonly used for grounding, and the other grounding terminal is used for grounding the third surface acoustic wave resonator included in the remaining basic resonator unit. The surface acoustic wave filter described. 6. The width of the wiring connecting the first and second surface acoustic wave resonators and the third surface acoustic wave resonator is 5
The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter has a thickness of 0 μm or more. 7. The surface acoustic wave filter according to claim 1, wherein the areas of the input terminal and the output terminal are each 0.14 square mm or less.