JPH09121136A - Ladder surface acoustic wave filter for resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sufficient and flat frequency characteristic with a wide pass band width without spurious within a pass band by defining the thickness of a metallic film as the specified percentage of an electrode period in IDt of parallel arm surface acoustic wave resonator. SOLUTION: In the parallel arm surface acoustic wave resonator 5 consisting of a pair of interdigital transducers IDTs which exciting surface acoustic wave, the width of the electrode finger of IDT is defined as Lm, the gap width of the electrode finger is as Lg and it is defined that η=Lm/(Lm+Lg). When ηbecomes larger than 0.5, a ripple within the pass band becomes larger. When ηbecomes smaller than 0.5, the ripple becomes smaller. When η becomes smaller than about 0.3, the electric resistance of IDT becomes larger with the rapid increase of an insertion loss and the frequency characteristic of the filter becomes inactive os that the ripple becomes large quickly. Therefore, it is necessary to set η to be 0.2<=η<=0 in order to make the ripple small with the ripple within the passage band where n is 0.5 as reference. At the time of this state, the sufficient frequency characteristic is obtained.

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 capable of selectively extracting a desired frequency band by exciting a surface acoustic wave on a substrate made of a piezoelectric material, and more particularly to a mobile communication device. The present invention relates to a resonator ladder type surface acoustic wave filter suitable for a high frequency filter.

[0002]

2. Description of the Related Art Conventionally, a surface acoustic wave filter has been widely used as a high frequency filter for mobile communication. In particular, a resonator ladder type surface acoustic wave filter has attracted attention because of its small insertion loss and excellent 50Ω matching.

Hereinafter, a conventional resonator ladder type surface acoustic wave filter will be described. The principle of the resonator ladder type surface acoustic wave filter is the same as that of the conventional ceramic ladder type filter, except that the resonator portion is replaced with a surface acoustic wave resonator. In this case, a surface acoustic wave resonator connected in series with the input / output terminals (hereinafter referred to as "series arm surface acoustic wave resonator").
That. ) And a surface acoustic wave resonator connected in parallel to the input / output terminals (hereinafter referred to as "parallel arm surface acoustic wave resonator").
That. By making the anti-resonance frequency of) almost match, the pass band is in the vicinity of this frequency, and it is higher than the anti-resonance frequency of the series arm surface acoustic wave resonator and higher than the resonance frequency of the parallel arm surface acoustic wave resonator. A filter characteristic having a low frequency as a stop band can be obtained.

The pass band width of the resonator ladder type surface acoustic wave filter is correlated with the electromechanical coupling coefficient of the piezoelectric substrate. That is, the greater the electromechanical coupling coefficient, the greater the difference between the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator, and the wider the pass band width. Electromechanical coupling coefficient is about 5
36% rotation Y-cut X-propagation lithium tantalate substrate, which is relatively large, has a specific pass bandwidth (pass bandwidth / center frequency) of about 3%. NTT, AMPS (North America region, etc.), GSM (Europe region, etc.) 80) for mobile communication such as
Since it is suitable for the 0 MHz band standard, it is generally used, and good filter characteristics are obtained.

[0005]

In recent years, there has been a demand for a filter having an extremely wide pass bandwidth so that one mobile communication device can use a plurality of standards. For example,
In order to make it possible to use both the NTT analog standard and the NTT digital standard, the reception filter uses NTT.
A pass band including the digital standard 810 MHz to 830 MHz and the NTT analog standard 860 MHz to 885 MHz is required. In this case, the specific pass bandwidth is about 9%, which cannot be dealt with by the conventional 36 ° rotation Y-cut X-propagation lithium tantalate substrate because the electromechanical coupling coefficient is small. FIG. 17 shows an example of frequency characteristics when a receiving filter of the NTT analog standard is manufactured on a conventional 36 ° rotation Y-cut X-propagation lithium tantalate substrate.

As a piezoelectric substrate having a larger electromechanical coupling coefficient than the 36 ° rotated Y-cut X-propagating lithium tantalate substrate, a 64 ° rotated Y-cut X-propagated lithium niobate substrate and a 41 ° rotated Y-cut X-propagated lithium niobate substrate. It has been known. The electromechanical coupling coefficient of these piezoelectric substrates is about 11% for the former and about 17% for the latter. Among the currently known substrates, the only substrate that can realize a specific pass bandwidth of about 9% is the 41 ° rotated Y-cut X-propagation lithium niobate substrate.

However, IEICE Transactions '84 / 1
Vol. J67-C No. 1 Pages 158-165
As described on the page, a 41 ° rotated Y-cut X-propagation lithium niobate substrate is coupled with a bulk wave by excitation of a interdigital transducer (hereinafter referred to as “IDT”) and its energy is radiated into the substrate. While a pseudo surface acoustic wave (hereinafter referred to as "leaky surface acoustic wave") propagating as a main wave propagates as a main wave, it is a bulk wave radiated substantially parallel to the substrate surface, and is an SSBW (Surface).
The exciting bulk wave) also propagates, and the excitation characteristics of these elastic waves are complicated due to the strong piezoelectricity. The velocity of the elastic wave propagating on the metal surface of which the surface is electrically short-circuited in the 41 ° rotation Y-cut X-propagation lithium niobate substrate is about 4790 [m / m in SSBW.
s] and the leaky surface acoustic wave is about 4370 [m / s]. Therefore, in the conventional surface acoustic wave resonator constituted by the IDT and the reflector using this substrate, the leaky surface acoustic wave and the SSBW resonance characteristics are observed at different frequencies, as shown in FIG. When a resonator ladder type surface acoustic wave filter is constructed using such surface acoustic wave resonators, the spurious which is the anti-resonance point of the leaky surface acoustic wave in the series arm surface acoustic wave resonator becomes the center of the pass band of the filter. Therefore, ripples occur in the pass band and the pass band width becomes narrow. In this case, JP-A-6-29
As described in Japanese Patent No. 1600, if the surface acoustic wave resonator is configured by only the IDT without providing reflectors on both sides along the surface acoustic wave propagation direction of the IDT, FIG.
, The propagation of leaky surface acoustic waves is suppressed,
Spurs are avoided.

As described above, a surface acoustic wave resonator using a 41 ° rotated Y-cut X-propagation lithium niobate substrate has already been proposed, but a resonator ladder on a 41 ° rotated Y-cut X-propagation lithium niobate substrate. There is no precedent for making a surface acoustic wave filter. When a 41 ° rotated Y-cut X-propagation lithium niobate substrate is used, the IDT has a large reflection coefficient of the surface acoustic wave at the electrode finger end.
Due to the internal reflection of surface acoustic waves inside, many fine spurs are generated in the resonance characteristics.

The present invention has been made in view of the above points, and is a resonator ladder having a wide pass band, no spurious in the pass band, a flat pass band, and good frequency characteristics. An object of the present invention is to provide a surface acoustic wave filter.

[0010]

In order to achieve the above object, the first structure of the resonator ladder type surface acoustic wave filter according to the present invention is such that an elastic wave is formed on a 41 ° rotating Y-cut X-propagating lithium niobate substrate. Resonance including at least one series arm surface acoustic wave resonator including a interdigital transducer that excites a surface wave and at least one parallel arm surface acoustic wave resonator including a interdigital transducer that excites a surface acoustic wave A ladder-type surface acoustic wave filter, wherein the interdigital transducer comprises a metal film of Al or an Al alloy containing Al as a main component, and the thickness of the metal film is the interdigital structure of the parallel arm surface acoustic wave resonator. The electrode period of the transducer is 2.5% or more and 7.5% or less.

In the first structure of the present invention, it is preferable that the film thickness of the metal film is 4% or more and 7% or less of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.

The second structure of the resonator ladder type surface acoustic wave filter according to the present invention comprises at least a interdigital transducer that excites surface acoustic waves on a 41 ° rotated Y-cut X-propagation lithium niobate substrate. A resonator ladder type surface acoustic wave filter comprising one series arm surface acoustic wave resonator and at least one parallel arm surface acoustic wave resonator composed of a interdigital transducer for exciting surface acoustic waves, The interdigital transducer is made of a metal film of any electrode material other than Al, and the density of _Al is ρ _Al ,
When the density of the arbitrary electrode material is ρ _Me , the film thickness of the metal film is 2.5 · ρ _Al / ρ _Me % or more and 7.5 · of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator. ρ
It is characterized in that it is _Al / ρ _Me % or less.

Further, in the second structure of the present invention, the thickness of the metal film is not less than 4 · ρ _Al / ρ _Me % of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
It is preferably ρ _Al / ρ _Me % or less.

Further, in the above-mentioned structure of the present invention, it is preferable that reflectors are further provided on both sides of the interdigital transducer of the parallel arm surface acoustic wave resonator along the surface acoustic wave propagation direction. Further, in this case, the center-to-center distance d between the closest electrode fingers where the interdigital transducer and the reflector of the parallel arm surface acoustic wave resonator face each other is set to d = (α +
n) L / 2 (n is an integer of 0 or more, α is a real number of 1 or less,
L is an electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator), and α is 0.8 or more and 1.0 or more.
It is preferred that:

Further, in the above-mentioned structure of the present invention, it is preferable that the series arm surface acoustic wave resonators are further provided with reflectors on both sides along the surface acoustic wave propagation direction of the interdigital transducer. Further, in this case, the center-to-center distance Pr of the adjacent electrode fingers in the reflector of the series arm surface acoustic wave resonator is Pr.
Is Pr = p · Pi (p is a real number, Pi is the distance between the centers of adjacent electrode fingers in the interdigital transducer of the series arm surface acoustic wave resonator), and p is 0.96 or more and 1.0 or more.
It is preferably 6 or less.

Further, in the above-mentioned structure of the present invention, the number of electrode fingers facing each other in the interdigital transducer is N.
It is preferable that N is 50 pairs or more. Further, in the configuration of the present invention, a value standardized by dividing the intersection width WD of the electrode fingers facing each other in the interdigital transducer by the electrode period L of the interdigital transducer is W.
When (= WD / L), W is preferably 8 or more.

In the structure of the present invention, when the width of the electrode fingers of the interdigital transducer is Lm and the gap width of the electrode fingers is Lg and η = Lm / (Lm + Lg) is defined, η is 0.2. It is preferably not less than 0.5 and not more than 0.5.

Further, in the above-described structure of the present invention, the cross width weight is applied to the facing electrode fingers in at least one interdigital transducer selected from the group consisting of a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator. It is preferably applied.

In the present invention, the leaky surface acoustic wave and SSBW (Surface), which are types of bulk waves, are used.
Both of the skimming bulk waves are called surface acoustic waves.

According to the first aspect of the present invention, at least one series arm surface acoustic wave resonance composed of interdigital transducers for exciting surface acoustic waves is provided on a 41 ° rotated Y-cut X-propagation lithium niobate substrate. A resonator ladder type surface acoustic wave filter comprising a resonator and at least one parallel arm surface acoustic wave resonator composed of a interdigital transducer that excites a surface acoustic wave, wherein the interdigital transducer comprises Al or Al. It is made of an Al alloy metal film as a main component, and the thickness of the metal film is 2.5% of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
By being characterized by being 7.5% or less,
Practically, spurious is not a problem and good frequency characteristics can be obtained.

In the first structure of the present invention,
According to the preferable example in which the film thickness of the metal film is 4% or more and 7% or less of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator, spurious can be completely suppressed, and a better frequency characteristic can be obtained. Is obtained.

According to the second configuration of the present invention,
41 ° rotation Y-cut X-propagation on lithium niobate substrate,
At least one series arm surface acoustic wave resonator composed of interdigital transducers for exciting surface acoustic waves and at least one parallel arm surface acoustic wave resonator composed of interdigital transducers for exciting surface acoustic waves are provided. A resonator ladder type surface acoustic wave filter, wherein the interdigital transducer comprises a metal film of an arbitrary electrode material other than Al, where Al has a density of ρ _Al and the arbitrary electrode material has a density of ρ _Me. , The metal film thickness is 2.5 · ρ of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
_{By being characterized by Al} / ρ _Me % or more and 7.5 · ρ _Al / ρ _Me % or less, spurious does not pose a problem in practical use, and good frequency characteristics can be obtained.

In the second structure of the present invention,
The thickness of the metal film is 4 · ρ _Al / ρ _Me % or more 7 · ρ of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
According to the preferable example of _Al / ρ _Me % or less, spurious can be completely suppressed, and more excellent frequency characteristics can be obtained.

According to a preferred example of the configuration of the present invention, in which the reflectors are further provided on both sides along the surface acoustic wave propagation direction of the interdigital transducer of the parallel arm surface acoustic wave resonator, the resonance frequency is increased. And the anti-resonance frequency are increased and the pass band width is widened, so that good frequency characteristics can be obtained. In this case, the distance d between the centers of the closest electrode fingers where the interdigital transducer and the reflector of the parallel arm surface acoustic wave resonator face each other is d =
When (α + n) · L / 2 (n is an integer of 0 or more, α is a real number of 1 or less, and L is an electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator), α is 0.8 or more According to the preferable example of 1.0 or less, the difference between the resonance frequency and the anti-resonance frequency is further increased, and the pass band width is widened, so that good frequency characteristics can be obtained.

According to a preferred example of the configuration of the present invention, in which the reflectors are further provided on both sides along the surface acoustic wave propagation direction of the interdigital transducer of the series arm surface acoustic wave resonator, the resonance frequency is increased. And the anti-resonance frequency are increased and the pass band width is widened, so that good frequency characteristics can be obtained. Further, in this case, the center-to-center distance Pr of adjacent electrode fingers in the reflector of the series arm surface acoustic wave resonator is Pr = p · Pi (p is a real number, Pi is the interdigital shape of the series arm surface acoustic wave resonator). According to a preferable example in which p is 0.96 or more and 1.06 or less, the difference between the resonance frequency and the anti-resonance frequency is further increased when the distance between the centers of adjacent electrode fingers in the transducer) Since the bandwidth is wide, good frequency characteristics can be obtained.

Further, in the above-mentioned configuration of the present invention, when the number of pairs of electrode fingers facing each other in the interdigital transducer is N, according to a preferred example in which N is 50 or more, surface acoustic wave propagation of the interdigital transducer is achieved. The surface acoustic wave does not leak in any direction, and the loss can be suppressed without providing a reflector.

Further, in the structure of the present invention, the cross width WD of the electrode fingers facing each other in the interdigital transducer WD.
According to a preferable example in which W is 8 or more, spurious appears in the pass band, where W (= WD / L) is a value standardized by dividing by the electrode period L of the interdigital transducer. However, since the pass band is flat, good frequency characteristics can be obtained.

In the structure of the present invention, when the width of the electrode fingers of the interdigital transducer is Lm and the gap width of the electrode fingers is Lg, and η = Lm / (Lm + Lg) is defined, η is 0.2 or more. According to the preferable example of 0.5 or less, the ripple in the pass band becomes small and the pass band width becomes wide, so that good frequency characteristics can be obtained.

Further, in the above-mentioned structure of the present invention, cross width weighting is applied to the facing electrode fingers in at least one interdigital transducer selected from the group consisting of a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator. According to the preferable example described above, the internal reflection efficiency of the surface acoustic wave in the interdigital transducer can be increased, and the loss and the ripple of the resonator ladder type surface acoustic wave filter can be reduced. Good frequency characteristics can be obtained.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION

<First Embodiment> FIGS. 1A and 1B are schematic views showing a resonator ladder type surface acoustic wave filter according to a first embodiment of the present invention. FIG. (A) I
FIG. 2 is a sectional view taken along the line I.

As shown in FIG. 1, 41 ° rotation Y cut X
On the propagating lithium niobate substrate 1, the input electrode portion 2
And a pair of interdigital transducers (hereinafter referred to as “IDT”), one of which is connected to the input electrode unit 2 and excites a surface acoustic wave.
That. ), An output electrode section 3 connected to the other IDT of the series arm surface acoustic wave resonator 5, and one connected to the output electrode section 3 to excite surface acoustic waves. A parallel arm surface acoustic wave resonator 6 including a pair of IDTs and a ground electrode portion 4 connected to the other IDT of the parallel arm surface acoustic wave resonator 6 are formed. The resonator ladder type surface acoustic wave filter is configured as described above. The series arm surface acoustic wave resonator 5 and the parallel arm surface acoustic wave resonator 6 are not provided with reflectors on both sides along the surface acoustic wave propagation direction of the IDT. The electrode film thickness (metal film thickness of the IDT) H shown in FIG. 1B is an important variable that determines the characteristics of the resonator. The resonator is required to have good frequency characteristics without spurious. Therefore, the optimum value of the electrode film thickness H was examined.

FIG. 2 shows the frequency characteristic of the imaginary part of the impedance in the surface acoustic wave resonator with the electrode film thickness as a variable. The structure of the surface acoustic wave resonator is as follows.
That is, the electrode material is Al, the number of pairs of electrode fingers of the IDT is 100, and the width W of the electrode fingers facing each other in the IDT is W.
D (see FIG. 1A) is the electrode period L of the IDT (see FIG.
(See (b); substantially equal to the wavelength of the excited surface acoustic wave). Here, the electrode film thickness is standardized by dividing the actual electrode film thickness H by the electrode period L of the IDT. As shown in FIG. 2, when the electrode film thickness (H / L) is as thin as about 2%, the resonator does not resonate sufficiently and spurious A appears, but the electrode film thickness (H / L) becomes thick. Accordingly, this spurious A gradually becomes smaller and disappears. However, when the electrode film thickness (H / L) increases to about 8%,
Spurious B due to the influence of bulk waves other than SSBW
Appears. When the resonance characteristics of the surface acoustic wave resonator were examined more in detail when the electrode film thickness (H / L) was between 2% and 8%, the range of the electrode film thickness (H / L) where spurious was not a problem in practical use Is 2.5% or more and 7.5% or less,
The range of the electrode film thickness (H / L) at which spurious was completely suppressed and the best frequency characteristics were obtained was 4% or more and 7% or less.

Although Al is used as the electrode material in the present embodiment, several wt% of different metals such as Cu, Ti, Si and Sc are mixed with Al in order to improve the power withstanding property. A metal film made of Al alloy or the plurality of Al
Even when the laminated film having the layered structure of the alloy metal film was used, the preferable range of the electrode film thickness was almost the same as that of the present embodiment. In this case, the proportion of Al contained in the Al alloy is preferably 70% or more.

The frequency characteristics of the surface acoustic wave resonator depending on the electrode film thickness depend on the mass of the metal film. That is, in order to obtain the optimum electrode film thickness when the electrode material is other than Al, it suffices to add the ratio of the density of Al and the density of the electrode material other than Al to the above electrode film thickness. Therefore, if the density of _Al is ρ _Al and the density of any electrode material is ρ _Me , the range of electrode film thickness where spurious is not a problem in practical use is 2.5 · ρ.
_Al / ρ _Me % or more and 7.5 · ρ _Al / ρ _Me % or less, the range of electrode film thickness is 4 · ρ _Al / ρ _Me % or more 7 where spurious is completely suppressed and the best frequency characteristics are obtained.・ Ρ _Al / ρ
_Me % or less. For example, when the electrode material is Au, ρ _Au
Is 18.9 and ρ _Al is 2.7, the range of the electrode film thickness in which spurious is not a problem in practical use is 0.36% or more and 1.07% or less, and spurious is completely suppressed, which is the most preferable. The range of electrode film thickness that can obtain frequency characteristics is 0.5
It is 7% or more and 1.0% or less.

FIG. 3 shows the frequency characteristics of an Al resonator ladder type surface acoustic wave filter whose electrode film thickness is 6% of the electrode period of the IDT of the parallel arm surface acoustic wave resonator. This resonator ladder type surface acoustic wave filter has a T-shaped two-stage structure as shown in FIG. That is, this resonator ladder type surface acoustic wave filter has an input electrode terminal 11 and an output electric terminal 12.
And four series arm surface acoustic wave resonators 14a, 14b, 14c, 14d arranged in series between the input electrode terminal 1 and
1 and the connection electrode connecting the series arm surface acoustic wave resonator 14a connected to 1 and the series arm surface acoustic wave resonator 14b adjacent to the series arm surface acoustic wave resonator 14a and the ground electrode terminal 1
Parallel arm surface acoustic wave resonator 15a arranged between
And a connection wire connecting the series arm surface acoustic wave resonator 14d connected to the output electrode terminal 12 and the series arm surface acoustic wave resonator 14c adjacent to the series arm surface acoustic wave resonator 14d and the ground electrode terminal 13. And a parallel arm surface acoustic wave resonator 15b disposed between and. Here, the series arm surface acoustic wave resonators 14a, 14b, 14c, 14d.
Has an IDT electrode finger width of 1.190 μm, an IDT electrode period of 4.760 μm, the number of facing electrode finger pairs in the IDT is 120.5, and the intersecting width of facing electrode fingers in the IDT is 40 μm. The parallel arm surface acoustic wave resonators 15a and 15b have an IDT electrode finger width of 1.340.
μm, the electrode period of the IDT is 5.360 μm, the number of pairs of facing electrode fingers in the IDT is 150.5, and the width of intersection of the facing electrode fingers in the IDT is 70 μm. As shown in FIG. 3, since this resonator ladder type surface acoustic wave filter is composed of a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator having good frequency characteristics without spurious, Spurious does not exist in the passband, ripple is small (1.5 dB), wide band (passband width is 1.5 MHz when it is 1.5 dB below the minimum insertion loss)
Excellent frequency characteristics of are obtained.

The influence of the number of pairs of electrode fingers of the IDT, the width of the opposing electrode fingers in the IDT and the IDT metallization ratio on the frequency characteristics of the resonator ladder type surface acoustic wave filter will be described below. These impacts are
N is the number of pairs of electrode fingers of N, and the cross width WD of facing electrode fingers in the IDT is divided by the electrode period L of the IDT to obtain a standardized value W (= WD / L), N · W = 150 Using the surface acoustic wave resonator so that the series arm and the parallel arm,
The resonator ladder type surface acoustic wave filter having the T-type two-stage structure of FIG. 4 was manufactured and studied. Series arm surface acoustic wave resonator 14a,
14b, 14c, 14d and parallel arm surface acoustic wave resonator 1
The electrode period of the IDTs 5a and 15b is the same as that of the resonator ladder type surface acoustic wave filter of FIG. That is, I of the series arm surface acoustic wave resonators 14a, 14b, 14c, 14d
The electrode period of the DT is 4.760 μm, and the electrode period of the IDT of the parallel arm surface acoustic wave resonators 15a and 15b is 5.360 μm.
m. The electrode film thickness (made of Al) is 6% of the electrode period of the IDT of the parallel arm surface acoustic wave resonators 15a and 15b.

First, the effect of the number of pairs of electrode fingers of the IDT on the frequency characteristics of the resonator ladder type surface acoustic wave filter will be described. Since the surface acoustic wave resonator used here does not have a reflector, some surface acoustic wave leaks in the surface acoustic wave propagation direction of the IDT, resulting in a large loss as compared with the case where a surface acoustic wave is provided. . However, this problem can be solved by increasing the number of pairs of electrode fingers of the IDT. In FIG. 5, N · W =
The relation between the number N of electrode fingers of the IDT and the minimum insertion loss at 150 is shown. As shown in FIG. 5, when the number N of electrode fingers of the IDT is smaller than 50, the minimum insertion loss of the resonator ladder type surface acoustic wave filter sharply increases. Therefore, if the number N of electrode fingers of the IDT is 50 or more, leakage of surface acoustic waves does not pose a problem.

Next, the influence of the crossing width of the facing electrode fingers in the IDT on the frequency characteristics of the resonator ladder type surface acoustic wave filter will be described. FIG. 6 shows the relationship between the normalized electrode finger crossing width W and the number of spurious in the pass band when N · W = 150. As shown in FIG. 6, spurious does not exist in the pass band when the electrode finger cross width W is 10 or more, but spurious appears in the pass band when the electrode finger cross width W is smaller than 10. This is because when the crossing width of the facing electrode fingers in the IDT becomes smaller, the diffraction of the surface acoustic wave at the tip of the IDT cannot be ignored and the resonance frequency shifts. When the allowable number of spurious components in the pass band is 2, it is sufficient that the electrode finger cross width W is 8 or more.

Finally, the effect of the IDT metallization ratio on the frequency characteristics of the resonator ladder type surface acoustic wave filter will be described. Where the width of the IDT electrode finger is L
The gap width between the m and IDT electrode fingers is Lg (see FIG. 1B), and is defined as η = Lm / (Lm + Lg). In Figure 7,
η and ripple in the pass band (the ripple is the difference between the minimum insertion loss and the insertion loss capable of ensuring the pass band width of 85 MHz;
(See) and shows the relationship with. As shown in FIG. 7, η is 0.5
Becomes larger, the ripple in the pass band becomes larger,
When η is smaller than 0.5, the ripple in the pass band is small. However, when η becomes smaller than about 0.3, the electrode resistance of the IDT becomes large, resulting in a sharp increase in insertion loss and a slow frequency characteristic of the filter. Become.
Therefore, in order to make the in-passband ripple smaller than this value with reference to the in-passband ripple value of η = 0.5, it is necessary to set η to 0.2 or more and 0.5 or less. Thus, if η is 0.2 or more and 0.5 or less,
Since the ripple in the pass band is small and the pass band width is wide, excellent frequency characteristics can be obtained.

The electrode fingers facing each other in the IDT of the series arm surface acoustic wave resonator 5 or the parallel arm surface acoustic wave resonator 6 are
If cross width weighting as shown in FIG. 8 is applied at least at one place, the internal reflection efficiency of the surface acoustic wave in the IDT is increased, and the loss and ripple of the resonator ladder type surface acoustic wave filter are reduced. Therefore, better frequency characteristics can be obtained.

As described above, according to the resonator ladder type surface acoustic wave filter of the present embodiment, as compared with the conventional resonator ladder type surface acoustic wave filter, spurious is not present in the pass band, Good frequency characteristics with a wide pass band and a small ripple in the pass band can be obtained.

<Second Embodiment> FIG. 9 shows a second embodiment of the present invention.
FIG. 3 is a schematic diagram showing a resonator ladder type surface acoustic wave filter according to the embodiment of the present invention. As shown in FIG. 9, 41 ° rotation Y
On the cut X-propagation lithium niobate substrate 1, an input electrode portion 2 and a series arm surface acoustic wave resonator 5 formed of a pair of IDTs, one of which is connected to the input electrode portion 2 and which excites a surface acoustic wave, A parallel arm surface acoustic wave resonator including an output electrode portion 3 connected to the other IDT of the series arm surface acoustic wave resonator 5 and a pair of IDTs, one of which is connected to the output electrode portion 3 and which excites a surface acoustic wave. 6 and a ground electrode portion 4 connected to the other IDT of the parallel arm surface acoustic wave resonator 6 are formed. On the 41 ° rotated Y-cut X-propagation lithium niobate substrate 1, the IDT of the parallel arm surface acoustic wave resonator 6 is arranged.
Reflectors 7 are formed on both sides in the propagation direction of the surface acoustic wave. The resonator ladder type surface acoustic wave filter is configured as described above. When the parallel arm surface acoustic wave resonator 6 is provided with the reflector 7 as described above, spurious due to the leaky surface acoustic wave appears in the frequency characteristic as shown in FIG. 15, but the resonance frequency and the antiresonance frequency are different from each other. Since the difference becomes large, the pass band width becomes wide when the filter configuration is adopted. As a result, good frequency characteristics can be obtained.

FIG. 10 shows the ID of the parallel arm surface acoustic wave resonator.
10 shows frequency characteristics of a resonator ladder type surface acoustic wave filter (FIG. 9) in which reflectors are provided on both sides along the surface acoustic wave propagation direction of T. This resonator ladder type surface acoustic wave filter is shown in FIG.
It has a T-shaped two-stage structure as shown in FIG. The broken line in FIG. 10 shows the frequency characteristic of the resonator ladder type surface acoustic wave filter of FIG. Here, the serial arm surface acoustic wave resonators 14a, 14b, 14c, 14d (see FIG. 4) are
The width of the electrode finger of T is 1.190 μm, the electrode period of the IDT is 4.760 μm, the number of pairs of facing electrode fingers in the IDT is 120.5, and the intersecting width of the facing electrode fingers in the IDT is 40 μm. In addition, the parallel arm surface acoustic wave resonator 15
a and 15b (see FIG. 4) have IDT electrode finger widths of 1.
340 μm, IDT electrode period is 5.360 μm, ID
The number of pairs of facing electrode fingers in T is 150.5, IDT
The crossing width of the electrode fingers facing each other is 70 μm, the width of the electrode fingers of the reflector is 1.340 μm, and the electrode period of the reflector is 4.7.
60 μm, the number of adjacent electrode fingers in the reflector is 50, and the intersecting width of adjacent electrode fingers of the reflector is 45 μm. Also,
The distance d between the centers of the closest electrode fingers where the IDT of the parallel arm surface acoustic wave resonators 15a and 15b and the reflector face each other is d.
= (Α + n) · L / 2, α = 0.9, n = 0
It is. As shown in FIG. 10, the resonator ladder type surface acoustic wave filter of the present embodiment is inferior in rising to the pass band as compared with the resonator ladder type surface acoustic wave filter of FIG. Get wider. That is, as the pass band width, 90 MHz is secured at a point 1.5 dB lower than the minimum insertion loss.

Here, in the resonator ladder type surface acoustic wave filter of the present embodiment, I of the parallel arm surface acoustic wave resonator is used.
The distance between the centers of the closest electrode fingers where the DT and the reflector face each other d = (α + n) L / 2 α, n is α = 0.9, n
The fact that = 0 is described. The spurious due to the leaky surface acoustic wave shown in FIG. 15 is caused by providing reflectors on both sides along the propagation direction of the surface acoustic wave of the IDT of the parallel arm surface acoustic wave resonator. When the center-to-center distance d between the closest electrode fingers where the IDT and the reflector of the parallel arm surface acoustic wave resonator face each other is equal to the center-to-center distance between the facing electrode fingers in the IDT, this spurious is a resonator ladder type. It is in the pass band of the surface acoustic wave filter and causes ripples. This spurious moves to the lower frequency side as the distance d between the centers of the closest electrode fingers where the IDT and the reflector of the parallel arm surface acoustic wave resonator face each other is increased, and near α = 0.5. Excluded outside the pass band. Also,
It becomes the same as when α = 1 and α = 0. That is, this spurious moves with a cycle of L / 2. In FIG.
The relationship between α and the pass bandwidth is shown. The passband width was defined 1.5 dB below the minimum insertion loss. FIG.
As shown in, the pass band width becomes maximum when α = 0.9, and becomes wider when α = 0.8 to 1.0 than when α = 0. However, in the vicinity of α = 0.2 to 0.5, the ripple in the pass band becomes 1.5 dB or more due to spurious in the pass band, and the pass band width cannot be determined.

The influence of the number of pairs of electrode fingers of the IDT, the crossing width of the facing electrode fingers in the IDT and the IDT metallization ratio on the frequency characteristics of the resonator ladder type surface acoustic wave filter is the same as in the first embodiment. It is similar to the case of.

The influence of the cross width weighting of the IDT on the frequency characteristics of the resonator ladder type surface acoustic wave filter is also as follows:
This is similar to the case of the first embodiment. Also in the present embodiment, if crossing width weighting is applied to at least one position on opposite electrode fingers in the IDT of the series arm surface acoustic wave resonator 5 or the parallel arm surface acoustic wave resonator 6 (see FIG. 8), Since the internal reflection efficiency of the surface acoustic wave in the IDT can be increased to reduce the loss and the ripple of the resonator ladder type surface acoustic wave filter, a better frequency characteristic can be obtained.

As described above, according to the resonator ladder type surface acoustic wave filter of the present embodiment, excellent frequency characteristics with a wider pass band width than that of the conventional resonator ladder type surface acoustic wave filter can be obtained. can get.

<Third Embodiment> FIG. 12 is a schematic view showing a resonator ladder type surface acoustic wave filter according to a third embodiment of the present invention. As shown in FIG. 12, on the 41 ° rotation Y-cut X-propagation lithium niobate substrate 1, an input electrode portion 2 and a pair of IDTs, one of which is connected to the input electrode portion 2 and which excites a surface acoustic wave, are formed. Series arm surface acoustic wave resonator 5 and the other IDT of series arm surface acoustic wave resonator 5
To the output electrode unit 3, one side of which is connected to the output electrode unit 3, and the parallel arm surface acoustic wave resonator 6 formed of a pair of IDTs for exciting surface acoustic waves, and the parallel arm surface acoustic wave resonator 6
And the ground electrode portion 4 connected to the other IDT. Further, on the 41 ° rotated Y-cut X-propagation lithium niobate substrate 1, I of the series arm surface acoustic wave resonator 5 is arranged.
Reflectors 7 are formed on both sides along the propagation direction of the surface acoustic wave of DT. The resonator ladder type surface acoustic wave filter is configured as described above. When the series arm surface acoustic wave resonator 5 is provided with the reflector 7 as described above, spurious due to leaky surface acoustic waves appears in the frequency characteristic as shown in FIG. 15, as in the second embodiment. However, since the difference between the resonance frequency and the anti-resonance frequency becomes large and the pass band width becomes wide when the filter structure is used, good frequency characteristics can be obtained. However, since the spurious due to the leaky surface acoustic wave of the series arm surface acoustic wave resonator 5 is located near the center of the pass band of the filter, if the number of electrode fingers of the reflector 7 is too large, the leaky elastic surface in the pass band is increased. Since spurious waves cause ripples, it is necessary to keep the number of electrode fingers of the reflector 7 within the allowable range of ripples. This allowable range depends on the required specifications of the filter, but may be about 50 or less.

FIG. 13 shows a T-type two-stage structure made of Al whose electrode film thickness is 6% of the electrode period of the IDT of the parallel arm surface acoustic wave resonator as in the resonator ladder type surface acoustic wave filter of FIG. The frequency characteristic of the resonator ladder type surface acoustic wave filter is shown. The series-arm surface acoustic wave resonator 5 of this resonator ladder type surface acoustic wave filter has an IDT electrode finger width of 1.185 μm, I
The electrode period of the DT is 4.740 μm, the number of pairs of facing electrode fingers in the IDT is 120.5, and the cross width of the facing electrode fingers in the IDT is 40 μm. Also, the reflector 7 is
The number of electrode fingers is 50, and the crossing width of adjacent electrode fingers is 45μ
m. Then, the center-to-center distance Pr (see FIG. 12) of adjacent electrode fingers in the reflector 7 of the series arm surface acoustic wave resonator 5 is Pr = p · Pi (p is a real number, Pi is the series arm surface acoustic wave resonator). Distance between centers of adjacent electrode fingers in the IDT of FIG.
(See)), p = 0.98. Further, in the parallel arm surface acoustic wave resonator 6, the width of the electrode finger of the IDT is 1.3.
40 μm, IDT electrode period is 5.360 μm, IDT
The number of pairs of electrode fingers facing each other is 150.5, and the intersecting width of the electrode fingers facing each other in the IDT is 70 μm. Here, in the resonator ladder type surface acoustic wave filter of the present embodiment, p of the reflector of the series arm surface acoustic wave resonator is 0.9.
The reason for changing to No. 8 will be described. In FIG. 14, in the resonator ladder type surface acoustic wave filter of the present embodiment, p and VSWR (voltage standing wave ratio) when p of the reflector 7 of the series arm surface acoustic wave resonator 5 is changed. And the relationship with the pass bandwidth. Pass band width is 1.5 dB from minimum insertion loss
I defined it when I went down. It should be noted that VSWR is a ratio between the incident wave and the reflected wave of the surface acoustic wave filter and takes a value of 1 or more. The closer the VSWR is to 1, the smaller the reflected wave and the better the matching with the external circuit. As shown in FIG. 14, VSWR has a maximum value when p of the reflector 7 is 1, and when p of the reflector 7 deviates from 1, VSWR becomes small. However, the pass band width is widest when p of the reflector 7 is 1, and when p of the reflector 7 deviates from 1, the pass band width becomes narrow. Therefore, an acceptable reduction in passband width of 0.
When set to 5%, p of the reflector 7 is 0.96 or more and 1.06
The following is sufficient.

When the series arm surface acoustic wave resonator 5 is provided with the reflector 7 as in the present embodiment, the resonance frequency moves to the low frequency side. Therefore, in order to obtain an in-band VSWR comparable to that of the resonator ladder type surface acoustic wave filter of FIG. 3, the electrode period of the IDT of the series arm surface acoustic wave resonator 5 is reduced to increase the resonance frequency and the anti-resonance frequency. By moving to the frequency side, the difference between the resonance frequency of the series arm surface acoustic wave resonator 5 and the anti-resonance frequency of the parallel arm surface acoustic wave resonator 6 is shown in FIG.
The same level as the resonator ladder type surface acoustic wave filter of. As a result, the passband width becomes wider than that of the resonator ladder type surface acoustic wave filter of FIG. 3, and 88 MHz is secured when the passband width is 1.5 dB lower than the minimum insertion loss (see FIG. 13).

The influence of the number of electrode fingers of the IDT, the crossing width of the facing electrode fingers in the IDT and the IDT metallization ratio on the frequency characteristics of the resonator ladder type surface acoustic wave filter is the same as in the first embodiment. It is similar to the case of.

Also, the influence of the cross width weighting of the IDT on the frequency characteristics of the resonator ladder type surface acoustic wave filter is
This is similar to the case of the first embodiment. Also in the present embodiment, if crossing width weighting is applied to at least one position on opposite electrode fingers in the IDT of the series arm surface acoustic wave resonator 5 or the parallel arm surface acoustic wave resonator 6 (see FIG. 8), Since the internal reflection efficiency of the surface acoustic wave in the IDT can be increased to reduce the loss and the ripple of the resonator ladder type surface acoustic wave filter, a better frequency characteristic can be obtained.

As described above, according to the resonator ladder type surface acoustic wave filter of the present embodiment, excellent frequency characteristics having a wider pass band width than the conventional resonator ladder type surface acoustic wave filter can be obtained. can get.

[0054]

As described above, according to the resonator ladder type surface acoustic wave filter of the present invention, the pass band width is wide,
A resonator-ladder surface acoustic wave filter having good frequency characteristics in which no spurious is present in the pass band and the pass band is flat is realized.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a resonator ladder type surface acoustic wave filter according to a first embodiment of the present invention, (a) is a configuration diagram on a substrate, and (b) is II of (a). FIG.

FIG. 2 is a frequency characteristic diagram of an imaginary part of impedance in which the electrode film thickness is a variable in the surface acoustic wave resonator according to the first embodiment of the present invention.

FIG. 3 is a frequency characteristic diagram of an Al resonator ladder type surface acoustic wave filter according to the first embodiment of the present invention.

FIG. 4 is a circuit configuration diagram of a resonator ladder type surface acoustic wave filter having a T-type two-stage structure according to the first embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between the number of pairs of electrode fingers of the IDT and the minimum insertion loss in the resonator ladder type surface acoustic wave filter according to the first embodiment of the present invention.

FIG. 6 is a diagram showing a relationship between a crossing width of electrode fingers facing each other in an IDT and a spurious number in a pass band in the resonator ladder type surface acoustic wave filter according to the first embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between an IDT metallization ratio and a ripple in a pass band in the resonator ladder type surface acoustic wave filter according to the first embodiment of the present invention.

FIG. 8 is a schematic view showing another example of the resonator ladder type surface acoustic wave filter according to the first embodiment of the present invention.

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

FIG. 10 is a frequency characteristic diagram of a resonator ladder type surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 11 is a diagram showing a relationship between a center-to-center distance between electrode fingers of an IDT and a reflector and a pass band width in the resonator ladder type surface acoustic wave filter according to the second embodiment of the present invention.

FIG. 12 is a schematic diagram showing a resonator ladder type surface acoustic wave filter according to a third embodiment of the present invention.

FIG. 13 is a frequency characteristic diagram of a resonator ladder type surface acoustic wave filter according to a third embodiment of the present invention.

FIG. 14 is a diagram showing a relationship between p of a reflector, a VSWR, and a pass band width in the resonator-ladder type surface acoustic wave filter according to the third embodiment of the present invention.

FIG. 15 is a frequency characteristic diagram of a surface acoustic wave resonator including a reflector according to a conventional technique.

FIG. 16 is a frequency characteristic diagram of a surface acoustic wave resonator provided with no reflector according to the related art.

FIG. 17 is a frequency characteristic diagram of a resonator ladder type surface acoustic wave filter according to a conventional technique.

[Explanation of symbols]

1 41 ° Rotation Y Cut X Propagation Lithium Niobate Substrate 2 Input Electrode Part 3 Output Electrode Part 4 Ground Electrode Part 5, 14a, 14b, 14c, 14d Series Arm Surface Acoustic Wave Resonator 6, 15a, 15b Parallel Arm Surface Acoustic Wave Resonator 7 Reflector 11 Input electrode terminal 12 Output electrode terminal 13 Earth electrode terminal

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Keiji Onishi Keiji Onishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Hiroki Sato, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 72) Inventor Osamu Kawasaki 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (12)

[Claims] 1. A 41 ° rotation Y-cut X-propagation lithium niobate substrate, and at least one series arm surface acoustic wave resonator composed of interdigital transducers for exciting surface acoustic waves, and a blind for exciting surface acoustic waves. A resonator ladder type surface acoustic wave filter comprising at least one parallel arm surface acoustic wave resonator comprising a linear transducer, wherein the interdigital transducer comprises Al or a metal film of Al alloy containing Al as a main component. The resonator ladder type surface acoustic wave filter, wherein the film thickness of the metal film is 2.5% or more and 7.5% or less of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator. . 2. The film thickness of the metal film is 4% or more and 7% or more of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
The resonator ladder type surface acoustic wave filter according to claim 1, which is as follows. 3. A 41 ° rotation Y-cut X-propagation lithium niobate substrate on which at least one series-arm surface acoustic wave resonator composed of interdigital transducers for exciting surface acoustic waves and a comb for exciting surface acoustic waves. A resonator ladder type surface acoustic wave filter comprising at least one parallel arm surface acoustic wave resonator made of a linear transducer, wherein the interdigital transducer is made of a metal film of any electrode material other than Al. Where ρ _{Al is} the density of the arbitrary electrode material and ρ _Me is the density of the arbitrary electrode material, the film thickness of the metal film is 2.5 · ρ _Al / ρ _Me of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator. % To 7.5 · ρ _Al / ρ _Me % or less, a resonator ladder type surface acoustic wave filter. 4. The film thickness of the metal film is 4 · ρ _Al / ρ of the electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator.
The resonator ladder type surface acoustic wave filter according to claim 3, wherein _Me % or more and 7 · ρ _Al / ρ _Me % or less. 5. The resonator ladder type elastic surface according to claim 1, further comprising reflectors on both sides of the interdigital transducer of the parallel arm surface acoustic wave resonator along the surface acoustic wave propagation direction. Wave filter. 6. The center-to-center distance d of the closest electrode fingers where the interdigital transducer and the reflector of the parallel arm surface acoustic wave resonator face each other is defined as d = (α + n) · L / 2 (n is an integer of 0 or more). , Α is a real number of 1 or less, and L is an electrode period of the interdigital transducer of the parallel arm surface acoustic wave resonator), α is 0.8 or more and 1.0 or less. Ladder type surface acoustic wave filter. 7. The resonator ladder type elastic surface according to claim 1, further comprising reflectors on both sides of the interdigital transducer of the series arm surface acoustic wave resonator along the surface acoustic wave propagation direction. Wave filter. 8. A center-to-center distance Pr between adjacent electrode fingers in a reflector of a series arm surface acoustic wave resonator is Pr = p · Pi (p is a real number, Pi is a comb shape of the series arm surface acoustic wave resonator). 8. The resonator ladder type surface acoustic wave filter according to claim 7, wherein p is 0.96 or more and 1.06 or less, where p is a distance between centers of adjacent electrode fingers in the transducer. 9. The resonator ladder type surface acoustic wave filter according to claim 1, wherein N is 50 or more, where N is the number of electrode fingers facing each other in the interdigital transducer. 10. When W (= WD / L) is a value standardized by dividing the intersection width WD of the electrode fingers facing each other in the interdigital transducer by the electrode period L of the interdigital transducer, W is 8 or more. A resonator ladder type surface acoustic wave filter according to any one of claims 1 to 8. 11. The width of the electrode finger of the interdigital transducer is Lm, and the gap width of the electrode finger is Lg, where η = Lm / (L
When defined as m + Lg), the resonator ladder type surface acoustic wave filter according to claim 1, wherein η is 0.2 or more and 0.5 or less. 12. The cross width weighting is applied to the facing electrode fingers in at least one of the interdigital transducers selected from the group consisting of a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator. 8. A resonator ladder type surface acoustic wave filter according to any one of 1.