JPH10335978A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an SAW(surface acoustic wave) filter which has a high damping region near the end section of the passing band and can obtain a large damping amount in a desired frequency band. SOLUTION: In this SAW filter, a balanced SAW filter element and a ladder- type SAW filter element are cascade-connected, and the balanced SAW filter element is constituted of two kinds of SAW resonators (SAW resonators 3 and 6 and 4 and 5) in which IDT electrodes are arranged at different finger pitches, so to satisfy a relation, k1 ×T1 <k2 ×T2 , where k1 , T1 , k2 and T2 respectively represent the electrode finger crossing width and electrode finger logarithm of the resonators 4 and 5 having the larger electrode finger pitch and the electrode finger crossing width and electrode finger logarithm of the resonators having a smaller electrode finger pitch.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter used for a mobile communication device such as a mobile phone.
The present invention relates to an unbalanced input-balanced output type or a balanced input-unbalanced output type in which a plurality of surface acoustic wave resonators are arranged in a symmetric lattice.

[0002]

2. Description of the Related Art Conventional ladder-type surface acoustic waves (Surface)
Acoustic Wave, hereinafter abbreviated as SAW) Filter F ₁
Will be described with reference to FIG. The drawing, there is shown a general configuration of a ladder type SAW filter F _1, a filter resonant frequency in the parallel arm series arm connecting the different two SAW resonators in a ladder shape, 4 These are SAW filters connected in cascade. In the figure, 1a
Is a first input terminal, 1b is a second input terminal serving as an input-side ground electrode, and 13, 14, 17, and 18 are serial S terminals.
The AW resonators 15, 16, 19 and 20 are parallel SAW resonators. Although FIG. 1 shows a four-stage cascade connection, a two-stage or three-stage cascade connection has been proposed.

A first input terminal 1a and a second input terminal 1
When a signal is input at b, the first output terminal 2a and the second output terminal (output) are output due to the frequency characteristics of the individual SAW resonators formed on the piezoelectric substrate (not shown) due to the difference in impedance. A signal having desired filter characteristics is output between the ground electrodes 2b on the side. At this time, the series SA
The resonance frequency of the W resonators 13, 14, 17, 18 and the parallel S
By making the anti-resonance frequencies of the AW resonators 15, 16, 19, and 20 approximately equal, a band-pass filter having good pass-band characteristics can be obtained.

It is desirable that such a SAW filter for a mobile communication device has low loss in order to reduce the size and power consumption of a portable terminal device. In addition, SAW filters used for reception and transmission each have a suppression band outside the pass band, and are strongly required to have high attenuation over a wide range from near the end of the pass band. Depending on the specifications of the mobile communication system, a considerably large amount of attenuation may be required only in a specific frequency band locally.

Conventionally, a general SAW filter is of an unbalanced input-unbalanced output type. Therefore, when an electronic circuit or the like subsequent to the SAW filter is of a balanced input type, the SAW filter has a SAW filter.
A circuit configuration in which an unbalance-balance converter such as a balun circuit is inserted between the filter and the electronic circuit at the subsequent stage has been adopted. on the other hand,
When the electronic circuit or the like in the preceding stage of the SAW filter is of a balanced output type, the circuit configuration is such that a balanced-unbalanced converter is inserted between the preceding electronic circuit and the SAW filter.

In recent years, in order to remove an unbalanced-balanced converter or a balanced-unbalanced converter from the above-mentioned circuit configuration, an S
An unbalanced input-balanced output type SAW filter or a balanced input-unbalanced output type SAW filter (hereinafter referred to as a balanced type) in which the AW filter itself has a function of a balance-unbalanced converter or an unbalanced-balanced converter. (Referred to as a type SAW filter) is being put to practical use.

FIG. 3 shows the balanced type SAW filter F _2.
In this method, unbalanced input-balanced output or balanced input-unbalanced output is possible, high attenuation outside the pass band, and low loss in the pass band. In the figure, the SAW resonator 23,
24, 25, and 26 are connected in a symmetrical grid pattern to form a first-stage balanced SAW filter, which includes a second-stage balanced SAW filter composed of SAW resonators 27, 28, 29, and 30. Filters are cascaded. In such a balanced SAW filter, low loss can be obtained in the pass band and high attenuation can be obtained outside the pass band.

[0008]

However, the ladder-type SAW filter has good filter characteristics in the pass band due to low loss, but has a balanced input / output because it is an unbalanced input-unbalanced output type. There are problems that it cannot be realized and that the amount of attenuation outside the pass band is inferior.

[0009] Further, in order to reduce the size, weight and cost of the portable terminal device, it is urgent to reduce the number of parts used. Therefore, it is required to add a new function to the SAW filter. One of them is, for example, RF (Radio Freq
uency: Radio frequency (IF) stage and IF (Intermediate Frequen)
(cy: intermediate frequency) There is a demand for a filter used between stages or the like to be configured as an unbalanced input-balanced output type or a balanced input-unbalanced output type.

However, in the above-mentioned conventional balanced SAW filter, although high attenuation is obtained as the distance from the pass band increases, the attenuation near the end of the pass band cannot be increased, and the cutoff characteristics inside and outside the pass band cannot be obtained. Is poor, and there is a problem that an arbitrary frequency band apart from the pass band cannot be largely attenuated.

Accordingly, the present invention has been completed in view of the above circumstances, and an object thereof is to have a high attenuation region near the end of a pass band and obtain a large amount of attenuation in a desired frequency band. It is another object of the present invention to reduce the size and weight of the device and to make it possible to manufacture the device at low cost.

[0012]

A surface acoustic wave filter according to the present invention is a balanced surface acoustic wave filter element comprising a plurality of surface acoustic wave resonators having a pair of comb-like electrodes connected in a symmetrical lattice. A ladder-type surface acoustic wave filter element formed by connecting a plurality of surface acoustic wave resonators in a ladder form, wherein the ladder-type surface acoustic wave filter element is cascaded on the same piezoelectric substrate; Is composed of two types of surface acoustic wave resonators having different electrode finger pitches of the comb-shaped electrodes, and the electrode finger intersection width of the surface acoustic wave resonator having the larger electrode finger pitch is k ₁ , The number of electrode finger pairs is T ₁ , and the electrode finger intersection width of the surface acoustic wave resonator having the smaller electrode finger pitch is k.
₂ , when the number of electrode finger pairs is T ₂ , k ₁ × T ₁ <k ₂ ×
Characterized by a T _2, the equivalent capacitance of the SAW resonator towards the electrode finger pitch is large, becomes smaller than the equivalent capacitance of the SAW resonator towards the electrode finger pitch is smaller, as a result, pass A steep cutoff characteristic is obtained at the band end, the attenuation outside the pass band is increased, and the frequency position of the attenuation pole at the end of the pass band can be adjusted.

[0013]

FIG. 1 shows a SAW filter F according to the present invention.
Shown in In the figure, 3, 4, 5, and 6 are balanced SAs.
SAW resonators constituting a W filter element, 7, 8, 9,
Reference numeral 10 denotes a SAW constituting a ladder-type SAW filter element.
The resonator is composed of a balanced SAW filter element and a ladder type SW filter element.
In this configuration, the AW filter elements are cascaded on one main surface of the same piezoelectric substrate (not shown). In addition, 1a, 1
Reference numerals b, 2a, and 2b denote the same parts as in FIGS. 2 and 3, which are first and second input terminals and first and second output terminals. 3a and 3b are a pair of interdigital electrodes (Inter Digital Tr).
3c, 3c is S
This is a reflector that reflects and resonates the AW.

With the above configuration, the potentials of the first input terminal 1a and the first output terminal 2a and the potentials of the second input terminal 1b and the second output terminal 2b do not become common, and the -Type unbalanced input, which is a characteristic of SAW filters
Balanced output or balanced input-unbalanced output becomes possible.
Further, the filter characteristic of the SAW filter F is a cascade connection of a balanced type and a ladder type, so that a large amount of attenuation outside the pass band is obtained while maintaining a low loss, and further, the vicinity of the end of the pass band. Steep cut-off characteristics, ie, signal level-
Steep rise and fall of the frequency characteristic can be obtained.

In the present invention, the balanced SAW filter element is composed of two types of SAW resonators having different resonance frequencies. The larger the electrode finger pitch of the IDT electrodes 3a and 3b, the lower the resonance frequency and the lower the electrode finger pitch. The smaller the pitch, the higher the resonance frequency. For example, in FIG. 1, the electrode finger pitch of the SAW resonators 4, 5 (and 9, 10) is larger than that of the SAW resonators 3, 6 (and 7, 8), and therefore, the SAW resonators 4, 5 (and 9). , 10) is represented by f _R1 , SA
If the resonance frequency of the W resonators 3, 6 (and 7, 8) is f _R2 , then f _R1 <f _R2 . Conversely, the configuration can be such that f _R1 > f _R2 . Therefore, in the equivalent circuit of FIG. 1, the SAW resonators at symmetric positions are of the same type, in other words, the adjacent SAW resonators are of different types. What should be done.

Then, the SAW resonators 4, 5 (and 9, 1)
0) The electrode finger intersection width is k ₁ , the number of electrode finger pairs is T ₁ , and the SAW
Assuming that the electrode finger intersection width of the IDT electrodes of the resonators 3 and 6 (and 7, 8) is k ₂ and the number of electrode finger pairs is T ₂ , k ₁ × T ₁ <k
₂ × T ₂ . At this time, the equivalent capacitance of the SAW resonators 4, 5 (and 9, 10) becomes smaller than the equivalent capacitance of the SAW resonators 3, 6 (and 7, 8).

At this time, 0.6 ≦ k ₁ × T ₁ / k ₂ × T
₂ <1 is preferable, and in the SAW resonators 3, 4, 5, and 6 constituting the balanced SAW filter element, the above range is set so that a sharp cutoff characteristic at the end of the pass band and a good pass band are obtained. Outer attenuation is obtained. Also, 0.
If it is less than 6, the attenuation will be as small as about 10 dB and the pass band width will be narrow. If it is more than 1, the attenuation pole at the end of the pass band will be lost, and the attenuation will be rapidly reduced, making it impossible to use as a filter. More preferably, 0.70 ≦ k
₁ × T ₁ / k ₂ × T ₂ ≦ 0.95.

In the present invention, the SAW resonators 7, 8, 9, and 10 constituting the ladder-type SAW filter element do not necessarily have to have the above-described configuration of the present invention, and are configured to be, for example, band-pass filters. It should just be. That is, in FIG. 1, the SAW resonators 7 and
It is sufficient that the resonance frequency of the SAW resonator 8 is higher than that of the SAW resonators 9 and 10 on the parallel side. Ladder type SAW
The filter may be provided on either the input side or the output side of the balanced type.

The equivalent capacitance is equivalent to C ₀ in the equivalent circuit diagram of the SAW resonator shown in FIG. 4, and is an electric capacitance (capacitance) determined by the structure of the SAW resonator. Since the equivalent capacitance is proportional to the product k × T of the intersection width k of the electrode fingers of the IDT electrode and the logarithm T of the electrode fingers, each SAW
By adjusting the intersection width k and logarithm T of the resonator to desired values, the equivalent capacitance can be controlled. It is also possible to change the frequency position of the attenuation pole at the end of the passband.

The pass bandwidth of the SAW filter of the present invention is
It is 1% to 5% (fractional bandwidth) of the center frequency (800 to 900 MHz). It is desirable that the piezoelectric substrate for realizing this has an electromechanical coupling coefficient of 2% or more. Specifically, LiTaO ₃ , 64 of 36 ° Y cut-X propagation
° Y cut-X propagation LiNbO ₃ , 41 ° Y cut-
X-propagating LiNbO _{3 and the} like can be mentioned. Also,
The thickness of the piezoelectric substrate is preferably about 0.3 to 0.5 mm.
If it is less than 3 mm, the piezoelectric substrate becomes brittle, and if it exceeds 0.5 mm, the material cost increases.

In the present invention, the IDT electrode of the SAW resonator is made of Al or Al alloy (Al-Cu based, Al-Ti
Al and alloys containing Al as a main component are particularly preferred because of their high excitation efficiency and low material cost. The IDT electrode is formed by a vapor deposition method, a sputtering method or a CVD method.
It is formed by a thin film forming method such as a method.

The logarithm of the IDT electrode is 50 to 200.
The width of the electrode finger is about 0.1 to 10.0 μm, the interval between the electrode fingers is about 0.1 to 10.0 μm, the opening width (cross width) of the electrode finger is about 10 to 150 μm, and the thickness of the IDT electrode is A thickness of about 0.2 to 0.5 μm is preferable for obtaining desired characteristics as a resonator or a filter. It is also preferable to form a piezoelectric material such as zinc oxide or aluminum oxide between the electrode fingers of the IDT electrode because the SAW resonance efficiency is improved.

Thus, the present invention has the effect of having a high attenuation region near the end of the pass band, obtaining a large amount of attenuation in a desired frequency band, and further reducing the size and weight.

It should be noted that the present invention is not limited to the above embodiment, and various changes may be made without departing from the scope of the present invention.

[0025]

Embodiments of the present invention will be described below. S in FIG.
The AW filter F was configured as follows. The electrode finger pitch of the SAW resonators 4, 5, 9, 10 (referred to as group A) is S
By making the AW resonators 3, 6, 7, and 8 (group B) larger, the resonance frequency f _{R1 of} group A can be increased to 800 MHz.
The resonance frequency f _R2 of the z and B groups was set to 832.7 MHz.
The resonance frequency f _{R2 of the} group B is equal to the anti-resonance frequency f _{A1 of the} group A = 83
It almost coincided with 0 MHz, and as a result, a flat passband characteristic was obtained. In the present embodiment, the same SAW resonator is used in each of the group A and the group B.
The resonance frequency of the resonator can be slightly shifted.

The intersection width of the electrode fingers of the IDT electrodes of the group A is k ₁ = 15λ μm (λ is the wavelength of the SAW), the number of electrode fingers is T ₁ = 76 pairs, and k ₁ × T ₁ = 1140 (λ μm ·
In the pair, the intersection width of the electrode fingers of the IDT electrode of the group B is k ₂ = 1.
5λ μm, the number of pairs of electrode fingers is T ₂ = 80 pairs, k ₂ × T ₂
= 1200 (λμm · pair), and k ₁ × T ₁ / k ₂ ×
T ₂ = 0.95. And the equivalent capacity of group A is C
L = 3.0 pF, Equivalent capacitance of group B is CH = 3.2 pF
And CL / CH ≒ 0.95.

Hereinafter, a specific method for manufacturing the SAW filter F will be described.

(1) 36 ° Y cut-X propagation LiTa
On one surface of a piezoelectric substrate made of an O ₃ crystal, a thin film of an Al—Cu alloy is formed by an evaporation method.

(2) A photoresist is applied on the piezoelectric substrate on which the alloy thin film has been formed.

(3) Using a photomask for forming a negative pattern such as an IDT electrode of a SAW resonator, exposure is performed with ultraviolet light.

(4) Immerse the piezoelectric substrate in a developing solution to form a negative pattern on the photoresist.

(5) An alloy thin film on the piezoelectric substrate is formed in a pattern of the SAW filter F by a wet etching method or a dry etching method using the patterned photoresist as a mask.

In this embodiment, the electrode is formed by etching the alloy thin film, but may be formed by the lift-off method.

In this embodiment, the center frequency of the pass band is about 815 MHz, the specific bandwidth is about 4%, the loss in the pass band is about 3 dB, and the attenuation outside the pass band of 1.2 GHz or more is 40 dB or more. A steep cutoff characteristic at the band edge and a high attenuation attenuation pole (50 dB or more) were obtained.

[0035]

According to the present invention, a balanced SAW filter element and a ladder type SAW filter element are cascaded, and the balanced SAW filter element is composed of two types of SAW resonators having different electrode finger pitches of IDT electrodes. The electrode finger intersection width of the SAW resonator having the larger electrode finger pitch is k ₁ , the number of electrode finger pairs is T ₁ , the electrode finger intersection width of the SAW resonator having the smaller electrode finger pitch is k ₂ , and the electrode finger is smaller. When the logarithm and T _2,
By setting k ₁ × T ₁ <k ₂ × T ₂ , it has a low insertion loss, has a high attenuation region near the end of the pass band, and can obtain a large amount of attenuation in a desired frequency band. This has the effect of reducing the weight, and allows unbalanced input-balanced output or balanced input-unbalanced output.

[Brief description of the drawings]

FIG. 1 is a plan view of a basic configuration of a SAW filter F of the present invention.

2 is a plan view of a basic configuration of the SAW filter F ₁ of the conventional ladder type.

3 is a plan view of a basic configuration of the SAW filter F ₂ of the conventional balanced.

FIG. 4 is an equivalent circuit diagram of a SAW resonator.

[Explanation of symbols]

 1a: first input terminal 1b: second input terminal 2a: first output terminal 2b: second output terminal 3: SAW resonator 3a: IDT electrode 3b: IDT electrode 3c: reflector 4: SAW resonator 5: SAW resonator 6: SAW resonator 7: SAW resonator 8: SAW resonator 9: SAW resonator 10: SAW resonator

Claims (1)

[Claims] 1. A balanced surface acoustic wave filter element comprising a plurality of surface acoustic wave resonators having a pair of comb-shaped electrodes connected in a symmetrical lattice shape, and a plurality of surface acoustic wave resonators in a ladder shape. A ladder-type surface acoustic wave filter element formed by connecting
A surface acoustic wave filter formed by cascade connection on the same piezoelectric substrate, wherein the balanced surface acoustic wave filter element is composed of two types of surface acoustic wave resonators having different electrode finger pitches of comb-like electrodes. , The electrode finger intersection width of the surface acoustic wave resonator having the larger electrode finger pitch is k ₁ , the number of electrode finger pairs is T ₁ , the electrode finger intersection width of the surface acoustic wave resonator having the smaller electrode finger pitch is k ₂ , When the number of electrode finger pairs is T ₂ , k ₁ × T ₁ <k
_A surface acoustic wave filter characterized by ₂ × T ₂ .