JP2006229754A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small surface acoustic wave filter capable of obtaining a wide band with low impedance.
A surface acoustic wave filter (10) includes an input electrode (16) and an output electrode (18) made of a comb-shaped electrode (20) on a piezoelectric substrate (12), and is reflected at a position sandwiching the input electrode (16) and the output electrode (18). The surface acoustic wave filter 10 is formed by connecting two sets of resonator filters 14 provided with the device 30 in parallel, and the interdigital electrode 20 and the electrode fingers 22 and 32 of the reflector 30 are obliquely bent. A pass band obtained by the first resonator filter 14a, and a second resonator. The connection portions 24 and 34 have straight portions 26 and 36 connected to both ends of the connection portions 24 and 34; While superposing a part of the pass band obtained by the filter 14b, the surface acoustic wave excited by the first resonator filter 14a and the elastic table excited by the second resonator filter 14b The phase difference from the surface wave is 180 degrees.
[Selection] Figure 1

Description

  The present invention relates to a surface acoustic wave filter.

  Conventionally, there are surface acoustic wave (SAW) filters in which two sets of resonator filters are arranged on a piezoelectric substrate and the resonator filters are connected in parallel to obtain a wide band. Each resonator filter has a configuration in which an input electrode and an output electrode made of interdigital electrodes (IDT) are arranged on a piezoelectric substrate, and a reflector is provided at a position between which the input electrode and the output electrode are sandwiched. This SAW filter obtains a high-pass band with one resonator filter, and obtains a low-pass band with the other resonator filter, and superimposes the resonances appearing in each resonator filter. This is a configuration that forms a pass band using the triple mode (see, for example, paragraphs 0005 to 0007 of FIG. 9 and FIG. 9).

In addition, the SAW filter is provided with a crank on the electrode fingers of the IDT and the reflector to reduce the equivalent mechanical coupling coefficient and the decrease in the Q value and to eliminate the influence of the high-order transverse mode spurious, and is divided by the crank. There is one in which the interval between two parallel portions is λ / 4 (λ: wavelength of surface acoustic wave) (see, for example, Patent Document 2).
JP-A-10-242801 JP 60-43912 A

  The SAW filter is required to have a low impedance in order to match 50 Ω, and it is known that the crossing width of the IDT may be increased in order to reduce the impedance. However, when the crossing width is increased, energy is distributed perpendicular to the direction in which the SAW propagates, and transverse mode resonance occurs. Due to the resonance of the transverse mode, transverse mode spurious is generated in the filter characteristic of the SAW filter, and the filter characteristic is deteriorated. The lateral mode spurious appears at a position closer to the center frequency of the filter characteristics as the crossing width is increased.

  For this reason, it has been difficult to design a SAW filter in which two sets of resonator filters are arranged on a piezoelectric substrate while obtaining low impedance and not being affected by transverse mode spurious.

  In order to reduce the impedance of the SAW filter, a plurality of resonator filters are arranged on the piezoelectric substrate to reduce the IDT crossing width in each resonator filter, and to increase the crossing width of the entire SAW filter. There is a configuration one. However, this configuration has a problem that the SAW filter cannot be reduced in size because a plurality of resonator filters are disposed on the piezoelectric substrate.

  Further, some SAW filters have a configuration in which IDT is weighted to suppress transverse mode spurious. Specifically, an apodized electrode was used for IDT. However, when the IDT is weighted, the crossing width is reduced, so that there is a problem that the impedance is increased.

When a SAW filter IDT was provided with a crank and the distance between two parallel parts divided by the crank was set to λ / 4, and the filter characteristics were evaluated, no filter characteristics were obtained.
An object of the present invention is to provide a small surface acoustic wave filter that can obtain a wide band with low impedance.

  In order to achieve the above object, a surface acoustic wave filter according to the present invention is provided with an input electrode and an output electrode made of interdigital electrodes on a piezoelectric substrate, and at a position sandwiching the input electrode and the output electrode. A surface acoustic wave filter in which two sets of resonator filters provided with reflectors are connected in parallel, wherein the interdigital electrode and the electrode fingers of the reflector are bent obliquely, and the connection portion And a part of the pass band obtained by the first resonator filter and the pass band obtained by the second resonator filter are overlapped with each other. The phase difference between the surface acoustic wave excited by the resonator filter and the surface acoustic wave excited by the second resonator filter is 180 degrees.

  Since the crossing width of each resonator filter is the sum of the crossing widths in each straight line portion, the crossing width of each straight line portion can be reduced without reducing the crossing width of the entire resonator filter. Therefore, the SAW filter can be reduced in impedance, can eliminate the influence of transverse mode spurious, and can be downsized. Further, since the pass band obtained by the first resonator filter and the pass band obtained by the second resonator filter are shifted and a part of these pass bands are overlapped, a broadband SAW filter can be obtained. Furthermore, since the phase difference of the surface acoustic wave is shifted by 180 degrees between the first resonator filter and the second resonator filter, the resonance point on the high frequency side in the low frequency band and the pass band on the high frequency side. The resonance point on the low frequency side can be in phase.

  Further, the electrode pitch of the interdigital electrodes constituting the first resonator filter is different from the electrode pitch of the interdigital electrodes constituting the second resonator filter. This changes the wavelength of the surface acoustic wave excited by each resonator filter, so that the passband of each resonator filter can be moved to the low frequency side or the high frequency side. By adjusting the amount of movement, the higher frequency side in the low-pass band can be superimposed on the lower frequency side in the high-pass band.

の関係を満たすことを特徴としている。 Further, the difference between the input electrode and the output electrode in the first resonator filter and the input electrode and the output electrode in the second resonator filter are:

It is characterized by satisfying the relationship.

  As a result, the phase difference of the surface acoustic wave excited by each resonator filter can be set to 180 degrees, so that the resonance point appearing on the high frequency side in the low-pass band and the high-pass band The resonance point appearing on the lower frequency side can be in phase. Since these resonance points do not cancel each other even if they are superposed, the filter characteristics of the SAW filter can provide a passband using three resonance points.

の関係を満たすことを特徴としている。すなわち、すだれ状電極の各電極指に設けられる各直線部の互いの距離Ｌ１と、反射器の各電極指に設けられる各直線部の互いの距離Ｌ２とは、数式５を満たす関係を有している。これによりＳＡＷフィルタは、横モードスプリアスの影響が無くなり、良好なフィルタ特性が得られる。 Further, the straight portions provided on the electrode fingers are separated from each other by a distance L with respect to the propagation direction of the surface acoustic wave, and the distance L is

It is characterized by satisfying the relationship. That is, the distance L1 between the straight portions provided on each electrode finger of the interdigital electrode and the distance L2 between the straight portions provided on each electrode finger of the reflector have a relationship satisfying Expression 5. ing. As a result, the SAW filter is free from the influence of the transverse mode spurious, and good filter characteristics can be obtained.

の関係を満たすことを特徴としている。 In addition, an elastic structure comprising a plurality of resonator filters connected in parallel with an input electrode and an output electrode made of interdigital electrodes disposed on a piezoelectric substrate, and a reflector disposed at a position sandwiching the input electrode and the output electrode. In the surface wave filter, the interdigital electrode and the electrode fingers of the reflector each have a connecting portion bent obliquely and a straight portion connected to the connecting portion, and the one resonator filter A pass band that is obtained by superimposing a high frequency side in the low pass band obtained and a low frequency side in the high pass band obtained by the other resonator filter as a whole. Changing the electrode pitch of the interdigital electrodes of each of the resonator filters so as to form, between the input electrode and the output electrode of one of the resonator filters where the passbands overlap, The difference between the between the input electrode and the output electrode of the resonator filter of square is

It is characterized by satisfying the relationship.

  As a result, the SAW filter can be reduced in impedance, can eliminate the influence of transverse mode spurious, and can be reduced in size. Further, since the passbands obtained by the respective resonator filters are shifted and a part of these passbands are overlapped, a broadband SAW filter can be obtained. Furthermore, since the phase difference of the surface acoustic wave excited by each resonator filter is shifted by 180 degrees, the resonance point on the high frequency side in the low pass band and the resonance on the low frequency side in the high pass band The point can be in phase.

  The best embodiment of a surface acoustic wave (SAW) filter according to the present invention will be described below. FIG. 1 is an explanatory diagram of a surface acoustic wave filter. The SAW filter 10 has a configuration in which two sets of resonator filters 14 (a first resonator filter 14 a and a second resonator filter 14 b) are provided on a piezoelectric substrate 12.

  Each resonator filter 14 is provided with an input electrode 16 and an output electrode 18 formed of interdigital electrodes (IDTs 20) on the piezoelectric substrate 12, and a reflector 30 is interposed between the input electrode 16 and the output electrode 18. It is the structure which was arrange | positioned. The two sets of resonator filters 14 are connected in parallel. Specifically, the input electrode 16 of the first resonator filter 14a and the input electrode 16 of the second resonator filter 14b are electrically connected, and the output electrode 18 of the first resonator filter 14a and the second resonance filter 14a. The output electrode 18 of the child filter 14b is electrically connected.

  The IDT 20 includes a pair of comb teeth that mesh with each other. The comb teeth have a plurality of electrode fingers 22 and one end thereof is short-circuited. Each electrode finger 22 is provided with a connecting portion 24 bent obliquely, and each electrode finger 22 is divided into a plurality of linear portions 26 (26a, 26b). That is, each electrode finger 22 includes a connection portion 24 and a straight portion 26 connected to both ends of the connection portion 24. The connecting portion 24 is preferably shorter in length. The straight portions 26 provided on one electrode finger 22 are separated along the SAW propagation direction, and the distance L1 satisfies the relationship of 3λ / 8 ≦ L1 ≦ 5λ / 8. . Here, λ represents the wavelength of SAW. In FIG. 1, the distance L1 = λ / 2. The intersection width of the resonator filter 14 is the sum of the intersection width a1 of the straight portion 26a and the intersection width a2 of the straight portion 26b.

  The reflector 30 has a configuration in which a plurality of electrode fingers 32 are arranged at a predetermined pitch in the SAW propagation direction, and both ends of the electrode fingers 32 are short-circuited. Each electrode finger 32 is provided with a connecting portion 34 that is bent obliquely, and each electrode finger 32 is divided into a plurality of linear portions 36. That is, each electrode finger 32 includes a connection portion 34 and straight portions 36 connected to both ends of the connection portion 34. The connecting portion 34 is preferably shorter in length. And the connection part 24 provided in IDT20 and the connection part 34 provided in the reflector 30 are arrange | positioned along the propagation direction of SAW. Each linear portion 36 provided on one electrode finger 32 of the reflector 30 is separated along the SAW propagation direction, and the distance L2 satisfies the relationship of 3λ / 8 ≦ L2 ≦ 5λ / 8. It has become. In FIG. 1, the distance L2 = λ / 2.

  In the SAW filter 10, the center frequency of the first resonator filter 14a and the center frequency of the second resonator filter 14b are made different from each other, so that the pass band obtained by the first resonator filter 14a and the second resonator filter can be obtained. Part of the passband obtained in 14b is superimposed. That is, when one of the first resonator filter 14a and the second resonator filter 14b is used as a reference, the other pass band is shifted to the high frequency side or the low frequency side. In order to change the center frequency of the first resonator filter 14a and the center frequency of the second resonator filter 14b, the wavelength of the SAW may be changed between the first resonator filter 14a and the second resonator filter 14b. Specifically, the electrode pitch of the IDT 20 that constitutes the first resonator filter 14a may be different from the electrode pitch of the IDT 20 that constitutes the second resonator filter 14b. The SAW wavelength can be changed even if the cross width of the first resonator filter 14a and the cross width of the second resonator filter 14b are different.

このような関係を満たす距離Ｄ１と距離Ｄ２とは、例えば距離Ｄ１＝λ／２であり、距離Ｄ２＝λであればよい。 In the SAW filter 10, the phase difference between the SAW excited by the first resonator filter 14a and the SAW excited by the second resonator filter 14b is 180 degrees. In order to obtain this phase difference, the difference between the distance D1 between the input electrode 16 and the output electrode 18 of the first resonator filter 14a and the distance D2 between the input electrode 16 and the output electrode 18 of the second resonator filter 14b is The relationship of Equation 7 is satisfied.

The distance D1 and the distance D2 that satisfy such a relationship are, for example, the distance D1 = λ / 2, and the distance D2 = λ may be satisfied.

  Next, the operation of the SAW filter 10 will be described. FIG. 2 is an explanatory diagram of filter characteristics obtained by each resonator filter. First, when an electric signal is applied to the input electrode 16, SAW is excited in each resonator filter 14. The SAW propagates in a direction intersecting with the straight portions 26 and 36 and is reflected by the reflector 30. Therefore, the vibration energy of SAW is confined without leaking outside. In each resonator filter 14, the SAWs that propagate in the direction intersecting the straight portions 26 and 36 are coupled to the portion having the intersection width a 1 and the portion having the intersection width a 2, so that one wave is excited as a whole. When such SAW reaches the output electrode 18, it is converted into an electrical signal and output.

  The first resonator filter 14a and the second resonator filter 14b have different center frequencies, and a part of the pass band obtained by the first resonator filter 14a and the second resonator filter 14b A part of the obtained pass band is overlapped. In addition, since the SAW phase difference between the first resonator filter 14a and the second resonator filter 14b is shifted by 180 degrees, the resonance point (arrow A in FIG. 2) on the low band side in the low band pass band The high-frequency resonance point (arrow A ′ in FIG. 2) in the high-frequency passband is in phase. Also, the resonance point on the high frequency side in the low frequency band (arrow B in FIG. 2) and the resonance frequency on the low frequency side in the high frequency band (arrow B ′ in FIG. 2) are in phase. .

  A specific example is as follows. That is, the center frequency of the first resonator filter 14a is shifted to the low frequency side to obtain a pass band indicated by a solid line in FIG. 2, and the phase of the low frequency resonance point (arrow A in FIG. 2) in this pass band is set to 0 degree. The phase of the resonance point on the high frequency side (arrow B in FIG. 2) is 180 degrees. Further, the center frequency of the second resonator filter 14b is shifted to the high frequency side to obtain a pass band indicated by a dotted line in FIG. 2, and the phase of the low frequency resonance point (arrow B ′ in FIG. 2) in this pass band is 180 degrees. And the phase of the resonance point on the high frequency side (arrow A ′ in FIG. 2) is 0 degree. The amount by which the center frequency of the first resonator filter 14a or the center frequency of the second resonator filter 14b is moved is such that the arrow B and the arrow B ′ shown in FIG. 2 overlap, that is, in the low-pass band. The amount of movement may be such that the resonance point on the higher frequency side overlaps the resonance point on the lower frequency side in the high-frequency passband.

  As another example, the low-pass band indicated by the solid line in FIG. 2 is obtained by the second resonator filter 14b, and the high-pass band indicated by the dotted line in FIG. 2 is obtained by the first resonator filter 14a. May be obtained. Furthermore, the phase of the resonance point indicated by arrows A and A ′ in FIG. 2 may be 180 degrees, and the phase of the resonance point indicated by arrows B and B ′ in FIG. 2 may be 0 degrees.

The filter characteristics obtained by the SAW filter 10 are obtained by connecting the output electrode 18 of the first resonator filter 14a and the output electrode 18 of the second resonator filter 14b. It is a composite of characteristics. FIG. 3 is an explanatory diagram of filter characteristics obtained by the SAW filter. Since the filter characteristics of the SAW filter 10 are obtained by synthesizing the filter characteristics output from the resonator filters 14, a pass band is formed at three resonance points and becomes a wide band. Since the arrow B and the arrow B ′ shown in FIG. 2 have the same phase, they do not cancel each other, and since the arrow B and the arrow B ′ overlap each other, no ripple occurs in the passband.
Further, the SAW filter 10 having a steep fall from the pass band to the attenuation band (arrow C in FIG. 3) is obtained.

  In such a SAW filter 10, the wavelength of the SAW excited by the first resonator filter 14a and the wavelength of the SAW excited by the second resonator filter 14b are changed, so that the center of the first resonator filter 14a is changed. The frequency and the center frequency of the second resonator filter 14b can be made different. Further, of the resonance points appearing in the low-pass band obtained in any one of these resonator filters 14, the resonance points appearing in the high-pass pass band obtained in the other, You can superimpose it on the low side.

  Further, by changing the distance D1 between the input electrode 16 and the output electrode 18 of the first resonator filter 14a and the distance D2 between the input electrode 16 and the output electrode 18 of the second resonator filter 14b, the first resonator filter is changed. The difference between the phase of 14a and the phase of the second resonator filter 14b can be 180 degrees. The resonance point on the high frequency side in the passband on the low frequency side and the resonance point on the low frequency side in the passband on the high frequency side can be in phase, and even if these resonance points are overlapped, they cancel each other out. Can be prevented. Therefore, the filter characteristics of the SAW filter 10 can form a pass band at three resonance points, and a wide band can be obtained.

  Furthermore, the intersection width of each resonator filter 14 is the sum of the intersection width a1 and the intersection width a2. Therefore, each resonator filter 14 can reduce the crossing widths a1 and a2 of the straight portions 16 without reducing the overall crossing width, so that the impedance can be reduced and the influence of the transverse mode spurious can be eliminated. And SAW filter 10 can be reduced in size.

  Furthermore, the filter characteristic of the SAW filter 10 has a sharp fall from the pass band to the attenuation band, and is about twice as good as the fall of the conventional SAW filter from the pass band to the attenuation band. . Therefore, the SAW filter 10 according to the present embodiment can obtain a filter characteristic with good performance.

  In the above-described embodiment, the configuration in which one connection portion is provided in each electrode finger 22 and 32 of each resonator filter 14 has been described. However, the present invention is not limited to this configuration. That is, the SAW filter can have a configuration in which two or more connecting portions are provided on each electrode finger of each resonator filter and three or more linear portions are provided. FIG. 4 is an explanatory view showing a modification of the SAW filter. FIG. 4 shows an example of a configuration in which two or more connection portions are provided on each electrode finger of each resonator filter, and three or more straight portions are provided. The connection portions 24, 32 are connected to the electrode fingers 22, 32, respectively. A configuration is shown in which two 34 are provided and three straight portions 26 and 36 are provided. In FIG. 4, the two connection portions 24 and 34 provided on the electrode fingers 22 and 32 are shown in a form in which the bending directions are alternately left and right. There may be.

  In the above-described embodiment, two resonator filters 14 are provided. However, three or more resonator filters may be provided. When three or more resonator filters are provided, the filter characteristics of the SAW filter can be further widened.

It is explanatory drawing of a surface acoustic wave filter. It is explanatory drawing of the filter characteristic obtained with each resonator filter. It is explanatory drawing of the filter characteristic obtained with a SAW filter. It is explanatory drawing which shows the modification of a SAW filter.

Explanation of symbols

  10 ... SAW filter, 14 ... Resonator filter, 20 ... IDT, 22 ... Electrode finger, 24 ... Connection part, 26 ... Linear part, 30 ... Reflector, 32 ......... Electrode finger, 34 ......... Connecting part, 36 ......... Linear part.

Claims (5)

Elasticity formed by connecting in parallel two sets of resonator filters in which an input electrode and an output electrode made of interdigital electrodes are disposed on a piezoelectric substrate, and a reflector is disposed at a position sandwiching the input electrode and the output electrode. A surface wave filter,
The interdigital electrode and the electrode fingers of the reflector have a connection part bent obliquely, and a straight part connected to both ends of the connection part,
Superimposing a part of the passband obtained by the first resonator filter and the passband obtained by the second resonator filter;
The phase difference between the surface acoustic wave excited by the first resonator filter and the surface acoustic wave excited by the second resonator filter is 180 degrees.
A surface acoustic wave filter characterized by that.   The electrode pitch of the interdigital electrodes constituting the first resonator filter is different from the electrode pitch of the interdigital electrodes constituting the second resonator filter. Surface acoustic wave filter. The difference between the input electrode and the output electrode in the first resonator filter and between the input electrode and the output electrode in the second resonator filter is:

The surface acoustic wave filter according to claim 1, wherein: The straight portions provided on the electrode fingers are separated from each other by a distance L with respect to the propagation direction of the surface acoustic wave,
The distance L is

The surface acoustic wave filter according to claim 1, wherein: An elastic surface formed by connecting a plurality of resonator filters connected in parallel on a piezoelectric substrate, each of which includes an interdigital electrode and an output electrode, and a reflector disposed between the input electrode and the output electrode. A wave filter,
The interdigital electrode and the electrode fingers of the reflector have a connecting portion bent obliquely, and a straight portion connected to the connecting portion,
Superimposing a high frequency side in the low pass band obtained by one of the resonator filters and a low frequency side in a high pass band obtained by the other resonator filter; and , Changing the pitch of the interdigital electrodes of each resonator filter so as to form a continuous passband as a whole,
The difference between the input electrode and the output electrode of one of the resonator filters where the passbands overlap, and the difference between the input electrode and the output electrode of the other resonator filter are:

A surface acoustic wave filter characterized by satisfying the relationship:

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