JP2002232264A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make attenuation in a low pass region larger than that of a conventional filter circuit in a surface acoustic wave filter in which a ladder filter circuit using surface acoustic wave electrodes is configured. SOLUTION: In this surface acoustic wave filter in which the ladder filter circuit is composed of surface acoustic wave electrodes 53 to 56 of a serial resonator group and surface acoustic wave electrodes 57 to 60 of a parallel resonator group, a low frequency structure is adopted, in which the antiresonance frequency of the surface acoustic wave electrode 55 of a serial resonator used conventionally to form the attenuation pole of a highpass side is made lower than the resonance frequency of parallel resonators 57 to 60.

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 comprising a plurality of surface acoustic wave electrodes formed on a piezoelectric substrate, and more particularly, to an elastic surface acoustic wave filter capable of sufficiently attenuating a lower pass band. The present invention relates to a surface acoustic wave filter.

[0002]

2. Description of the Related Art In a mobile communication device, a surface acoustic wave filter having a plurality of surface acoustic wave electrodes formed on a piezoelectric substrate is known as a band filter for a high frequency band. For example, Japanese Patent Application Laid-Open No. 5-183380 discloses a surface acoustic wave filter in which a ladder-type filter circuit is formed by a plurality of surface acoustic wave electrodes on a piezoelectric substrate.

FIG. 7 is a schematic circuit diagram for explaining the surface acoustic wave filter disclosed in the above prior art. The conventional surface acoustic wave filter 510 is configured using a rectangular piezoelectric substrate 520. A resonator 5 having a surface acoustic wave electrode (not shown) formed on a piezoelectric substrate 520
30, 540, 550, and 560 are provided. That is, as shown in the figure, the input terminal 570 and the output terminal 58
0, the resonators 530, 5
40 are connected in series (each resonator 530, 540
Is called a series resonator. The resonators 530 and 540 are collectively referred to as a series resonator group. same as below. ). Further, resonators 550 and 560 are provided between the series arm and the ground electrode 590.
Are connected in parallel (each of the resonators 550 and 560 is referred to as a parallel resonator. The resonators 550 and 560 are collectively referred to as a parallel resonator group. The same applies hereinafter). Note that the series resonators 530 and 540 and the parallel resonators 550 and 560 are alternately arranged between the input and the output, and each parallel resonator 55
0 and 560 are connected to a ground electrode 590 via inductances 555 and 565, respectively. One set of the series resonator 530 and the parallel resonator 550 constitutes a one-stage ladder filter. Similarly, one set of the series resonator 540 and the parallel resonator 560 constitutes a one-stage ladder filter. ing.

The operation principle of the conventional surface acoustic wave filter 510 is as follows. FIG. 8 shows a series / parallel resonator 530.
It is a figure explaining the structure of each surface acoustic wave electrode formed in -560. In the figure, only the electrode portion of the one-port type surface acoustic wave resonator is schematically shown.

In FIG. 1, reference numeral 700 denotes a surface acoustic wave electrode. The surface acoustic wave electrode 700 has an IDT 7 disposed at the center.
10 has a structure in which reflectors 720 and 730 are arranged on both sides. In the IDT 710, a comb-shaped electrode 710a having a plurality of electrode fingers 711 and a comb-shaped electrode 710b having a plurality of electrode fingers 712 are arranged so as to intersect so that the electrode fingers 711 and 712 are interposed. Having a structure. In addition,
For example, the comb-shaped electrode 710a is connected to the input / output electrode,
The comb-shaped electrode 710b is connected to a ground electrode.

A surface wave excited by a signal input to the IDT 710 of the surface acoustic wave electrode 700 having such a structure is reflected by the reflectors 720 and 730 to become a standing wave.
It operates as a resonator having a high Q value confined between the reflectors 720 and 730. As is well known, the surface acoustic wave electrode 700 has a resonance characteristic in which a pole having a low impedance near the resonance frequency and a pole having a high impedance at the anti-resonance frequency appear.

A series / parallel resonator 530 having such a structure
In the surface acoustic wave filter 510 having 560, a pass band having a desired bandwidth is obtained by using the impedance characteristics of the surface acoustic wave electrode 700. That is, the resonance frequency of each series resonator 530, 540 and the anti-resonance frequency of each parallel resonator 550, 560 are substantially matched, so that the input / output impedance is matched with the characteristic impedance in the vicinity of the frequency between them. This constitutes a pass band. Particularly, in the ladder type filter circuit, since the surface acoustic wave electrode 700 has a predetermined impedance characteristic, the impedance becomes very high near the anti-resonance frequency of the series resonators 530 and 540, and conversely, the resonance frequency of the parallel resonators 550 and 560 Since the impedance is very low in the vicinity, the ladder-type filter circuit uses this characteristic to obtain a wide filter characteristic that forms a low-frequency stopband from a high-frequency stopband via a passband. become.

As a method of improving the attenuation of the attenuation pole in such a ladder-type filter circuit, an LC circuit formed of a surface acoustic wave electrode is provided on the resonator (Japanese Patent Laid-Open No. 9-23).
2906), a technique has been disclosed in which the inductance value of the wire itself is changed by changing the wire length at the time of external connection to change the position of the attenuation pole and adjust the amount of attenuation (JP-A-11-55067).

[0009]

However, in recent mobile communication systems, standards have been determined for effective use of radio waves. For example, in the US PCS standard, the reception band is 1930 to 1990 MHz, the transmission band is Is 1
850 to 1910 MHz, the interval between the pass bands of the transmitting and receiving filters is 20 MHz, and the transmitting band is formed adjacent to the receiving band. Therefore, in the receiving filter, a sufficient attenuation and pass band are maintained while maintaining a wide band. , It is necessary to ensure a low insertion loss.

In a conventional surface acoustic wave filter constituting a ladder type filter circuit, since a filter is constituted by two attenuation poles formed by both a series resonator and a parallel resonator, both ends of a pass band are formed. Is relatively large, whereby the surface acoustic wave filter has characteristics suitable as a bandpass filter of a mobile communication device.

[0011] However, although the attenuation is very large in the vicinity of the attenuation pole near the pass band, there is a problem that the attenuation decreases rapidly outside the frequency of the attenuation pole.
In other words, in the stop band near the pass band, a large amount of attenuation can be obtained near the frequency of the attenuation pole, but since the frequency region where the amount of attenuation is large is narrow, the transmission / reception interval in the transmission / reception interval in recent mobile communication standards is small. It is difficult to secure the amount of attenuation in the adjacent channel.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to make it possible to sufficiently attenuate the lower band of a pass band and to prevent interference between adjacent transmission / reception channels. It is another object of the present invention to provide a surface acoustic wave filter that can perform the above method.

[0013]

According to the present invention, there is provided a series resonator having a plurality of surface acoustic wave electrodes connected in series between an input terminal and an output terminal on a surface of a piezoelectric substrate. And a parallel resonator group in which a plurality of surface acoustic wave electrodes are connected in parallel between the input terminal side or output terminal side of each surface acoustic wave electrode of the series resonator group and the ground, and A surface acoustic wave filter in which a pass band of a filter is formed by substantially matching a resonance frequency formed by surface acoustic wave electrodes of a resonator group and an anti-resonance frequency formed by surface acoustic wave electrodes of a parallel resonator group, Of the surface acoustic wave electrodes of the series resonator group, some surface acoustic wave electrodes have a low anti-resonance frequency of the surface acoustic wave electrode lower than the resonance frequency of the surface acoustic wave electrode of the parallel resonator group. Features a frequency structure To provide an elastic surface wave filter.

Here, the low frequency structure means that the anti-resonance frequency of the surface acoustic wave electrodes of the series resonators set to be higher than the anti-resonance frequency of the surface acoustic wave electrodes of the parallel resonators Is a structure in which the shape, thickness, material and the like of the electrode are changed so as to be lower than the resonance frequency of the surface acoustic wave electrode.

According to the structure of the present invention, since a part of the surface acoustic wave electrodes of the series resonators has a low-frequency structure, the transmission of the filter formed at the resonance frequency of the surface acoustic wave electrodes of the parallel resonators is performed. Since a new attenuation pole can be formed on the lower side of the attenuation pole on the low frequency side of the pass band in the characteristics, a sufficient amount of attenuation can be secured in a wide frequency range.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a structure of a surface acoustic wave filter according to an embodiment of the present invention, and FIG. 2 is an enlarged view of a parallel resonator for explaining features of the present invention. In FIG. 1, the surface acoustic wave filter A is a piezoelectric substrate 1
A series resonator in which a plurality of surface acoustic wave electrodes 53, 54, 55, 56 are connected in series between an input terminal (IN) and an output terminal (OUT); ,
A parallel resonator group is formed in which 58, 59, 60 are connected in parallel to the input / output terminals (IN, OUT) and the ground (GND). Hereinafter, each of the surface acoustic wave electrodes 53 to 56 of the series resonator group is referred to as a series resonator, and each of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group is referred to as a parallel resonator.

The surface acoustic wave electrodes 53, 54,
55, 56 and surface acoustic wave electrodes 57, 5 of parallel resonators
Reference numerals 8, 59, and 60 denote interdigital transducers (hereinafter, IDTs) 53a to 60a each having a comb-like shape at the center.
And ladder-like reflectors 53b, 53c, 5 on both sides thereof
4b, 54c... 60b, 60c are formed.

The piezoelectric substrate 1 is made of quartz, lithium niobate, lithium tantalate, lithium tetraborate, or the like, which is cut into a rectangular shape so as to have a predetermined cut angle and a predetermined propagation direction. The surface acoustic wave electrodes 53, 54, 55, 56 and the surface acoustic wave electrodes 57, 58, 59, 60 are, for example,
It is made of aluminum thin film and its thickness is 0.1-0.3
It is formed in a predetermined pattern with a thickness of μm. Also, I
DTs 53a-60a and reflectors 53b, 53 on both sides thereof
The electrode finger widths and electrode finger intervals of c, 54b, 54c... 60b, 60c are, for example, approximately λλ with respect to the wavelength λ of the surface acoustic wave.

As an example of the structure of the IDTs 53a to 60a, taking a surface acoustic wave electrode of a parallel resonator group as an example, as shown in FIGS. 2A and 2B, an electrode finger A1 and an electrode finger A2 crossing the electrode finger A1. Are arranged so as to cross each other, and are arranged orthogonal to the propagation direction of the generated surface acoustic wave.

Reference numeral 61 denotes an input terminal, and 62 denotes an output terminal. IDTs 53a to 56a are connected in series from an input terminal 61, and are connected to an output terminal 62. ID for these
One side of T57a-60a is each reflector 57b, 58c,
The other side is connected to each ground electrode (GND) by each reflector 57c, 58b, 59b, 60c.

Here, in the present invention, as shown in FIG.
A low-frequency structure in which the anti-resonance frequency Fa of the surface acoustic wave electrode 55 among the surface acoustic wave electrodes 53 to 56 of the series resonator group is lower than the resonance frequency Fr of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group. Has become. Thereby, in the filter characteristic, the anti-resonance frequency of the surface acoustic wave electrode 55 of the series resonator group located further lower than the attenuation pole which can be set to the resonance frequency Fr of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group. A new attenuation pole is formed at Fa, and the two attenuation poles can ensure a sufficient attenuation over a wide frequency range.

A specific method of the low frequency structure is to change the electrode finger pitch of the surface acoustic wave electrode 55. The electrode finger pitch is composed of the electrode finger width Pw and the electrode finger pitch Pp between the electrode fingers as shown in FIGS. That is, the electrode finger pitch (Pw + Pp) of the surface acoustic wave electrode 55 of the series resonator group is formed longer than the electrode finger pitch (Pw + Pp) of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group. As a result, the anti-resonance frequency of the surface acoustic wave electrode 55 is changed to the surface acoustic wave electrodes 57 to 60 of the parallel resonator group.
Lower than the resonance frequency.

As means for changing the electrode finger pitch, the distance Pp between the adjacent electrode fingers A1 and A2 of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group and the surface acoustic wave electrode 5
5, the distance Pp between the adjacent electrode fingers A1 and A2 is substantially the same, and the width Pw of each electrode finger of the surface acoustic wave electrode 55 is longer than the width Pw of each electrode finger of the surface acoustic wave electrodes 57 to 60. Is achieved.

Conversely, each electrode finger width P of the surface acoustic wave electrode 55
w and the electrode finger widths Pw of the surface acoustic wave electrodes 57 to 60 are substantially the same, and the distance Pp between the adjacent electrode fingers A1 and A2 of the surface acoustic wave electrode 55 is
This can be achieved even if the distance between the adjacent electrode fingers A1 and A2 is longer than the distance Pp between the adjacent electrode fingers A1 and A2.

Further, each electrode finger width P of the surface acoustic wave electrode 55 is
w is formed longer than each electrode finger width Pw of the surface acoustic wave electrodes 57 to 60, and each electrode finger A adjacent to the surface acoustic wave electrode 55 is formed.
This can be achieved even if the distance Pp between the electrodes A1 and A2 is longer than the distance Pp between the adjacent electrode fingers A1 and A2 of the surface acoustic wave electrodes 57 to 60.

On the other hand, as a low-frequency structure, the metallization ratio of the surface acoustic wave electrode 55 (referred to as the ratio of the electrode finger width Pw to the electrode finger pitch) means that the surface acoustic wave electrodes 57 to 60 of the parallel resonator group have a lower frequency. There is a method of reducing the metallization ratio.

As a low frequency structure, the electrode thickness of the surface acoustic wave electrode 55 is set to
There is also a method of forming the electrode 60 thicker than the electrode thickness of 60.

In the above description, the surface acoustic wave electrode 55 of one series resonator group may form an attenuation pole further lower than the attenuation pole formed by the surface acoustic wave electrodes 57 to 60 of the parallel resonator group. Although the configuration is such that it is possible, the present invention is not limited to this, and it may be formed by two or more surface acoustic wave electrodes of a series resonator group. Even with such a configuration, a similar effect can be obtained.

Thus, the surface acoustic wave filter A of the present invention
According to the conventional technique, the surface acoustic wave electrodes 53 to 5 of the series resonator
6, the anti-resonance frequency Fa of one surface acoustic wave electrode 55 of the surface acoustic wave electrodes 53 to 56 is used as a parallel resonator. Resonance frequency Fr formed by group surface acoustic wave electrodes 57-60
By setting it lower than that, it is possible to form an attenuation pole further lower than the lower attenuation pole formed by the parallel resonator group, so that the attenuation is secured over a wide range. In other words, the problem that the problematic amount of attenuation cannot be secured and interference occurs can be solved. Moreover,
Since the passband characteristic hardly changes, the conventional design method can be used as it is.

Conventionally, by changing the length of an aluminum wire or a gold wire, which is an external routing electrode, the inductance value of the wire is changed to secure the amount of attenuation in the lower attenuation region. The position of the poles is slightly shifted depending on the state, which causes a decrease in the yield of the characteristic inspection. However, according to the present invention, the attenuation range can be widened, so that a stable yield can be secured.

[0031]

Next, in order to confirm the operation and effect of the present invention, an embodiment of the present invention is shown in the structure of FIG. A 42 ° YX lithium tantalate substrate is used as the piezoelectric substrate 1, and the surface acoustic wave electrodes 53 to 56 of a series resonator group and the surface acoustic wave electrodes 57 to 57 of a parallel resonator group formed of Al or an Al alloy on the surface thereof. 60 were formed. Thus, a ladder SAW filter having a center frequency of 1.9 GHz was manufactured.

In this case, the surface acoustic wave electrodes 53 to 56 of the series resonators and the surface acoustic wave electrodes 57 to 6 of the parallel resonators are used.
The cross width W of 0 was W = 20λ. Surface acoustic wave electrode 53
75 to 75 electrode pairs, surface acoustic wave electrodes 57 to 60
Are 12 pairs of electrodes.

Here, the surface acoustic wave electrode 5 of the series resonators
The electrode finger width of 3, 54, 56 is set to Pw = 0.5 μm, and only the surface acoustic wave electrode 55 of the series resonator group is set to Pw = 0.55.
The electrode width was set to be long in μm. Note that the electrode finger pitch Pp is substantially the same in the series resonator group.

The surface acoustic wave filter A thus formed
FIG. 4 shows the filter characteristics of. At this time, the vertical axis in FIG. 4 is the attenuation (5 dB / div), and the horizontal axis is the frequency (50 dB / div).
MHz / div). In FIG. 4, an attenuation pole formed by the surface acoustic wave electrode 55 of the series resonator is formed near 1860 MHz. In the vicinity of 1910 MHz, an attenuation region is formed by the surface acoustic wave electrodes 57 to 60 of the parallel resonator. Further, the attenuation pole near 2040 MHz is the attenuation pole due to the surface acoustic wave electrodes 53, 54 and 56 of the series resonator. As shown in FIG. 4, the pole formed by the surface acoustic wave electrode 55 of the series resonator is formed on the lower band side of the surface acoustic wave electrodes 57 to 60 of the parallel resonator group. It can be seen that the attenuation is secured.

On the other hand, as a comparative example, FIG. 6 shows the filter characteristics of a surface acoustic wave filter in which the electrode widths Pw of the series surface acoustic wave electrodes 53 to 56 are all the same. Also in FIG. 6, the vertical axis indicates the attenuation (5 dB / div),
The horizontal axis is frequency (50 MHz / div). According to this filter characteristic, it can be seen that a sufficient amount of attenuation is not obtained on the lower band side of the attenuation pole on the lower band side.

As described above, according to the present invention, one electrode pitch (Pw + P
By making p) longer than the electrode pitch (Pw + Pp) of the surface acoustic wave electrodes of the other series resonators, the anti-resonance frequency of the series resonator is shifted to a lower frequency side, and a wide frequency range on the lower side of the pass band is obtained. It is understood that a sufficient amount of attenuation can be secured.

[0037]

According to the structure of the present invention, a part of the surface acoustic wave electrodes of the series resonators has a low-frequency structure. Thus, it is possible to provide a surface acoustic wave filter that can secure a sufficient amount of attenuation over a wide frequency range by using these two attenuation poles.

Further, since only the attenuation in the lower band can be improved without increasing the number of stages of the surface acoustic wave electrode, the characteristics of the pass band are not changed, the loss in the pass band is low, and the attenuation in the lower band of the pass band is reduced. The present invention can provide a surface acoustic wave filter having a high attenuation range.

[Brief description of the drawings]

FIG. 1 is a plan view of a surface acoustic wave filter according to the present invention.

2A is a diagram illustrating a configuration of surface acoustic wave electrodes 53, 54, and 56, and FIG. 2B is a diagram illustrating a configuration of a surface acoustic wave electrode 55.

FIG. 3 is a diagram showing resonance characteristics of the surface acoustic wave electrodes 53 to 60 of the present invention.

FIG. 4 is a characteristic diagram showing a filter characteristic of the present invention.

FIG. 5 is a characteristic diagram showing a filter characteristic of the present invention.

FIG. 6 is a characteristic diagram illustrating filter characteristics of a comparative example.

FIG. 7 is a schematic plan view for explaining a conventional surface acoustic wave filter.

FIG. 8 is an enlarged plan view for explaining a one-port type surface acoustic wave electrode.

[Explanation of symbols]

 A: Surface acoustic wave filter 1: Piezoelectric substrate 53 to 56: Surface acoustic wave electrode of series resonator group 57 to 60: Surface acoustic wave electrode of parallel resonator group 61: Input terminal 62: Output terminal

Claims (7)

[Claims] 1. A series resonator group having a plurality of surface acoustic wave electrodes connected in series between an input terminal and an output terminal on a surface of a piezoelectric substrate, and an input terminal of each surface acoustic wave electrode of the series resonator group. A parallel resonator group in which a plurality of surface acoustic wave electrodes are connected in parallel between the side or the output terminal side and the ground, and a parallel with the resonance frequency formed by the surface acoustic wave electrodes of the series resonator group In a surface acoustic wave filter that forms a pass band of a filter by substantially matching anti-resonance frequencies formed by surface acoustic wave electrodes of a resonator group, a part of the surface acoustic wave electrodes of the series resonator group is The surface acoustic wave filter has a low frequency structure in which the anti-resonance frequency of the surface acoustic wave electrode is lower than the resonance frequency of the surface acoustic wave electrodes of the parallel resonator group. 2. The low-frequency structure according to claim 1, wherein the surface acoustic wave electrode is constituted by comb-shaped electrodes arranged so that the electrode fingers cross each other. 2. The surface acoustic wave according to claim 1, wherein an electrode finger pitch comprising a width and an electrode finger interval between electrode fingers is formed longer than an electrode finger pitch of the surface acoustic wave electrodes of the parallel resonator group. filter. 3. The electrode finger pitch of the part of the surface acoustic wave electrodes is such that each electrode finger pitch is substantially the same as the electrode finger pitch of the surface acoustic wave electrodes of the parallel resonator group, and The surface acoustic wave filter according to claim 2, wherein each electrode finger width of the surface acoustic wave electrode is formed longer than each electrode finger width of the surface acoustic wave electrodes of the parallel resonator group. 4. The electrode finger pitch of the part of the surface acoustic wave electrodes is such that each electrode finger width is substantially the same as the electrode finger width of the surface acoustic wave electrodes of the parallel resonator group, and 3. The surface acoustic wave filter according to claim 2, wherein the distance between the electrode fingers of the surface acoustic wave electrode is longer than the distance between the electrode fingers of the surface acoustic wave electrodes of the parallel resonator group. 5. The electrode finger pitch of the part of surface acoustic wave electrodes is such that each electrode finger width is formed longer than each electrode finger width of the surface acoustic wave electrodes of the parallel resonator group, and 3. The surface acoustic wave filter according to claim 2, wherein the distance between the electrode fingers of the surface acoustic wave electrode is longer than the distance between the electrode fingers of the surface acoustic wave electrodes of the parallel resonator group. 6. The low-frequency structure according to claim 1, wherein the surface acoustic wave electrodes are composed of comb-shaped electrodes arranged so as to face each other such that the electrode fingers cross each other. The ratio of the width of the electrode finger to the electrode finger pitch consisting of the electrode finger spacing between fingers is formed so that the surface acoustic wave electrodes of the parallel resonator group are smaller than the part of the surface acoustic wave electrodes. The surface acoustic wave filter according to claim 1, wherein: 7. The low-frequency structure according to claim 1, wherein the surface acoustic wave electrode comprises a comb-shaped electrode arranged so that electrode fingers cross each other, and a thickness of the part of the surface acoustic wave electrode is reduced. 2. The surface acoustic wave filter according to claim 1, wherein the thickness of the surface acoustic wave electrode of the group of parallel resonators is larger than that of the surface acoustic wave electrode.