JP2002198769A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the attenuation in the lower frequency of the pass-band attenuation pole in a surface acoustic wave filter where a ladder circuit is formed by using a surface acoustic wave electrode. SOLUTION: The surface acoustic wave filter consists of IDTs 53a-57a and reflectors 53b-57c having electrode fingers, and a relation of P1=P2=PR=PW is obtained, where P1 is a pitch between the electrode fingers of the IDTs 53a-57a, P2 is an electrode finger width of the IDTs 53a-57a, PR is a pitch between the electrode fingers of the reflectors 53b-57c and PW is the electrode finger width of the reflectors 53b-57c, and surface acoustic wave electrodes 55-57 of a parallel resonator group are formed to satisfy conditions of N1<=50 and NR>=40, where N1 is the number of pairs of all the electrode fingers of the IDTs and NR is the number of all the electrode fingers of the reflectors.

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. 6 is a schematic circuit diagram for explaining a 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. Resonators 530 and 54 composed of surface acoustic wave electrodes are provided on the piezoelectric substrate 520.
0, 550, 560 are provided. That is, as shown in the figure, resonators 530 and 540 are connected in series in a series arm formed between input terminal 570 and output terminal 580 (each resonator 530 and 540 is referred to as a series resonator). Further, the resonators 530 and 540 are collectively referred to as a series resonator group. Resonators 550 and 560 are connected in parallel between the series arm and the ground electrode 590 (each of the resonators 550 and 560 is referred to as a parallel resonator. In addition, the resonators 550 and 560 are combined in parallel. This is referred to as a resonator group. Note that the series resonators 530,
540 and the parallel resonators 550 and 560 are alternately arranged between the input and the output, and the respective parallel resonators 550 and 560 are arranged.
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, the series resonator 540 and the parallel resonator 56
One set of zeros constitutes a one-stage ladder filter.

The operation principle of the conventional surface acoustic wave filter 510 is as follows. FIG. 7 shows a series / parallel resonator 530.
It is a figure explaining the structure of each surface acoustic wave electrode formed in -560. Only the electrode portions of the surface acoustic wave electrode 700 of the one-port in the drawing is shown schematically.

The surface acoustic wave electrode 700 has a structure in which reflectors 720 and 730 are arranged on both sides of an IDT 710 arranged at the center. The IDT 710 combines 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 with each other.
12 have a structure in which they are arranged so as to intersect with each other. Note that, for example, the comb-shaped electrode 710a is connected to an input / output electrode, and 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 frequencies of the series resonators 530 and 540 and the parallel resonator 55
By matching the anti-resonance frequency of 0,560,
The input / output impedance is matched with the characteristic impedance in the vicinity of the frequency during this period, thereby forming a pass band. Especially in the ladder type filter circuit,
Since the surface acoustic wave electrode 700 has a predetermined impedance characteristic, it has a very high impedance near the anti-resonance frequency of the series resonators 530 and 540, and has a very low impedance near the resonance frequency of the parallel resonators 550 and 560. Therefore, in the ladder filter circuit utilizing this characteristic, a wide filter characteristic in which a low-pass band is formed from a high-pass band through a pass band.

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 composed of two attenuation poles formed by both surface acoustic wave electrodes of a series resonator and a parallel resonator, a pass-through filter is required. The amount of attenuation at both shoulders of the band becomes relatively large, so that the surface acoustic wave filter has characteristics suitable for a band filter of a mobile communication device.

However, in a surface acoustic wave filter having a ladder type circuit, the attenuation is very large near the attenuation pole near the pass band, but the frequency range of the attenuation pole is narrow. There is a problem that the amount of attenuation is small. 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 range 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. A group 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, In a surface acoustic wave filter in which a pass band of a filter is formed by substantially matching a resonance frequency formed by a series resonator group and an anti-resonance frequency formed by a parallel resonator group, the surface acoustic wave electrode includes an electrode finger. And a comb reflector having a plurality of electrode fingers provided on both sides of the comb-like electrode along a path along which a surface acoustic wave propagates. ,
The pitch between adjacent electrode fingers of the comb-shaped electrode is P
1. The electrode finger width of the comb-shaped electrode is P2, the pitch between adjacent electrode fingers of the grating reflector is PR, the electrode finger width of the grating reflector is PW, and P1 = P2 = PR.
= PW, and some of the surface acoustic wave electrodes of the parallel resonator group have N1 as the number of all electrode fingers of the comb-like electrode, and a reflector. N1 ≦ 50, NR ≧, where NR is the number of all electrode fingers of
A surface acoustic wave filter characterized by being formed so as to satisfy the condition (40).

As a design of a general ladder-type surface acoustic wave filter, as shown in FIG. 5, the surface acoustic wave is reflected at two frequencies by adjusting the frequency な る a at which the reflection coefficient 大 き く increases as shown by the solid line to the resonance frequency fr. It is designed to improve the reflection efficiency at the vessel.

However, in the present invention, at least some of the surface acoustic wave electrodes of the parallel resonator group are formed so as to satisfy N1 ≦ 50 and NR ≧ 40 as described above. As shown by the dotted line, the frequency of Δb at which the reflection coefficient becomes a minimum approaches the resonance frequency fr, whereby some surface acoustic wave electrodes may increase the impedance on the lower side than the pass band. it can.

That is, by setting N1 ≦ 50 and NR ≧ 40, the surface acoustic wave is reflected only in the comb-shaped electrodes of some surface acoustic wave electrodes, and the leaked surface acoustic wave is reflected by the reflector. It is considered that since the light is not reflected, the energy loss is increased, the impedance is increased, and spurious is generated on the lower frequency side than the resonance frequency fr.

Therefore, a part of the surface acoustic wave electrodes makes it possible to obtain a sufficient attenuation by lowering the insertion loss on the lower side of the attenuation band on the left shoulder side of the pass band as a whole filter. The interference of the adjacent channel can be prevented.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an electrode layout of a surface acoustic wave filter according to a first embodiment of the present invention.
FIG. 3 is an enlarged view of a reflector of a parallel resonator in order to explain a feature of the present invention. In FIG. 1, a surface acoustic wave filter A includes a plurality of surface acoustic wave electrodes 53 and 5 between an input terminal (IN) and an output terminal (OUT) on a main surface of a piezoelectric substrate 1.
4 and a parallel resonator group in which the surface acoustic wave electrodes 55, 56, and 57 are connected in parallel to the input / output terminals (IN, OUT) and the ground (GND). Have been. Hereinafter, each of the surface acoustic wave electrodes 53 and 54 of the series resonator group is referred to as a series resonator, and each of the surface acoustic wave electrodes 55 to 57 of the parallel resonator group is referred to as a parallel resonator.

A surface acoustic wave electrode 53 comprising a series resonator
54 and surface acoustic wave electrodes 55, 5 comprising parallel resonators
6 and 57 each have a comb-shaped interdigital transducer (hereinafter, IDT) 53a to 57a at the center, and grating reflectors (hereinafter, referred to as reflectors) 53b to 57c formed on both sides thereof. Having a structure.

The piezoelectric substrate 1 is made of quartz, lithium niobate, which has been cut so as to have a predetermined cut angle and a predetermined propagation direction.
It consists of lithium tantalate, lithium tetraborate and the like. Further, the surface acoustic wave electrodes 53 and 54 composed of series resonators and the surface acoustic wave electrodes 55 to 57 composed of parallel resonators are made of, for example, an aluminum thin film and have a thickness of 0.1 to 0.1.
It is formed in a predetermined pattern with a thickness of 0.3 μm. The IDTs 53a to 57a and the reflectors 53b on both sides thereof
The electrode finger widths and electrode finger intervals of ５７57c are, for example, １ ／ λ with respect to the wavelength λ of the surface acoustic wave.

As shown in FIG. 2, the IDTs 53a to 57a have an electrode finger A1 and an electrode finger A2 intersecting the electrode finger A1.
A2 is aligned perpendicularly to the propagation direction of the surface acoustic wave generated. The electrode finger pitch between the electrode fingers A1 and A2 is set to P1, the electrode widths of the electrode fingers A1 and A2 are set to P2, and P1 and P2 are set to substantially the same length. Have been.

The reflectors 53b to 57c are arranged on both sides of each of the IDTs 53a to 57a in the propagation path of the generated surface acoustic wave, and are formed perpendicular to the propagation direction of the surface acoustic wave generated by the plurality of electrode fingers B. I have. Here, the pitch between the adjacent electrode fingers B is set to PR, and the width of the electrode finger B is set to PW having the same length as the pitch PR. Thus, the surface acoustic wave electrodes 53 to 57 are configured such that the relationship of P1 = P2 = PR = PW is established. Reference numeral 61 denotes an input terminal, and 62 denotes an output terminal. Ground electrode (GND) of each parallel resonator 55, 56, 57
Is grounded by each of the reflectors 55c, 56c, 57c. That is, the input terminal 61 is connected to the ground electrode through the parallel resonator 57. The input terminal 61 is connected to the series resonator 53 in series, and
They are connected in the order of 5, 56 and are connected to the output terminal 62.

Here, the features of the present invention will be described. FIG. 5A shows resonance characteristics obtained in a part of the surface acoustic wave electrodes of the parallel resonator group, where N1 = 60 and NR = 1.
FIG. 5B is a diagram for comparing the resonance characteristics in the case of 0 and the resonance characteristics of N1 = 50 and NR = 40, and FIG. 5B shows the respective reflection characteristics. As a design of a general ladder-type surface acoustic wave filter, as shown by the solid line, the reflection efficiency of the surface acoustic wave at the two reflectors can be reduced by adjusting the frequency な る a at which the reflection coefficient 大 き く increases to the resonance frequency fr. Designed to be better.

However, in the present invention, at least a part of the surface acoustic wave electrodes 55 to 57 of the parallel resonator (hereinafter, the surface acoustic wave electrode 57 is described as an example) is
By forming so as to satisfy N1 ≦ 50 and NR ≧ 40 as described above, the frequency of Γb at which the reflection coefficient Γ becomes minimum as shown by the dotted line approaches the resonance frequency fr. The surface acoustic wave electrode of the portion can increase the impedance on the lower side than the pass band.

That is, when the above range is set, the surface acoustic wave is reflected only in the IDT 57a of the surface acoustic wave electrode 57, and the leaked surface acoustic waves are reflected by the reflectors 57b and 57b.
It is considered that since the light is not reflected by c, the energy loss increases, the impedance increases, and spurious components occur on the lower side than the resonance frequency fr.

Therefore, a part of the surface acoustic wave electrodes can reduce the insertion loss on the lower side of the attenuation band on the left shoulder side of the pass band of the filter as a whole filter, thereby obtaining a sufficient attenuation. -It is possible to prevent interference of a reception adjacent channel.

Here, if N1 is more than 50 pairs, IDT
Becomes less susceptible to the frequency characteristic of the reflection coefficient that increases the reflection of surface acoustic waves within the antenna, and if the NR is less than 40, the reflection coefficient of the reflector becomes smaller as a whole, causing insertion loss to deteriorate. Not preferred.

In the above description, one of the three surface acoustic wave electrodes 55, 56, and 57 of the three parallel resonators can form a spurious component on the lower side than the attenuation pole, but is not limited to this. It may be formed of two or more surface acoustic wave electrodes.
Even with such a configuration, a similar effect can be obtained.

[0028]

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. As the piezoelectric substrate 1, a XY lithium tantalate substrate having a cut angle of 42 ° XY was used, and on the surface thereof, series resonators 53 and 54 and parallel resonators 55, 56 and 57 made of Al or an Al alloy were formed. Thus, the ladder SA having the center frequency of 1.9 GHz
A W-type filter was manufactured.

In this case, the electrode finger period is 2.00 μm, the pitch P1 between the electrode fingers of the surface acoustic wave electrodes 53 and 54 of the series resonator is 1.038 μm, and the electrode finger width is 0.51 μm.
9 μm, each electrode finger pitch PR of the reflector is 1.038 μm
m, the electrode finger width PW is 0.519 μm, the pitch P1 between the electrode fingers of the surface acoustic wave electrodes 55, 56, 57 of the parallel resonator is 1.068 μm, and the electrode finger width is 0.53 μm.
4 μm, the pitch PR of each electrode finger of the reflector is 1.068 μm
m, and the electrode finger width PW was 0.534 μm.

In the surface acoustic wave electrodes 53 and 54 of the series resonator, the number of IDT electrode fingers is 50 pairs, the number of reflector electrode fingers is 40, and the parallel resonator surface acoustic wave electrodes 55 to 5 are used.
7, the number of electrode pairs is 20, and the number of electrode fingers of the reflectors 55b to 57c is 40.

The surface acoustic wave filter A thus formed
FIG. 3A shows the resonance characteristics of the series resonator and the parallel resonator of FIG.
FIG. 3B shows the filter characteristics. At this time,
In FIG. 3A, the vertical axis represents impedance, and the horizontal axis represents normalized frequency. The vertical axis in FIG. 3B is the attenuation (5
dB / div), and the horizontal axis is the normalized frequency.

In FIG. 3, a pole x due to spurious waves is formed on the lower side of the resonance frequency fr in the parallel row resonator, but this parallel resonator causes the filter to have a pass band in the pass band of the filter as shown in FIG. It can be seen that an attenuation pole appears on the lower band side than the left shoulder attenuation pole, and a sufficient amount of attenuation is secured in a wide frequency range.

Next, as a comparative example, the surface acoustic wave electrodes of the series vibrator were set with the number of pairs of all the electrode fingers of the surface acoustic wave electrodes 55 to 57 in the parallel resonator being 20, and the total number of the electrode fingers of the reflector being 20. Except having made it the same, it was set as the structure similar to an Example. FIG. 4A shows the resonance frequency of each resonator at that time.
FIG. 4B shows the filter characteristics. In FIG. 4, the vertical and horizontal axes are the same as in FIG.

According to the filter characteristics, it is understood that the amount of attenuation is smaller on the lower side than the left shoulder side attenuation pole of the pass band of the filter, and the frequency range in which the amount of attenuation can be secured is narrowed.

[0035]

As described above, according to the present invention, at least some of the surface acoustic wave electrodes of the parallel resonator group satisfy N1 ≦ 50 and NR ≧ 20 as described above. By forming so that some surface acoustic wave electrodes can increase the impedance lower than the pass band, in the attenuation band lower than the left shoulder attenuation pole of the pass band of the filter. A sufficient amount of attenuation can be secured.

Further, since only the attenuation in the vicinity of the lower attenuation band in the pass band of the filter can be improved without increasing the number of stages of the surface acoustic wave electrode, the characteristics in the pass band are not changed.
It is possible to provide a surface acoustic wave filter having a low loss in a pass band and a high attenuation in a lower attenuation side of the pass band.

[Brief description of the drawings]

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

FIG. 2 is an enlarged view of a surface acoustic wave electrode according to the present invention.

FIG. 3A is a diagram illustrating respective resonance characteristics of a series resonator and a parallel resonator according to the present invention, and FIG. 3B is a diagram illustrating a ladder-type filter including the series resonator and the parallel resonator according to the present invention. Shows the filter characteristics.

FIG. 4A is a diagram illustrating respective resonance characteristics of a series resonator and a parallel resonator of a comparative example, and FIG. 4B is a diagram of a ladder-type filter including a series resonator and a parallel resonator of a comparative example. Shows the filter characteristics.

FIG. 5A shows resonance characteristics obtained in a parallel resonator group, a solid line represents resonance characteristics when N1 = 60 and NR = 10, and a dotted line represents resonance characteristics when N1 = 50 and NR = 20. FIG. 3B is a diagram showing each reflection characteristic.

FIG. 6 is a schematic plan view for explaining a conventional surface acoustic wave filter.

FIG. 7 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, 54: Series resonator 55-57: Parallel resonator 61: Input terminal 62: Output terminal

Claims (1)

[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 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 terminal side or the output terminal side and the ground, and a resonance frequency formed by the series resonator group and a parallel resonator group In a surface acoustic wave filter in which a pass band of the filter is formed by making the formed anti-resonance frequencies substantially coincide with each other, the surface acoustic wave electrode includes a comb-shaped electrode that is arranged to face each other so that electrode fingers cross each other, A grating reflector having a plurality of electrode fingers provided on both sides of the comb-shaped electrode along a path along which the surface acoustic wave propagates, wherein the pitch between the adjacent electrode fingers of the comb-shaped electrode is P1, of the comb-shaped electrode Gokuyubi width P2, Bragg gratings reflector PR pitch between electrode fingers adjacent to each other, Bragg gratings reflector electrode finger width as PW P1 = P2 = PR = PW
Is satisfied, and at least a part of the surface acoustic wave electrodes of the parallel resonator group has N1 as the logarithm of all electrode fingers of the comb-like electrode, and has a grating reflection. When the number of all electrode fingers of the vessel is NR,
A surface acoustic wave filter formed so as to satisfy the following conditions. N1 ≦ 50 NR ≧ 40