JP2001177370A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave device for which a low loss, a broadband and a steep skirt characteristic are required. SOLUTION: Transducers, each consisting of RSPUDTs 5, 9, 13, 16 by oppositely arranging SPUDTs 3, 4, 7, 8, 11, 12, 14, 15 with a unidirectional electrode structure, having directivity in a propagation characteristic in a way that the propagation directions are opposite to each other, are connected in parallel on a piezoelectric substrate 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device in which surface acoustic wave elements are connected in parallel.

[0002]

BACKGROUND OF THE INVENTION As is well known, as is well known, generally,
A surface acoustic wave filter of a surface acoustic wave device used for mobile communication is required to have low loss and steep out-of-band cutoff characteristics. In particular, GSM (Global System for Mobi
le Communication)-IF (Intermediate Frequency)
Like a filter, a surface acoustic wave element for a system in which a steep filter characteristic is required because low-loss property is emphasized and an adjacent channel is close to a wide band, for example, is a subordinate type. , A resonator type filter connected in multiple stages is used.

However, in a resonator type filter that is connected in multiple stages, it is necessary to increase the number of stages in order to ensure steepness, and the loss increases. Further, in order to achieve a wide band, it is necessary to tune between the stages of each resonator filter, which makes it difficult to mount.

[0004] Other surface acoustic wave filters include:
For example, JP-A-62-43204 and JP-A-9-2
A configuration described in, for example, Japanese Patent No. 14284 is known.
These realize a wide band by the frequency intervals of the multiple modes of the resonators in each stage.

However, in these filters, since the multiple resonance frequency interval mainly depends on the electromechanical coupling coefficient inherent to the piezoelectric substrate, there is a limit in widening the band. Unless the phase of the unnecessary higher-order mode outside the mode band is set to be opposite to the phase of the other resonator type filter, the unnecessary higher-order mode cannot be suppressed. For this reason, it is practically impossible to design the in-band characteristics while controlling the out-of-band characteristics.

Further, in recent years, a single phase uni-directional (SPUDT) comprising a comb-shaped electrode formed in a unidirectional electrode structure so as to give directionality to the propagation characteristics of a surface acoustic wave.
A surface acoustic wave filter using a surface acoustic wave element R (Resonant) SPUDT, in which a directional transducer is disposed so that the main propagation directions of the excitation waves are opposite to each other, has a high degree of design freedom and a low surface acoustic wave filter. Since it is easy to achieve miniaturization due to loss, it has been widely used.

However, in this type of surface acoustic wave filter, since the internal reflection of the electrode finger is actively used to obtain one-way characteristics, both the bandwidth and the skirt characteristic are in one direction in the positive direction. Since it depends on the ratio and reflectance of the unidirectional electrode and the unidirectional electrode in the opposite direction, and the number of electrode fingers,
There are limits to designing bandwidth and skirt characteristics independently. In particular, it is difficult to achieve both a wide bandwidth and a steep skirt characteristic.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and has as its object to provide a surface acoustic wave device which requires a low loss, a wide band and a steep skirt characteristic.

[0009]

A surface acoustic wave device according to the present invention is formed on a piezoelectric substrate, includes a pair of comb-shaped electrodes, and has a plurality of surface waves having directions opposite to each other. Having two or more transducers with
At least two of these transducers are connected in parallel with each other.

[0010] The inventors of the present invention have proposed a surface acoustic wave device having an RSPUDT structure in which a positive SPU inside a transducer is provided.
The ratio of the DT area to the reverse SPUDT area (each S
It was found that changing the ratio of the number of electrode fingers in the PUDT region) changed the frequency characteristics of the transducer, and in particular, the resonance point interval.

That is, a surface acoustic wave device using an RSPUDT-structured transducer behaves intermediately between a so-called transversal type filter and a resonator type filter, and a positive direction SPUDT inside the transducer.
A resonance cavity is formed with the boundary between the region and the reverse SPUDT region as an end, exhibits a multimode resonance frequency characteristic, and has a plurality of peaks (resonance points) in frequency-amplitude characteristics.

In the filter having the RSPUDT structure, the resonance cavity length changes as the ratio of the number of electrode fingers in the forward SPUDT to the number of electrode fingers in the reverse SPUDT inside the transducer changes. As a result, the position of the resonance point on the frequency axis also changes. For example, 45 ° XZ lithium tetraborate (Li2B4O
7: LBO) It was found that when the number of electrode fingers in the positive SPUDT region of the transducer having the RSPUDT structure formed on the piezoelectric substrate was increased (the ratio of the positive SPUDT region was increased), the resonance point interval was increased. . See the 1999 IEEE ULTRASONICS SYMPOSIUM PR
See OCEEDING Vol.1 P351-356.

Utilizing this phenomenon, two (or two or more) RSSPUDTs are connected in anti-phase parallel, and both RSPUDTs are connected.
A filter is configured such that the resonance points of the DT are desirably arranged at equal intervals on the frequency axis, and the output is impedance-matched, so that a wide band pass characteristic obtained by combining the frequency characteristics of the two RP SPUDTs can be realized. . At this time, the desired characteristic can be realized by controlling the resonance point interval of the RSPUDT, and the resonance point interval can be controlled only by changing the ratio of the number of electrode fingers in the forward SPUDT and the reverse SPUDT. Therefore, it is possible to arrange the pass bandwidth relatively freely by making small changes in the electrode design.

The resonance frequency of one of the transducers connected in parallel is represented by Fl.
Assuming that the resonance frequencies of the other transducers are Fl2, Fc2, and Fu2, Fl1 <Fl2 <Fc2 <Fc1 <Fu1
<Fu2.

For this reason, the out-of-band characteristics, especially the skirt characteristics, are maintained in opposite phase relations on the low frequency side near the resonance frequency F11 and on the high frequency side near the resonance frequency Fu2.
The passing characteristics of one transducer and the other transducer cancel each other out, and a steep skirt characteristic can be realized, and steepness is realized.

Further, the phase of Fl1 and the phase of Fl2 are reversed,
Since the phase of Fc1 and the phase of Fc2 are reversed and the phase of Fu1 and the phase of Fu2 are reversed, a wider band can be achieved.

Further, the resonance frequencies Fl1, Fc1, Fu1, Fl2,
Since the intervals between at least four resonance frequencies of Fc2 and Fu2 are substantially equal, a wide band can be achieved.

The resonance frequencies Fl1, Fc1, Fu1, Fl2, Fc
Since the insertion-side chamber values of at least four of the resonance frequencies of Fu2 and Fu2 are substantially equal, a wide band can be uniformly obtained regardless of the frequency.

Further, one transducer and the other transducer are both formed on the same chip.

Further, one transducer and the other transducer are formed on different chips.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. That is, in FIG. 1, reference numeral 1 denotes a surface acoustic wave device.

The surface acoustic wave device 1 includes a surface acoustic wave filter A as a first surface acoustic wave element on the same chip.
And a surface acoustic wave filter B as a second surface acoustic wave element
Are connected in parallel.

These surface acoustic wave filters A and B are composed of two RSPs arranged in the main propagation direction of the surface acoustic wave.
UDTs 5, 9, 13, and 16 are provided. Each of the RSPUDTs has a positive SPUDT 3,8,11,1 for transmitting SAW in the forward direction, assuming that the traveling direction of the surface acoustic wave (SAW) is a forward direction in the right direction and a reverse direction in the left direction in FIG.
5, and reverse SPUDTs 4, 7, which transmit SAW in the reverse direction.
12 and 14.

FIG. 2 shows an example of the SPUDT electrode structure. That is, an electrode finger 3a having a λ / 8 width (λ is a surface wave wavelength) connected to one of a pair of bus bars formed on the piezoelectric substrate 10 and a 3λ connected to the other bus bar.
It has a comb-tooth electrode structure in which / 8-width electrode fingers 3b and λ / 8-width electrode fingers 3c are alternately combined.

In this comb-teeth electrode structure, the phase relationship between the excitation wave and the internally reflected wave generated at the end of the electrode differs between the right and left directions in the figure. Due to the phase relationship between the excitation wave and the internally reflected wave, the surface acoustic wave excited is strengthened in one direction and weakened in the opposite direction, and as a result, one-way property is obtained.

In this embodiment, an RSPUDT structure is obtained by arranging a comb-shaped electrode structure having a line-symmetrical structure in which the comb-shaped electrode structure is just folded to form one transducer.

That is, the normal SPUDT and the reverse SPUDT shown in FIG. 1 are configured as a single transducer having comb-shaped electrodes connected to a common bus bar, and the electrode arrangement is line-symmetrical across the boundary. It has a structure.

Here, the line-symmetric structure does not mean perfect line symmetry. For example, normal SPUDT and reverse S
The number of electrode fingers of the PUDT does not always match, and the ratio of the number of electrode fingers can be changed according to required characteristics as described later.

Further, one transducer does not necessarily need to be composed of a pair of normal SPUDT area and reverse SPUDT area, and even if it is configured such that several normal SPUDT areas and reverse SPUDT areas are arranged inside one transducer. good.

In actually constructing the filter,
For example, in order to realize a desired filter characteristic by performing weighting using this directionality, the normal SPUDT and the inverse SPUDT are used.
Although the arrangement of T is often more complicated, the arrangement is not particularly limited to the RSPUDT structure shown in FIG. 1, and it is of course possible to use an RSPUDT structure having such a complicated structure.

Each of the filters A and B has a resonance cavity having the boundary between the normal SPUDT and the reverse SPUDT as an end, and performs an intermediate operation between the transversal filter and the resonator filter. In this configuration, the electrode fingers forming the positive SPUDT inside the transducer and the reverse S
When the ratio of the electrode fingers constituting the PUDT is changed, the resonance cavity length also changes. As a result, the resonance mode interval of these filters also changes.

The surface acoustic wave filters A and B each have a triple mode resonance frequency. In this case, the surface acoustic wave filter A
If the resonance frequencies of the surface acoustic wave filter B are Fl2, Fc2, and Fu2, and the resonance frequencies of the surface acoustic wave filter B are Fl1, Fc1, and Fu1,
<Fc2 <Fc1 <Fu1 <Fu2.

As shown in FIGS. 3 and 4, the phase relationship of the resonance frequency Fl1 is opposite to the phase of the resonance frequency Fl2, and the phase of the resonance frequency Fc1 is the resonance frequency Fc2.
The phase of the resonance frequency Fu1 is opposite to the phase of the resonance frequency Fu2.

Further, the resonance frequencies Fl1, Fc1, Fu1, Fl2,
Of Fc2 and Fu2, the intervals between at least four resonance frequencies are set to be approximately equal, and the insertion loss values of at least four resonance frequencies are set to be approximately equal.

The total of six resonance modes, ie, three resonance modes of the surface acoustic wave filter A and three resonance modes of the surface acoustic wave filter B are all coupled, and as shown in FIG. Two large bands can be formed.

Outside the band, the surface acoustic wave filter A and the surface acoustic wave filter B maintain the opposite sign relationship even in the vicinity of the band.
The steepness of the skirt characteristic increases as compared with the case of the surface acoustic wave filter A and the surface acoustic wave filter B alone. In this state, if the insertion loss levels of the filter A and the filter B are equal, the attenuation near the band becomes infinite.

Here, FIG. 6 shows a surface acoustic wave device 21 in which resonator filters are connected in anti-phase parallel as a comparative example of the surface acoustic wave device 1 shown in FIG.

That is, the surface acoustic wave device 21 includes a surface acoustic wave filter A as a first surface acoustic wave element and a surface acoustic wave filter B as a second surface acoustic wave element on the same chip. It is configured to be connected in parallel.

The surface acoustic wave filter A is connected to an input terminal 22 formed on the piezoelectric substrate 20.
DT (Inter Digital Transducer) 25 and piezoelectric substrate 2
IDT 29 connected to an output terminal 26 formed on
It consists of

The IDT 25 has a configuration in which the comb-shaped electrode 23 and the comb-shaped electrode 24 cross each other. The IDT 29 has a configuration in which the comb-tooth electrode 27 and the comb-tooth electrode 28 cross each other.

On the other hand, the surface acoustic wave filter B is connected to an input terminal 22 formed on the piezoelectric substrate 20.
DT 33 and output terminal 26 formed on piezoelectric substrate 20
And an IDT connected to the IDT.

The IDT 33 has a configuration in which the comb-shaped electrodes 31 and 32 intersect each other. The IDT 36 has a configuration in which the comb-tooth electrode 34 and the comb-tooth electrode 35 intersect each other.

The surface acoustic wave filter A and the surface acoustic wave filter B are respectively provided on both sides thereof with a reflector 3
7.

The surface acoustic wave filter A and the surface acoustic wave filter B each have a dual mode type resonance frequency characteristic. In this case, even if the resonance frequencies of the surface acoustic wave filter A are Fl1 and Fu1 and the resonance frequencies of the surface acoustic wave filter B are Fl2 and Fu2, the frequency characteristics depend only on the coupling constant and the reflectance of the substrate. As shown in FIG. 8 or FIG. 8, unlike the surface acoustic wave device 1 shown in FIG. 1, it is difficult to widen the band.

In the above-described embodiment, the surface acoustic wave filter A and the surface acoustic wave filter B are formed on the same chip. However, similar effects can be obtained by forming them on different chips. be able to.

Further, since the RSPUDT structure is used, the out-of-band characteristic can be freely designed by controlling the excitation or reflection distribution of the IDT with a weighting function. It is possible to greatly improve the surface acoustic wave device 21 connected in anti-phase parallel.

That is, it is possible to realize a wider band than a single unit, obtain a steep skirt characteristic, freely design band characteristics and out-of-band characteristics, and achieve downsizing.

Next, using an LBO substrate as a piezoelectric substrate, a 210 MHz band PCS (Personal Communication) is used.
s System)-Two RSPs formed of aluminum (Al) film on the same piezoelectric substrate by forming an IF filter
The results of experiments on the surface acoustic wave filters A and B having the UDT structure will be described.

FIG. 9 shows the surface acoustic wave filter A having a resistance of 50Ω.
FIG. 10 shows the frequency characteristic of the surface acoustic wave filter B of the 50Ω system.

FIG. 11 shows a composite waveform of a surface acoustic wave filter A having the frequency characteristic shown in FIG. 9 and a surface acoustic wave filter B having the frequency characteristic shown in FIG. 10 connected in parallel. 9 shows frequency characteristics.

Next, FIG. 12 shows that a resistor R1, a capacitor C1 and an inductor L1 are connected to the input side of the surface acoustic wave device 1.
FIG. 1 shows a state in which an external circuit composed of a resistor R2, a capacitor C2, and an inductor L2 is connected to the output side of the surface acoustic wave device 1, and matching is performed by these external circuits. I have.

As shown in FIG. 12, when the matching is performed, the simulation of the surface acoustic wave device 1 is performed by the first simulation.
As shown in FIG. 3, it is possible to obtain the same frequency characteristics as in the case shown in FIG.

Further, the actual result has a frequency characteristic as shown in FIG. 14, and the same result as the simulation shown in FIG. 13 can be obtained.

In the above embodiment, LBO is used for the piezoelectric substrate, but the same effect can be obtained with other piezoelectric substrates.

In the above-described embodiment, an experiment was performed on an IF filter that requires tuning in an external circuit. However, a similar effect can be obtained with an RF (Radio Frequency) filter driven by a pure 50Ω.

It should be noted that the present invention is not limited to the above-described embodiment, and can be variously modified and implemented without departing from the scope of the invention.

[0057]

As described in detail above, according to the present invention,
First, the band characteristic is such that the propagation characteristic has directionality,
An RS in which a pair of SPUDTs each having a unidirectional electrode structure are arranged so as to face each other so that the propagation directions are opposite to each other.
It is formed by the frequency characteristics of the PUDT.

The bandwidth is defined as the ratio of the SPUDT comb-shaped electrode having the propagation characteristics in the forward direction to the SPUDT comb-shaped electrode having the propagation characteristics in the reverse direction, ie, the resonance cavity. By changing, if within the trap defined by the logarithm of the comb-shaped electrode,
It can be controlled freely, and it is possible to obtain low loss, wide band and steep skirt characteristics.

[Brief description of the drawings]

FIG. 1 is a view for explaining an embodiment of a surface acoustic wave device according to the present invention.

FIG. 2 is a plan view illustrating an example of a comb-shaped electrode structure of the SPUDT in the embodiment.

FIG. 3 is a diagram illustrating a phase relationship between resonance frequencies of two surface acoustic wave filters according to the embodiment.

FIG. 4 is a view for explaining another phase relationship between resonance frequencies of two surface acoustic wave filters according to the embodiment.

FIG. 5 is a view for explaining frequency characteristics of the surface acoustic wave device according to the embodiment.

FIG. 6 is a view for explaining a surface acoustic wave device as a comparative example of the surface acoustic wave device according to the embodiment.

FIG. 7 is a view for explaining frequency characteristics of the surface acoustic wave device of the comparative example.

FIG. 8 is a view for explaining another frequency characteristic of the surface acoustic wave device of the comparative example.

FIG. 9 is a view for explaining frequency characteristics of one surface acoustic wave filter of the surface acoustic wave device according to the embodiment.

FIG. 10 is a view for explaining frequency characteristics of the other surface acoustic wave filter of the surface acoustic wave device according to the embodiment.

FIG. 11 is a diagram illustrating a frequency characteristic obtained by combining two surface acoustic wave filters of the surface acoustic wave device according to the embodiment.

FIG. 12 is a block diagram showing a state where an external circuit is connected to the surface acoustic wave device according to the embodiment.

FIG. 13 is a view for explaining a state in which the frequency characteristics of the surface acoustic wave device to which the external circuit is connected in the embodiment are simulated;

FIG. 14 is a view for explaining actual frequency characteristics of the surface acoustic wave device to which an external circuit is connected in the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave device, 2 ... Input terminal, 3 ... Normal SPUDT, 4 ... Reverse SPUDT, 5 ... RSPUDT, 6 ... Output terminal, 7 ... Reverse SPUDT, 8 ... Regular SPUDT, 9 ... RSPUDT, 10 ... Piezoelectric substrate 11: Normal SPUDT, 12: Reverse SPUDT, 13: RSPUDT, 14: Reverse SPUDT, 15: Normal SPUDT, 16: RSPUDT.

Claims (8)

[Claims] 1. A semiconductor device comprising two or more transducers formed on a piezoelectric substrate and provided with a plurality of regions each having a pair of comb-shaped electrodes and having a direction in which the propagation direction of a surface wave is opposite to each other. A surface acoustic wave device wherein at least two of the transducers are connected in parallel with each other. 2. The surface acoustic wave device according to claim 1, wherein each of the transducers has a triple mode resonance frequency characteristic. 3. The resonance frequency of one of the transducers connected in parallel to Fl.
Assuming that the resonance frequencies of the other transducers are Fl2, Fc2, and Fu2, Fl1 <Fl2 <Fc2 <Fc1 <Fu1
3. The surface acoustic wave device according to claim 2, wherein the surface acoustic wave device has a relationship of Fu2. 4. The resonance frequency of one of the transducers connected in parallel is set to Fl.
Assuming that the resonance frequencies of the other transducers are Fl2, Fc2, and Fu2, the phases of Fl1 and Fl2 are opposite, the phases of Fc1 and Fc2 are opposite, and Fu1
3. The surface acoustic wave device according to claim 2, wherein the phase of Fu2 is opposite to the phase of Fu2. 5. The resonance frequency of one of the transducers connected in parallel to F1
Assuming that the resonance frequencies of the other transducers are Fl2, Fc2 and Fu2, six resonance frequencies Fl1 and Fc
3. The surface acoustic wave device according to claim 2, wherein at least four resonance frequencies among 1, Fu1, Fl2, Fc2, and Fu2 are substantially equal. 6. The resonance frequency of one of the transducers connected in parallel to Fl.
Assuming that the resonance frequencies of the other transducers are Fl2, Fc2 and Fu2, six resonance frequencies Fl1 and Fc
3. The surface acoustic wave device according to claim 2, wherein at least four resonance frequencies of 1, Fu1, Fl2, Fc2, and Fu2 have substantially equal insertion loss values. 7. The surface acoustic wave device according to claim 1, wherein one of the transducers connected in parallel and the other transducer are formed on the same chip. 8. The surface acoustic wave device according to claim 1, wherein one of the transducers connected in parallel and the other transducer are formed on different chips.