JP2000151356A - Resonator-type surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set the band width of a resonator-type SAW (surface acoustic wave) filter to a desired frequency band. SOLUTION: Serial arm SAW resonators 41a, 42a, and 43a pass signals of frequencies above a resonance frequency and below an antiresonance frequency well among given signals and do not pass well the signals of frequencies which are higher than the antiresonance frequency. Parallel arm SAW resonators 44a and 45a connected in ladder style to the SAW resonators 41a to 43a pass well the signals of frequencies above the resonance frequency and below the antiresonance frequency among given signals and do not pass well the signals of frequencies below the resonance frequency. Inductors 41b to 45b which are in series with the serial arms SAW resonators 41a to 43a and parallel arm SAW resonators 44a and 45a widen intervals of the resonance frequency and anitresonance frequency of the SAW resonators 41a to 45a respectively, so that the band width of the resonance type SAW filter is also widened.

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 used for various circuits such as communication equipment.
e, hereinafter referred to as “SAW”),
The present invention relates to a resonator type SAW filter.

[0002]

2. Description of the Related Art A SAW device has a transducer (Interdigital Transducer; hereinafter, referred to as an IDT) composed of interdigital electrodes. By processing the IDT, the SAW device has various functions and characteristics. Can be. Conventionally, SAW devices are mainly SAW devices.
It often refers to a filter, and among the SAW filters, a multi-electrode SAW filter has played a leading role. However, in recent years, research and development of a resonator-type SAW filter in addition to a multi-electrode-type SAW filter has been actively conducted, and the SAW filter does not necessarily mean a multi-electrode-type SAW filter. A resonator-type SAW filter is a filter configured using a SAW resonator based on a classic electric filter design method. The main body of the SAW resonator is an IDT, and in some cases, reflectors are provided on the left and right of the IDT. Both the IDT and the reflector are formed on a piezoelectric substrate. The reflector, like the IDT, is made up of interdigitated electrode fingers, which may be electrically short-circuited or open. The SAW excited by the IDT leaks right and left, and the reflector is provided to acoustically reflect the SAW.

FIG. 2 is a plan view showing a conventional SAW resonator. FIGS. 3A to 3C show the reflector 15 in FIG.
FIGS. 2A and 2B are schematic diagrams showing L and 15R. FIG. 1A is a schematic diagram, FIG. 1B is a short-circuit type, and FIG. 1C is an open type. The SAW resonator 10 includes an input terminal 12, an output terminal 13, and an IDT 14 having a plurality of electrode fingers formed on a piezoelectric substrate 11. Reflectors 15L and 15R are formed on the left and right sides of the IDT 14, respectively. As the reflectors 15L and 15R, there are short-circuit type in which all the electrode fingers 15a are short-circuited as shown in FIG. 3B, or open-type reflectors in which the electrode fingers 15a are opened as shown in FIG. 3C. Is generally used. In any case, it is appropriate that the number of the electrode fingers 15a is about 50 to 100.

There are various possible locations of the reflectors 15L and 15R in order to obtain a desired impedance. However, the reflectors are spaced apart from the outermost part of the IDT 14 by a distance corresponding to a quarter wavelength of the SAW to be excited. Generally, they are arranged before and after the position. If the reflectors 15L and 15R are unnecessary, they are not formed. SAW resonator 10
In the manufacturing process, the IDT 14 and the reflectors 15L and 15L
Since R and R are formed at the same time, their film thickness and material are the same. The standard of the film thickness is several hundred to several thousand angstroms. As a material, for example, pure Al (aluminum) or an alloy mainly containing Al is generally used. In some cases, the material is pure Au (silver), pure Ti
(Titanium) or an alloy containing these as a main component may be used. The SAW resonator configured as described above has
To show impedance characteristics very similar to LC resonators,
The equivalent circuit is often approximately represented by an LC resonator.

FIGS. 4A and 4B are diagrams showing equivalent circuits and reactance characteristics of the SAW resonator shown in FIG. FIG.
4A is represented by an equivalent circuit composed of an inductor 16, a capacitor 17 and a resistor 18 connected in series between terminals, and a capacitor 19 in parallel with the SAW resonator 10 as shown in FIG. The characteristic curve of the reactance X ₀ is as shown in FIG. 4B, and has a resonance frequency Fr and an anti-resonance frequency Fa. The basic circuit in the case where a filter is formed by a SAW resonator is a one-stage ladder-type circuit.

FIGS. 5A and 5B are configuration diagrams showing a one-stage ladder-type circuit. The one-stage ladder circuit 20A shown in FIG. 5A and the one-stage ladder circuit 20B shown in FIG. 5B are both a parallel arm (arm) SAW resonator 1.
0p and a series arm SAW resonator 10s.
The parallel arm SAW resonator 10p and the series arm SAW resonator 10s are SAW resonators formed on a common piezoelectric substrate 11 as shown in FIG. Here, the configurations of the ladder circuits 20A and 20B are symmetrical to each other. That is, the impedance of the one-stage ladder circuit 20A viewed from its left terminals Ta1 and Ta2 is equal to the impedance of the one-stage ladder circuit 20B viewed from its right terminals Tb1 and Tb2, and the one-stage ladder circuit 20A is connected to the right. Impedance seen from the terminals Ta3 and Ta4 of
The impedance of the one-stage ladder circuit 20B is equal to the impedance when viewed from the left terminals Tb3 and Tb4.

When a filter is formed using a SAW resonator, one-stage ladder-type circuit 20A or 20B is selected and connected in multiple stages while considering the impedance between the ladder-type circuits. In each one-stage ladder circuit, if the anti-resonance frequency of the parallel arm SAW resonator and the resonance frequency of the series arm SAW resonator are very close to or coincide with each other, the matching state at the input terminal and output terminal of the entire system Is very good, and good bandpass filter characteristics can be obtained.

FIG. 6 shows a one-stage ladder circuit 20A of FIG.
FIG. 10 is a characteristic diagram illustrating a reactance characteristic and a transmission characteristic of the 20B. As shown by G1 in FIG. 6, the parallel arm SAW resonator 1
Anti-resonance frequency Fa at 0p reactance characteristic Xp
If the resonant frequency Frs in reactance characteristics Xs of p series-arm SAW resonator 10s for example to match, the insertion loss characteristic I ₀ of stage ladder type circuit and the reflection loss characteristic R _0, as shown in G2 in FIG. 6 Become.

When the number of stages is increased by alternately cascading the one-stage ladder-type circuits shown in FIGS. 5A and 5B, the attenuation on both the left and right sides of the pass band increases significantly, but the insertion loss in the pass band also increases. To increase. Therefore, the number of stages of the one-stage ladder circuit required for the configuration of the filter is determined by the characteristics of the filter, for example, the level of the insertion loss in the pass band and the amount of out-of-band attenuation. Further, the number of stages contributes to the filter bandwidth (pass bandwidth) to some extent, but is not a decisive factor.
The bandwidth of the filter is mainly determined by the reactance characteristics Xp and Xs of the series arm SAW resonator 10s and the parallel arm SAW resonator 10p that constitute the filter. The band of the filter in which the serial arm SAW resonator 10s is connected between the input terminal and the output terminal and the parallel arm SAW resonator 10p is connected in a ladder shape has a resonance frequency Frp of the parallel arm SAW resonator 10p and a serial arm S.
It falls within the range of the half resonance frequency Fas of the AW resonator 10s, and does not exceed this range.

FIGS. 6A and 6B show examples in which the anti-resonance frequency Fap of the parallel arm SAW resonator 10p matches the resonance frequency Frs of the series arm resonator 10s. If the frequencies do not match and, for example, the anti-resonance frequency Fap of the parallel arm SAW resonator 10s is lower than the resonance frequency Frs of the series arm SAW resonator 10s, the bandwidth of the filter becomes wide, and conversely, the parallel arm SAW resonance If the anti-resonance frequency Fap of the resonator 10s is higher than the resonator frequency Frs of the series arm SAW resonator 10s, the bandwidth of the filter becomes narrow. In any of these cases, as the amount of frequency mismatch increases, the matching state in the filter circuit deteriorates, and as a result, the filter characteristics deteriorate.

While the resonator type SAW filter has the above-described characteristics, the operating frequency range legally assigned to various communication devices is not necessarily limited to the case where the matching state in the resonator type SAW filter is good. Does not match the bandwidth of Therefore, the bandwidth of the filter must be adjusted to match the usable frequency band determined by law or the like. As a result, in the case of the resonator type SAW filter, the anti-resonance frequency Fa of the parallel arm SAW resonator 10p can be reduced even at the expense of the matching state in the filter circuit.
and the resonance frequency Frs of the series arm SAW resonator 10s
Must be shifted from each other so that the bandwidth of the filter matches the assigned working frequency. However, such adjustment deteriorates the characteristics of the insertion loss I ₀ and the reflection loss R ₀ of the resonator type SAW filter.
Low loss of the filter is prevented. On the other hand, in a resonator type SAW filter actually used, a ladder type circuit 2 shown in FIG.
Although 0A and 20B are connected in three or more stages, the basic filter configuration is the same as in the case of one stage. The number of stages forming this filter affects the bandwidth but is not a decisive factor. Hereinafter, an outline of a resonator type SAW filter constituted by a multi-stage ladder type circuit will be described.

FIG. 7 shows a ladder type circuit 20A, 20 of FIG.
FIG. 4 is a configuration diagram showing a conventional four-stage resonator type SAW filter using B. This resonator type SAW filter has the structure shown in FIG.
The one-stage ladder-type circuits 20A and 20B shown in FIGS.
The terminals are connected in four stages, and terminals having the same impedance are connected so that the impedance between the stages is matched. That is, the one-stage ladder type circuits 20A, 20A
B are connected alternately and have a total of eight resonators. Here, a SAW resonator that is connected in series between the input terminal IN and the output terminal OUT to form a serial arm is called a serial arm SAW resonator, and a ladder-type connection is made to the serial arm SAW resonator. When the SAW resonator constituting the parallel arm is called a parallel arm resonator, the serial arm is composed of four serial arm SAW resonators 10s ₁ , 10s ₂ , 10s ₃ ,
It consists of 10s _4, parallel arm, four parallel arms SAW resonator _{10p 1, 10p 2, 10p 3} , 10p 4
It consists of. As a result, one set of two series-connected SAW resonator systems is formed on the serial arm, and two sets of two parallel-connected SAW resonator systems are formed on the parallel arm. In general,
Two SAW resonators connected in series or a parallel SA
The W resonator can be combined into one SAW resonator. This composite resonator is characterized in that it has almost the same impedance characteristics as two SAW resonator systems.

For example, a series arm SAW resonator 10s_Two
And series arm SAW resonator 10s_ThreeAnd the parallel
SAW resonator 10p₁And parallel arm SAW resonator 1
0p _TwoAnd a parallel arm SAW resonator 10
p_ThreeAnd parallel arm SAW resonator 10p_FourCombine with
Is as shown in FIG.

FIG. 8 is a circuit diagram showing a resonator type SAW filter after the SAW resonator shown in FIG. 7 is synthesized. Series arm S
AW resonator 10s ₂ and series arm SAW resonator 10s ₃
By combining the bets are made a new series arm SAW resonator 21, by combining the parallel arm SAW resonator 10p ₁ and parallel arm SAW resonator 10p _2, a new parallel arm SAW resonator 22 the produced, by combining the parallel arm SAW resonator 10p ₃ and the parallel arm SAW resonator 10p _4, a new parallel-arm SAW resonator 23 is fabricated. Therefore, in FIG.
In FIG. 8, a resonator type SAW filter having the same transmission characteristics and impedance characteristics as those of the filter of FIG. 7 can be obtained with five SAW resonators.

FIG. 9 is a plan view showing a configuration example of the resonator type SAW filter of FIG. This resonator type SAW filter is formed on a piezoelectric substrate 30 of, for example, lithium tantalate (LiTaO ₃ ), and has an input terminal IN and an output terminal O.
And UT, the respective SAW resonators 10s _1, 10s _4, 21~2
3 and these SAW resonators 10s ₁ , 10s ₄ , 21
To 23, and an earth bonding pattern 33.

A piezoelectric substrate 30 made of LiTaO ₃ 80
In a resonator type SAW filter in the 0 MHz band, the out-of-band attenuation is about 28 to 30 dB when the number of stages of the ladder type circuit is four. If this attenuation is required to be 35 dB or more, the number of stages of the ladder circuit must be increased to five or six, but the insertion loss of the pass band also increases in proportion to this. Therefore, the conventional resonator type SAW
If a filter is used and it is necessary to reduce the pass band to a low insertion loss, reduce the number of stages in the ladder circuit.
Out of band attenuation was sacrificed. Conversely, if a high out-of-band attenuation is required, increase the number of ladder-type circuits,
At the expense of insertion loss in the passband of the filter. When the anti-resonance frequency of the parallel arm SAW resonator and the resonance frequency of the series arm SAW resonator are matched as shown in FIG. 6, the bandwidth of the filter becomes about 20 MHz. However, when the bandwidth requirement of the filter is 25 MHz, in order to satisfy this condition, the anti-resonance frequency of the parallel arm SAW resonator constituting the filter is set to a low band of 2.5 MHz or more, and the resonance of the series arm SAW resonator is reduced. 1. Increase frequency to high frequency
It was shifted by 5 MHz or more. At this time, the insertion loss in the pass band is degraded by about 1.0 dB, and the reflection loss in the pass band is degraded by 5.0 dB or more. As described above, in the conventional resonator-type SAW filter, an essential characteristic is preferentially realized depending on a use condition by sacrificing another characteristic.

[0017]

However, the conventional resonator-type SAW filters shown in FIGS. 7 to 9 have the following problems. There are various problems in improving the performance of the SAW filter, and particularly importance is placed on reducing the loss (reducing the insertion loss in the pass band), increasing the amount of attenuation outside the pass band, and increasing the power durability. . However, in recent years, with the development of mobile communication technology, types of mobile communication devices have increased,
The management of frequency bands, which are already in short supply, has become more stringent.In addition to the performance that filters used in various communication devices have low loss, high out-of-band attenuation, and high power durability, they have been strictly defined. It is required to maintain these performances in the frequency band. In this regard, SAW
Filters have disadvantages over other filters. In other words, the pass band of the SAW filter depends on the piezoelectric substrate 3 used.
Since it is proportional to the electromechanical coupling coefficient of 0, the degree of freedom in design is limited.

At present, the types of the piezoelectric substrate 30 used for the substrate of the SAW device are limited to several types. In addition to LiTaO ₃ , lithium niobate (LiNbO ₃ ) and quartz (Si
O ₂ ), potassium niobate (KNbO ₃ ), langasite (La ₃ Ga ₅ SiO ₁₄ ) and the like are used. Since the electromechanical coupling coefficient is determined by the material of the piezoelectric substrate 30, the bandwidth of the band of the SAW filter is also substantially determined by the used piezoelectric substrate 30. Here, when the piezoelectric substrate 30 is cut in the process of processing the substrate, by adjusting the angle between the cut surface and the crystal axis, the electromechanical coupling coefficient can be slightly changed, so that the bandwidth of the SAW filter is also small. Adjustable. However, since the relationship between the cut plane and the angle of the crystal axis plays a dominant role in the insertion loss I ₀ of the SAW filter, it cannot be easily changed. Adjusting the relationship between the angles of the crystal axes may lead to deterioration of insertion loss in the pass band. According to the present invention, when a resonator type SAW filter is manufactured, the pass band of the filter can be changed without changing the angle between the cut surface of the piezoelectric substrate 30 and the crystal axis, which causes deterioration of insertion loss in the filter pass band. It is an object of the present invention to provide a resonator-type SAW filter having a configuration that can be set to a width.

[0019]

In order to solve the above problems, a first aspect of the present invention is to form a serial arm formed on a piezoelectric substrate and connected in series between input and output terminals. A serial arm SAW resonator exhibiting a unique impedance characteristic, and a parallel arm elastic surface formed on the piezoelectric substrate and connected to the serial arm by a ladder to form a parallel arm, and exhibiting a unique impedance characteristic. A resonator type SAW filter having a wave resonator and passing a signal in a frequency range based on the bandwidth of the series arm surface acoustic wave resonator and the parallel arm SAW resonator has the following configuration. . That is, the serial arm SAW resonator is connected in series to the serial arm SAW resonator.
A series arm inductor for increasing the bandwidth of the W resonator and a parallel arm inductor connected in series to the parallel arm SAW resonator for increasing the bandwidth of the parallel arm SAW resonator are provided.

In the second invention, the resonator type S of the first invention is used.
In the AW filter, the series arm inductor and the parallel arm inductor may be an inductor externally attached to the piezoelectric substrate, a spiral inductor formed on the piezoelectric substrate, or a meander inductor formed on the piezoelectric substrate. It consists of inductors selected from According to the first and second aspects of the invention, the resonator type SAW filter is configured as described above.
The resonator and the parallel arm SAW resonator pass a signal having a bandwidth based on their unique impedance characteristics, and the resonator type SAW filter uses the resonance frequency of the parallel arm SAW resonator and the anti-resonance of the series arm SAW resonator. Pass signals in the frequency range up to the frequency. Here, the series arm inductor and the parallel arm inductor are:
Since the resonance frequency and the antiresonance resonance frequency in each of the series arm SAW resonator and the parallel arm SAW resonator are changed to increase the bandwidth, respectively, the signal passing frequency bandwidth as a filter is expanded and adjusted to a desired value. .

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIGS. 1A and 1B are diagrams showing the configuration of a resonator type SAW filter according to an embodiment of the present invention. FIG. (B) is a circuit diagram. FIG. 10 (a),
(B) shows the series and parallel arm circuits 41 to 45 in FIG.
FIG. 3A is a plan view, and FIG. 3B is an equivalent circuit. The resonator type SAW filter shown in FIG. 8 in which a four-stage ladder type circuit is synthesized.
It corresponds to a filter, and the AL
And three serial arm circuits 41, 42, 43 connected in series between the input terminal IN and the output terminal OUT to form a serial arm. The parallel arm includes two parallel arm circuits 44 and 45 connected to the serial arm in a ladder form to form a parallel arm, and an earth bonding pad 46.

The serial arm circuit 41 has a serial arm SAW
It comprises a resonator 41a and a series arm inductor 41b connected thereto. The series arm circuit 42
The series arm circuit 43 includes a series arm SAW resonator 42a and a series arm inductor 42b connected thereto. The series arm circuit 43 also includes a series arm SAW resonator 43a and a series arm inductor 43b connected thereto. I have. The parallel arm circuit 44 includes a parallel arm SAW resonator 4
4a and parallel arm inductor 44b connected thereto
And the parallel arm circuit 45 also has the parallel arm SA
It comprises a W resonator 45a and a parallel arm inductor 45b connected thereto. The series arm SAW resonator 42a and the parallel arm SAW resonators 44a, 45a
It is a resonator synthesized as shown in FIG. The serial arm circuits 41 to 43 and the parallel arms 44 and 45 are connected by a transmission line pattern 47.

Each of the SAW resonators 41a to 45a is provided on a piezoelectric substrate 40 as shown in FIG.
It comprises an IDT 48 formed of L and reflectors 49 formed on both sides thereof. Inductors 41b to 45b
Are, for example, each composed of a meander-type inductor formed on the piezoelectric substrate 40, and each of the SAW resonators 41a to 41a
One end is connected to each of the IDTs 48a of 5a. Each of the inductors 41b to 45b may be constituted by a spiral type (spiral type) inductor according to the design convenience, and the dimensions of the substrate 40 forming the filter and the SAW resonator 41a
Depending on the arrangement and size of the inductors 41b to 45a,
All of b may be a spiral type, or a meander type and a spiral type may be mixed. Further, the inductor 41b
45b may be external inductors without being formed on the substrate 40. These inductors 41b
45b are connected to the input side of the IDT 48,
48 may be connected to the output side.

The equivalent circuit of each of the series and parallel arm circuits 41 to 45 is, as shown in FIG. 10B, an inductor 50 having one end connected to the terminal T1 and a circuit between the other end of the inductor 50 and the terminal T2. And the SAW resonator 60 connected to the other. The inductor 50 includes inductors 41b-
45b, and the SAW resonator 60
This corresponds to the resonators 41a to 45a. SAW resonator 60
Further, an inductor 61, a capacitor 62 and a resistor 63 connected in series between the other end of the inductor 50 and the terminal T2, and the inductor 61 and the capacitor between the other end of the inductor 50 and the terminal T2. 62 and resistor 6
3 can be represented by a capacitor 64 connected in parallel.

Next, the operation of the resonator type SAW filter will be described. First, the serial and parallel arm circuits 41 to 45
Will be described with reference to the equivalent circuit of FIG. When a high-frequency input signal is input from the terminal T1, the input signal is applied to an inductor 50 corresponding to the inductors 41b to 45b.
To reach the SAW resonator 60 via. The operating principle of the SAW resonator 41a~45a is similar to the prior art, the inductance of the inductor 50 and L _50, when the angular frequency of the input signal and omega, the impedance of the inductor 50 becomes (L ₅₀ omega) Therefore, the impedance (L ₅₀ ω) is added to the impedance of the SAW resonator 60. On the other hand, a voltage difference is generated between the electrode fingers of all the IDTs constituting each of the SAW resonators 41a to 45a to excite the SAW, and the SAW resonators 41a to 45a are formed of a crystal resonator or a conventional LC resonator. 4 shows impedance characteristics.

FIG. 11 is a characteristic diagram showing the reactance characteristics of the series and parallel arm circuits 41 to 45 of FIG.
However, in FIG. 11, the characteristic curve X ₀ in FIG. 4B is also displayed for comparison. The characteristic curve X in FIG.
_v indicates the reactance characteristic of the series or parallel arm circuits 41 to 45 in FIG. The resonance frequency Fr1 and the anti-resonance frequency Fa1 are different from those of the conventional SAW resonator 10 shown in FIG.
And the resonance frequency Fr2 and the anti-resonance frequency Fa2
Are the values of the serial and parallel arm circuits 41 to 45 in FIG. Since the impedance of the inductor 50 is positive, if this is connected in series to the SAW resonator 60, the system (that is, the serial arm circuit or the parallel arm circuit 41 to 4) is connected.
5) The overall reactance will rise in the positive direction. As a result, as shown in FIG. 11, (Fr2-Fa
2) is larger than (Fr1-Fa1). That is, S
By connecting the inductor 50 to the AW resonator 60 in series, the bandwidth (bandwidth) of this system is reduced to the conventional SA of FIG.
It is enlarged more than the W resonator 10. That is, the piezoelectric substrate 40
Resonators 41 a to 45 a determined by the electromechanical coupling coefficients of
The bandwidth (Fr1-Fa1) of a is changed to the bandwidth (Fr2-Fa2) by connecting the inductor 50 in series.
Each is spread.

Next, the operation of FIG. 1 will be described. FIG.
FIG. 2 is a characteristic diagram illustrating insertion loss characteristics of the resonator type SAW filter of FIG. 1. In FIG. 12, for the sake of comparison, the inductors 41b to 45b are connected to the SAW resonators 41a to 45a in FIG.
A characteristic curve I ₀ when b is not connected is also displayed.
The inductors 41b to 45b are connected to the SAW resonators 41a to 45b.
In the series and parallel arm circuits 41 to 45 configured to be connected in series to b, respectively, the bandwidths are wider than when the inductors 41b to 45b are not connected.
The insertion loss characteristic Iv of the resonator type SAW filter of FIG. 1 set by these bandwidths is a curve different from the characteristic curve _IO as shown in FIG. That is, in the resonator type SAW filter of FIG. 1, the bandwidth is widened as compared with the case where the inductors 41b to 45b are not connected. The bandwidth of the resonator type SAW filter shown in FIG. 1 is increased by about twice as much as the bandwidth of the SAW resonator 60 shown in FIG.

As described above, for example, even if a resonator type SAW filter is formed on the piezoelectric substrate 40 having the determined electromechanical coupling coefficient, the inductor 41b is connected to the series arm SAW resonators 41a to 43a and the parallel arm SAW resonators 44a and 45a.
To 45b can be connected in series to increase the bandwidth of the filter. Therefore, the resonator type S
Even with an AW filter, a desired bandwidth can be obtained without sacrificing insertion loss and reflection loss. As a result, this resonator type SAW filter allows signals having frequencies from the resonance frequency of the parallel arm circuits 44 and 45 to the anti-resonance frequency of the series arm circuits 41 to 43 to pass, and functions as a bandpass filter.

As described above, in the first embodiment,
Series arm SAW resonator 4 formed on piezoelectric substrate 40
1a to 43a and parallel arm SAW resonators 44a and 45a
Arm inductors 41b-43b and parallel arm inductors 44
b and 45b are connected to each other to connect the series arm circuits 41 to 4
3 and the parallel arm circuits 44 and 45, respectively, the bandwidths of the series arm circuits 41 to 43 and the parallel arm circuits 44 and 45 are widened, and the relationship between the cut surface of the substrate 40 and the angle of the crystal axis is adjusted. Instead, the resonator type SAW filter has a desired bandwidth. Therefore, a desired bandwidth can be set in a desired frequency band without deteriorating insertion loss in a pass band.

Taking advantage of such features, it can be further applied as follows. For example, in a timing extraction SAW filter, a quartz substrate is used as the piezoelectric substrate 40. Since this crystal substrate has a very small electromechanical coupling coefficient, the bandwidth of the filter is also very narrow in proportion to this, and it has been difficult to match the pass band with a desired frequency. As a result, without adjusting the frequency of the filter, the manufacturing yield was poor and the manufacturing cost was high. Similarly, in a mobile communication resonator type SAW filter,
When lithium tantalate is used as the piezoelectric substrate 40, it does not have an electromechanical coupling coefficient enough to satisfy the specifications and conditions of the filter. Then, the anti-co-frequency of the parallel arm SAW resonator is shifted to the lower frequency side, and the bandwidth is secured even at the expense of the matching state in the pass band. However, by connecting the inductor to the SAW resonator by applying the technique of this embodiment, the bandwidth of the filter can be increased, so that the above-described problem can be solved.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, there are the following modifications. (A) Although the four-stage resonator type SAW filter has been described, the number of stages may be increased or decreased according to the required specification of the amount of attenuation. (B) Although the resonator type SAW filter of FIG. 1 shows a filter in which resonator synthesis is performed, a configuration in which resonator synthesis is not performed as shown in FIG. 7 may be employed. Even when resonator combining is not performed, the same effect as in the above embodiment can be obtained by connecting an inductor to each SAW resonator. (C) The reflector 49 in FIG. 10A may be deleted if the insertion loss may be large.

[0032]

As described above in detail, according to the first and second aspects of the present invention, each series arm SAW resonator and each parallel arm SAW resonator constituting a resonator type SAW filter have a series arm inductor. And a parallel arm inductor, so that the bands of the series arm SAW resonator and each parallel arm SAW resonator are expanded. Thereby, the bandwidth of the resonator type SAW filter is widened, and a desired bandwidth can be set in a desired frequency range.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a resonator type SAW filter showing an embodiment of the present invention.

FIG. 2 is a plan view showing a conventional SAW resonator.

FIG. 3 is a configuration diagram showing reflectors 15L and 15R in FIG. 2;

4 is a diagram showing an equivalent circuit and reactance characteristics of the SAW resonator shown in FIG.

FIG. 5 is a configuration diagram showing a one-stage ladder-type circuit.

FIG. 6 is a characteristic diagram illustrating transmission characteristics of a one-stage ladder-type circuit.

FIG. 7 is a configuration diagram showing a conventional four-stage resonator type SAW filter using the ladder type circuits 20A and 20B of FIG. 5;

8 is a circuit diagram showing a resonator type SAW filter after the SAW resonator shown in FIG. 7 is synthesized.

FIG. 9 is a plan view illustrating a configuration example of the resonator type SAW filter of FIG. 8;

FIG. 10 shows series and parallel arm circuits 41 to 45 in FIG.
FIG.

FIG. 11 shows the serial and parallel arm circuits 41 to 45 of FIG. 10;
FIG. 4 is a characteristic diagram showing the reactance characteristics of the first embodiment.

FIG. 12 is a characteristic diagram showing insertion loss characteristics of the resonator type SAW filter of FIG.

[Explanation of symbols]

 Reference Signs List 40 piezoelectric substrate 41-43 series arm circuit 41a-43a series arm SAW resonator 41b-43b series arm inductor 44,45 parallel arm circuit 44a, 45a parallel arm SAW resonator 44b, 45b parallel arm inductor IN input terminal OUT output Terminal

Claims (2)

[Claims] 1. A series arm surface acoustic wave resonator formed on a piezoelectric substrate and connected in series between input and output terminals to form a serial arm and exhibiting a unique impedance characteristic, and formed on the piezoelectric substrate. And a ladder-shaped connection to the series arm to form a parallel arm, and a parallel arm surface acoustic wave resonator exhibiting a unique impedance characteristic. The series arm surface acoustic wave resonator and the parallel arm surface acoustic surface In a resonator type surface acoustic wave filter that passes a signal in a frequency range based on the bandwidth of a wave resonator, a bandwidth of the series arm surface acoustic wave resonator connected in series to the series arm surface acoustic wave resonator A parallel arm surface acoustic wave resonator connected in series with the parallel arm surface acoustic wave resonator, Resonator type surface acoustic wave filter, characterized in that an inductor for parallel arm widening the de width provided. 2. The inductor for a serial arm and the inductor for a parallel arm, wherein the inductor is externally attached to the piezoelectric substrate, a spiral inductor formed on the piezoelectric substrate, or a meander inductor formed on the piezoelectric substrate. 2. The resonator type surface acoustic wave filter according to claim 1, wherein said surface acoustic wave filter comprises an inductor selected from inductors.