JP2003332883A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a surface acoustic wave filter of which the attenuation characteristics and pass characteristics can be set arbitrarily concerning the surface acoustic wave filter which is used for cellular phones and other mobile communication equipment. <P>SOLUTION: The surface acoustic wave filter has resonators 71, 72, 73, and 74. A transmission line 77 equivalent to that the phase of which is rotated arbitral angle at the series resonance frequency of a series arm consisting of the resonators 74, 72, or the parallel resonance frequency of a parallel arm consisting of the resonators 73, 71 is connected between the connection point of the resonator 74 to the resonator 73, and the input terminal of the resonator 72. <P>COPYRIGHT: (C)2004,JPO

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 used in mobile communication equipment such as mobile phones.

[0002]

2. Description of the Related Art FIG. 10 shows a circuit configuration of a conventional surface acoustic wave filter. In FIG. 10, 101, 102, 10
Reference numeral 3 is a surface acoustic wave resonance element formed on a piezoelectric substrate, and 104 and 105 are an input terminal and an output terminal, respectively. Here, a band elimination filter is configured using a resonator of three elements.

When the conventional circuit configuration shown in FIG. 10 is used to construct a three-element band elimination filter using LiTaO ₃ (lithium tantalate) as the piezoelectric substrate, the characteristic becomes the characteristic shown in FIG. The equivalent circuit of the surface acoustic wave resonant element is shown in FIG. In addition,
In the figure, 111 is a parallel capacitance C0, 112 is a series equivalent inductor L1, 113 is a series equivalent capacitor C1, 114 is an equivalent resistance R1.

[0004]

However, in the above configuration, the physical constant of the capacitance ratio η = C0 / C1 in the circuit shown in FIG. 12 is determined by the material characteristics of the piezoelectric substrate. (In case of LiTaO ₃ , η = about 13) At that time, the series resonance frequency Fs and the parallel resonance frequency Fp of the surface acoustic wave resonator are Fs = 1 / (2π (L1 * C1) ^ 0.5) Fp = 1 / (2π (L1 * C1 * C2 / (C1 + C2)))
0.5). Therefore, since there is a relationship between C1 and C2 determined by the capacitance ratio η, the frequency interval between the series resonance frequency and the parallel resonance frequency of the surface acoustic wave resonance element cannot be arbitrarily set as an electrical characteristic. As a result, the attenuation characteristic and the pass characteristic, which are important in the filter characteristic, have limitations as shown in FIG.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to realize a surface acoustic wave filter whose attenuation characteristic and pass characteristic can be arbitrarily set.

In order to achieve this object, the surface acoustic wave filter of the present invention has an output terminal of a first resonator having comb-shaped interdigital electrodes facing each other on the same piezoelectric substrate and having an input terminal and an output terminal. Comb-shaped interdigital electrodes formed on the piezoelectric substrate are installed to face each other, an input terminal of a second resonator having an input terminal and an output terminal is connected, and further comb-shaped interdigital electrodes are installed to face each other on the piezoelectric substrate. An output terminal of a third resonator having an input terminal and an output terminal is connected to a comb-shaped interdigital electrode formed on the piezoelectric substrate to connect the input terminal of the fourth resonator having the input terminal and the output terminal. And the first
Between the connection point between the second resonator and the second resonator and the input terminal of the third resonator, the series resonance frequency of the series arm including the first and third resonators or the second and the third resonators. The parallel arm of the four resonators has a configuration in which a transmission line equivalent to the phase rotation at an arbitrary angle at the parallel resonance frequency is connected.

As a result, a band elimination filter or a band pass filter whose attenuation characteristic and pass characteristic can be arbitrarily set can be constructed.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The surface acoustic wave filter of the present invention comprises:
Comb-shaped interdigital electrodes are provided on the same piezoelectric substrate so as to face each other, and at least two resonators having an input terminal and an output terminal are provided, and between the output terminal of the first resonator and the input terminal of the second resonator. A transmission line is connected which is equivalent to rotate the phase at an arbitrary angle within the range of 5 degrees to 175 degrees at the series resonance frequency of the resonator, whereby a band elimination having a relatively arbitrary attenuation and pass characteristic is achieved. A filter can be constructed.

Further preferably, at least two resonators having comb-shaped interdigital electrodes facing each other are provided on the same piezoelectric substrate and having an input terminal and an output terminal, and each output terminal of the resonator is grounded. A transmission line is connected between the input terminals of the resonator so that the phase rotates at an arbitrary angle within the range of 5 degrees to 175 degrees at the parallel resonance frequency of the resonator. And a bandpass filter having a pass characteristic can be configured.

Preferably, comb-shaped interdigital electrodes are provided on the same piezoelectric substrate so as to face each other, the output terminal of the first resonator having an input terminal and an output terminal, and the comb-shaped interdigital electrode formed on the piezoelectric substrate. A third resonator having an input terminal and an output terminal connected to each other, and an input terminal of a second resonator having an input terminal and an output terminal connected to each other; Output terminal and a comb-shaped interdigital electrode formed on the piezoelectric substrate are installed so as to face each other, and an input terminal and an input terminal of a fourth resonator having an output terminal are connected to each other, and the first resonator and the second resonator are connected. Between the connection point of the resonator and the connection point of the third resonator and the fourth resonator, or the series resonance frequency of the first or third resonator or the second or fourth resonance. 5 degrees at the parallel resonance frequency It is obtained by connecting an equivalent transmission line to an arbitrary angular rotation in the range of et 175 °, which makes it possible to constitute a bandpass filter having a relatively arbitrary attenuation and pass characteristics.

It is also desirable that comb-shaped interdigital electrodes are provided on the same piezoelectric substrate so as to face each other, the output terminal of the first resonator having an input terminal and an output terminal, and the comb-shaped interdigital electrode formed on the piezoelectric substrate. A third resonator having an input terminal and an output terminal connected to each other, and an input terminal of a second resonator having an input terminal and an output terminal connected to each other; Output terminal and a comb-shaped interdigital electrode formed on the piezoelectric substrate are installed so as to face each other, and an input terminal and an input terminal of a fourth resonator having an output terminal are connected to each other, and the first resonator and the second resonator are connected. Between the connection point of the resonator and the input terminal of the third resonator at the series resonance frequency of the first or third resonator or the parallel resonance frequency of the second or fourth resonator. Ranges from 5 to 175 degrees In it is obtained by connecting an equivalent transmission line to an arbitrary angle, which makes it possible to constitute a bandpass filter having a relatively arbitrary attenuation and pass characteristics.

Further, it is desirable that the transmission line is realized by a π-type or T-type circuit in which a shunt is connected to a capacitor and a series is connected to an inductor, whereby the frequency of the pass band is doubled. It is possible to use a filter capable of removing an unnecessary signal at a high frequency of about a certain level or higher.

It is also desirable that the transmission line is realized by a π-type or T-type circuit in which an inductor is connected to a shunt and a capacitor is connected in series, whereby the frequency of the pass band is halved. A filter that can remove unnecessary frequencies at low frequencies of about 1 or less can be provided.

It is also desirable that in a circuit in which a plurality of the above circuits are connected in series, some of the transmission lines are π type or T type.
The shunt is connected to the capacitor, the series is connected to the inductor to form an equivalent structure, and the other transmission line is connected to the π type or T type with the shunt to the inductor, and the series is connected to the capacitor. This allows
High frequency about 2 times higher than passband frequency and 2
It is possible to use a filter that can remove both unnecessary signals included in low frequencies of about one-half or less.

Further, it is desirable that when a capacitor forming the transmission line is connected to the shunt, an inductor is connected in series with the capacitor so that the combined impedances thereof are equal in the pass band, or the transmission line is connected. When the capacitors to be configured are connected in series, the inductors are connected in parallel with the capacitors so that their combined admittances are equal in the pass band, or when the inductors that form the transmission line are connected to the shunt. Is a configuration in which a capacitor is connected in series with the inductor so that the combined impedance becomes equal in the pass band, or when the inductors that form the transmission line are connected in series, the combined admittance becomes equal in the pass band. As described above, the present invention is characterized by comprising one of the configurations in which a capacitor is connected in parallel with the inductor, whereby a new attenuation pole can be provided outside the band, and unnecessary signal components are more efficiently It can be a filter that can be removed selectively.

Further, preferably, the transmission line has a relative dielectric constant of 1
It is characterized in that it is formed in a laminated body of 0 or less dielectrics, and the laminated body is used as a part of an airtight sealing material for the piezoelectric substrate. A surface acoustic wave filter can be obtained.

Further, it is desirable that the transmission line is formed by printing a paste of silver or copper using the main material as a main material, whereby a surface acoustic wave filter with low loss can be obtained.

(Embodiment 1) FIGS. 1 and 2 are a circuit diagram and a characteristic diagram of a surface acoustic wave filter according to Embodiment 1 of the present invention. In the figure, 11, 12, and 13 are surface acoustic wave resonators formed on the same piezoelectric substrate, and 14
Reference numerals a and 15a are transmission lines, and 16 and 17 are input terminals and output terminals. In this embodiment, lithium tantalate is used as the piezoelectric substrate.

To explain the circuit operation here, at the attenuation frequency, the surface acoustic wave resonator 11 resonates in series and is 0 ohm, and the connection point between the surface acoustic wave resonator 11 and the transmission line 14a is grounded. Is. If the transmission line 14a
If the electrical length of is equal to 90 degrees, the impedance seen from the opposite side of the transmission line 14a (the side connected to the surface acoustic wave resonator 12) is open.

On the other hand, however, since the surface acoustic wave resonator 12 is set to almost the same frequency, the connection point between the surface acoustic wave resonator 12 and the transmission line 14a is also short-circuited. Therefore, the impedances on both sides of the connection point between the transmission line 14a and the surface acoustic wave resonator 12 are significantly different. Normally, in a high-frequency circuit, the matching condition is met when the impedance when looking at both sides from a certain point has a complex conjugate relationship, but the further it deviates from this relationship, the greater the attenuation characteristic due to mismatch in the pass characteristic. be able to. Therefore, the same effect can be expected by connecting the transmission line between the surface acoustic wave resonators.

In an actual circuit, the impedance relationship becomes complicated, and it is not necessary to simply set the phase to 90 degrees, but to optimize the characteristic impedance and the electrical length of the transmission line that functions as an impedance changing element. By adjusting the impedance conversion method, a filter with high attenuation and low loss can be obtained.

In this circuit, the transmission lines 14a and 1
The characteristic impedance of 5a and the line length at the center frequency of the pass band are 50 Ω 68 degrees and 50 Ω, respectively.
When it is set to 55 degrees, the filter characteristic as shown in FIG. 2 is obtained, and the loss in the pass band is 1.9 dB,
Good characteristics with an attenuation of 33 dB can be obtained.

A feature of the present invention is that the transmission lines 14a and 14a are
5a and the surface acoustic wave resonators 11, 12 and 13 are combined. Since these transmission lines operate as impedance conversion elements, good filter characteristics can be obtained by optimizing their characteristic impedance and electrical length. . The present embodiment shows a combination as an example. The electrical length of the transmission line is
The impedance conversion element remarkably functions as a line length of 5 to 175 degrees in electrical phase, and does not operate as an impedance conversion element at 0 and 180 degrees.

(Second Embodiment) FIGS. 3 and 4 are a circuit diagram and a characteristic diagram of a surface acoustic wave filter according to a second embodiment of the present invention. In the figure, 31 to 35 are circuit elements that function electrically equivalent to the transmission lines 14a and 15a shown in FIG. 1. In the present embodiment, a π-type low-voltage circuit in which a capacitor is connected to a shunt and an inductor is connected in series. A pass band type circuit configuration is formed.

The filter characteristic of this circuit is as shown in FIG. 4, and good characteristics almost equal to those of the first embodiment are obtained in the vicinity of the pass band, and at a high frequency, 40 dB or more. Great attenuation is obtained. The reason is that in the present invention, the transmission line is finally equivalently converted by the lumped elements of five elements, and the impedance frequency characteristics of these lumped elements act as attenuation characteristics at higher harmonics.

As described above, by using the elements whose transmission lines are equivalently connected in the π type, it is possible to easily change the elements, improve the development efficiency, and improve the yield in mass production. It is possible to have a damping characteristic on the road. The same effect can be obtained by using a T-type circuit configuration or a high-pass type circuit configuration in which an inductor is connected to the shunt and a capacitor is connected in series, but in this case, the attenuation characteristic on the low frequency side is increased. be able to.

(Third Embodiment) FIGS. 5 and 6 are a circuit diagram and a characteristic diagram of a surface acoustic wave filter according to a third embodiment of the present invention. In the figure, 51, 52 and 53 are surface acoustic wave resonators formed on the same piezoelectric substrate, and 54,
55 is a transmission line, and 16 and 17 are input terminals and output terminals. In this embodiment, lithium tantalate is used as the piezoelectric substrate.

In this circuit, transmission lines 54 and 55
The characteristic impedance and the line length at the center frequency of the pass band are 66Ω 79 degrees and 38Ω 142, respectively.
6, the filter characteristics shown in FIG. 6 are obtained, and good characteristics with a loss in the pass band of 2.7 dB and an attenuation amount of 38 dB are obtained.

A feature of the present invention is that the transmission lines 54 and 55 are
Is a combination of the surface acoustic wave resonators 51, 52 and 53. Since these transmission lines operate as impedance conversion elements, it is possible to obtain good filter characteristics by optimizing the characteristic impedance and electrical length. The present embodiment shows a combination as an example. In addition, as the electrical length of the transmission line, it is remarkable that the impedance conversion element exhibits its function.
The line length is 5 to 175 degrees as an electrical phase, and does not operate as an impedance conversion element at 0 and 180 degrees.

In this embodiment, the series resonance point of the surface acoustic wave resonator is set in the pass band, and the parallel resonator of the surface acoustic wave resonator is set in the damping region. Since the series resonance point has a frequency lower than that of the parallel resonance point, it is effective when the characteristic of mainly attenuating the upper side of the pass band is required, unlike the case of the first embodiment.

(Fourth Embodiment) FIGS. 7 and 8 are a circuit diagram and a characteristic diagram of a surface acoustic wave filter according to a fourth embodiment of the present invention. In the figure, 71 to 76 are surface acoustic wave resonators formed on the same piezoelectric substrate, 77 and 78 are transmission lines, and 16 and 17 are input terminals and output terminals. In this embodiment, lithium tantalate is used as the piezoelectric substrate.

In this circuit, the transmission lines 77 and 78
The characteristic impedance and the line length at the center frequency of the pass band are 43Ω102 degrees and 67Ω81, respectively.
8, the filter characteristic is as shown in FIG. 8, and a good bandpass characteristic with a loss in the pass band of 2.7 dB and an attenuation amount of 35 dB is obtained.

A feature of the present invention is that transmission lines 77 and 78 are provided.
And the surface acoustic wave resonators 71 to 76 are combined. Since these transmission lines operate as impedance conversion elements, good filter characteristics can be obtained by optimizing their characteristic impedance and electrical length. The present embodiment shows a combination as an example. As for the electrical length of the transmission line, the impedance conversion element remarkably exhibits its function when the line length is 5 to 175 degrees in electrical phase, and does not operate as an impedance conversion element at 0 and 180 degrees. .

Further, in the present embodiment, a resonator set assembled in a ladder type, for example, a surface acoustic wave resonator 71.
And 72 are designed as a unit, and the attenuation characteristics on the lower side of the pass band are the surface acoustic wave resonators 71, 73 and 7
5, and the design is based on the idea that the surface acoustic wave resonators 72, 74 and 76 have attenuation characteristics above the pass band.

Here, the transmission lines 77 and 78 shown in FIG. 7 are, as shown in the first and second embodiments, a π type or T type low pass type or high pass type using a capacitor and an inductor. Similar effects can be obtained by using a type equivalent circuit. In that case, it is possible to obtain an attenuation characteristic at a frequency relatively distant from the pass characteristic.

Furthermore, if one of the transmission lines is of a high-pass type and the other is of a low-pass type, a bandpass filter having a broad characteristic can be combined to obtain a characteristic that is far from the band. It is possible to more efficiently remove unnecessary signal components of. That is, since it is possible to obtain attenuation at a frequency that cannot be attenuated only by the characteristics of the surface acoustic wave resonator, conventionally, it was necessary to prepare another filter specially in some cases. There is no need.

(Fifth Embodiment) FIG. 9 is an exploded view of a surface acoustic wave filter according to a fifth embodiment of the present invention.
In the figure, reference numerals 91, 92 and 93 are laminated sheets of dielectrics, 94 is a piezoelectric substrate, and 95a and 95b are laminated bodies 9
2 is a transmission line printed on the printed circuit board, 96 is a shield electrode printed on the same, and 97 is a cap provided so as to cover the piezoelectric substrate 94.

This embodiment embodies the circuit shown in FIG. 1 in terms of a circuit, and the surface acoustic wave resonator formed on the piezoelectric substrate 94 and the transmission lines 95a and 95b are via holes. Connected at 99.

The characteristic impedances and line lengths of the transmission lines 95a and 95b are designed values by the line width and the distance between the upper and lower ground electrodes, that is, the thickness of the dielectric sheets 91 and 92 and the physical print length. ing.

Further, by providing a shield electrode in the lower portion, it is preferable to provide the shield electrode in order to stabilize the characteristic impedance and to avoid the influence of the outside, for example, the pattern of the mother substrate.

With the above structure, the dielectric sheets are laminated, so that the surface acoustic wave filter having a smaller size and higher reliability can be obtained. In addition, for example, when used in a mobile phone or the like, since a filter having a size of several millimeters is assumed, the thickness of the laminated body is about 1 mm and the line width is 0.1 mm.
If a stripline transmission line with a triplate structure is assumed, the characteristic impedance of the transmission line is 50Ω.
In the vicinity, the relative dielectric constant of the material used for lamination is 10
Those near or below are good. For example, aluminum oxide,
Forsterite, glass ceramics, etc. Further, by using silver or copper or a metal material having a high content thereof as a material forming the transmission line, a low loss filter can be obtained.

[0042]

As described above, according to the present invention, a transmission line serving as an impedance conversion element or a lumped constant circuit obtained by equivalent conversion of the transmission line and a surface acoustic wave resonator are combined to reduce the attenuation range with a small number of stages. Since the attenuation can be increased, a band stop or band pass filter having a large attenuation characteristic and a low-loss pass characteristic can be constructed.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a surface acoustic wave filter according to a first embodiment of the present invention.

FIG. 2 is a diagram showing electrical characteristics of the filter.

FIG. 3 is a circuit diagram of a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 4 is a diagram showing electrical characteristics of the filter.

FIG. 5 is a circuit diagram of a surface acoustic wave filter according to a third embodiment of the present invention.

FIG. 6 is a diagram showing electrical characteristics of the filter.

FIG. 7 is a circuit diagram of a surface acoustic wave filter according to a fourth embodiment of the present invention.

FIG. 8 is a diagram showing electrical characteristics of the filter.

FIG. 9 is an exploded configuration diagram of a surface acoustic wave filter according to a fifth embodiment of the present invention.

FIG. 10 is a circuit diagram of a conventional surface acoustic wave filter.

FIG. 11 is a diagram showing electrical characteristics of the conventional filter.

FIG. 12 is an equivalent circuit diagram of the conventional filter.

[Explanation of symbols]

11-13, 51-53, 71-76 surface acoustic wave resonators 14a, 15a, 14b, 15a, 54, 55, 77,
78 transmission line 16,17 input terminal, output terminal 91-93 dielectric sheet 94 piezoelectric substrate 95a, 95b transmission line 96 shield electrode 97 sealing cap 98 bonding wire 99 via hole

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kozo Murakami             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electronics             Parts Co., Ltd. F term (reference) 5J097 AA16 BB01 BB17 KK04 LL01

Claims (2)

[Claims] 1. A surface acoustic wave filter for increasing the amount of attenuation in the vicinity of a pass band, the first resonator having comb-shaped interdigital electrodes facing each other on the same piezoelectric substrate and having an input terminal and an output terminal. And an input terminal of a second resonator having an output terminal and a comb-shaped interdigital electrode formed on the piezoelectric substrate facing each other are connected to each other, and the comb-shaped interdigital electrode is further formed on the piezoelectric substrate. An output terminal of a third resonator having electrodes opposed to each other and having an input terminal and an output terminal;
Comb-shaped interdigital electrodes formed on the piezoelectric substrate are installed to face each other, the input terminal of a fourth resonator having an input terminal and an output terminal is connected, and the first resonator and the second resonator are connected. Between the point and the input terminal of the third resonator, the series resonance frequency of the series arm including the first and third resonators or the parallel resonance frequency of the parallel arm including the second and fourth resonators. 2. A surface acoustic wave filter having a configuration in which a transmission line equivalent to that the phase rotates by an arbitrary angle is connected. 2. A phase rotates at an arbitrary angle at a series resonance frequency of a series arm composed of the first and third resonators or a parallel resonance frequency of a parallel arm composed of the second and fourth resonators. A surface acoustic wave filter characterized by being an equivalent transmission line.