JPH0865094A - Surface acoustic wave filter device - Google Patents

Abstract

PURPOSE: To give a high out-of-band attenuation by forming an interdigital transducer (IDT) transducing the surface acoustic wave which is not connected with the IDT by electrodes and is excited by the IDT into an electric signal. CONSTITUTION: In a symmetrical grid circuit part, a resonator 20 a is arranged between an input terminal 21a and an output terminal 22a, and a resonator 20b which is the same as this resonator 20a is arranged between an input terminal 21b and an output terminal 22b. Each of the resonators 20a to 20d is formed by the surface acoustic wave resonator having a reflector at the both ends of an IDT. In this case, grid circuit output terminals 22a and 22b are connected with the IDT 11 and IDT 13 of a resonator filter part 24 composed of three IDT. By these IDT, the surface acoustic wave is excited and the wave is again transformed into an electric signal in an IDT 12. Reflectors 7 and 8 are placed on the external side of the IDT 11 and the IDT 13. The reflectors reflect surface waves to be propagated in the outside direction in the IDT 11 and the IDT 13 and prevent the loss degradation within a passing band.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication filter using surface acoustic waves, and more particularly to a surface acoustic wave filter having a surface acoustic wave resonator arranged in a symmetrical lattice.

[0002]

2. Description of the Related Art In recent years, many surface acoustic wave devices have been used as elements such as filters, delay lines, and oscillators of electronic equipment that uses radio waves. In particular, surface acoustic wave filter devices, which are small and lightweight and have high sharp cutoff performance as filters, have become widely used as RF stage and IF stage filters for mobile terminal devices in the mobile communication field. There is a demand for a surface acoustic wave filter device having a high attenuation band near the pass band. As a structure of a surface acoustic wave filter used for mobile phones and other mobile communications, LADDER with resonators connected in a ladder
Type and comb-shaped converters (hereinafter abbreviated as IDT)
The IIDT type and the resonator type in which several IDTs are sandwiched between reflectors are used. In particular, when the degree of out-of-band suppression is emphasized, a resonator-type filter structure is often used to match the input / output impedance of the filter to 50 Ω.

A conventional resonator type filter structure will be described with reference to FIG. FIG. 6 shows a surface acoustic wave filter device in which two resonator-type filters are connected in mirror symmetry in order to improve the out-of-band attenuation in the resonator-type filter structure. In FIG. 6, pad 1 is connected to an input signal terminal, and pads 2 and 3 are grounded on the input side (hereinafter abbreviated as GND).
I am terrified. When an electric signal is input to the IDT 12 with the pads 1 and 3, surface acoustic waves are generated on the piezoelectric substrate,
This is received by IDT11 and IDT13 and converted into an electric signal again. This electric signal becomes a surface acoustic wave again by the IDT 11a and IDT 13a, is finally received by the IDT 12a and converted into an electric signal, and is output to the output signal terminal via the pad 6. The pads 4 and 5 are connected to the output side GND, and the input and output GNDs are connected to each other and have a common potential. The reflector 7 is placed outside the IDT 11 and IDT 13.
And 8 are reflectors 7 outside the IDT 11a and IDT 13a.
a and 8a are provided respectively. The frequency characteristic diagram obtained with this configuration is shown in FIG.

Such mobile communication filters are used both for reception and for transmission, and each have a stop band in the vicinity of the pass band, and high attenuation is required as the frequency characteristic here. In addition, filters have been strongly required to have lower loss in order to reduce the size and power consumption of systems such as car phones. The two-stage resonator filter shown in FIG. 6 described above has a problem in that although the out-of-band attenuation amount is good, the in-band attenuation amount is inferior.

In order to realize low loss in-band characteristics, L
There is a method of arranging a surface acoustic wave filter on the ADDER type.
The structure is shown in FIG. 8 and the frequency characteristic is shown in FIG. As shown in FIG. 9, in this case, it is easy to provide a sharp high attenuation region (notch) in the vicinity of the band, but it is difficult to obtain a high out-of-band attenuation amount over a wide frequency range to some extent. In order to obtain a high out-of-band attenuation in a wide frequency range, it is necessary to add an inductance component to the resonators arranged in parallel via bonding wires or the like. But,
The addition of the inductance component is largely restricted by the shape of the surface acoustic wave filter substrate in terms of how the bonding wire is stretched, and does not contribute to the miniaturization of the envelope.

As a method of realizing a high attenuation in the out-of-band region and a low loss in the in-band, there is a system in which an impedance element including a surface acoustic wave resonator 20 is formed in a symmetric lattice type. The structure is shown in FIG. 10 and the frequency characteristic is shown in FIG. As shown in FIG. 11, low loss is realized within the band.

[0007]

However, this symmetric lattice type filter is basically balanced input / output, and when the input / output GND is common in the package in which this filter is mounted, and when the input / output of the package is the same. Even if the GND is galvanically isolated, if it is set to a common GND on the board on which the package is mounted, this symmetric grid type is effectively a LADDER.
Is equivalent to the type. As a result, there is a problem that the out-of-band characteristic deteriorates as shown in the frequency characteristic of FIG.
Therefore, there is a problem that a surface acoustic wave filter device capable of unbalanced input / output driving while maintaining the low loss and high out-of-band attenuation of the symmetric lattice type cannot be obtained.

The present invention has been made to solve such a problem, and provides a surface acoustic wave filter device having a band having a high attenuation amount in the vicinity of a pass band, particularly a filter device for mobile communication such as an automobile telephone. The purpose is to provide.

[0009]

In a surface acoustic wave filter device of the present invention, a surface acoustic wave resonator composed of at least one IDT formed on a piezoelectric substrate is arranged in a grid pattern having input and output terminals. The IDT is formed between at least one of the input and output terminals, and the surface acoustic wave excited by the IDT that is not connected to the IDT is converted into an electric signal. It is characterized by forming an IDT.

Further, the surface acoustic wave filter device of the present invention is characterized in that the IDT formed between at least one of the input and output terminals is not connected to the IDT by an electrode, and is excited by the IDT. Another IDT that converts the generated surface acoustic wave into an electric signal forms a resonator type surface acoustic wave filter or a transversal type surface acoustic wave filter.

According to the present invention, the surface acoustic wave resonators are arranged in a lattice having input and output terminals. As shown in FIG. 1, for example, an input signal from the lattice circuit input terminal 21a is transmitted through the resonator 20a. Through the grating circuit output terminal 22a and the resonator 20c to the grating circuit output terminal 22b, and the input signal from the grating circuit input terminal 21b passes through the resonator 20b and the grating circuit output terminal 22b and the resonator 2.
It is arranged so as to output to the grid circuit output terminal 22a via 0d.

The surface acoustic wave resonator may be composed of one IDT or a plurality of IDTs. In addition, as shown in Figure 2, reflectors on both sides of the IDT
It may have Gr.

The band of the RF filter in mobile communication in which the surface acoustic wave filter device of the present invention is mainly used is 1%.
From 5%. To realize this, the piezoelectric substrate according to the present invention preferably has an electromechanical coupling coefficient of 2% or more. Specific examples include 36 ° YX lithium tantalate, 64 ° YX lithium niobate, 41 ° YX lithium niobate and the like.

[0014]

As described above, in the present invention, the GND of one of the input and output of the symmetric grid circuit is connected to the GND of the envelope through the IDT that is not electrically connected to the connected IDT, and thus the symmetric grid The type filter part does not have a common GND for input and output. Therefore, a surface acoustic wave filter device having high attenuation characteristics outside the band can be obtained.

[0015]

【Example】

Example 1 A surface acoustic wave filter device of Example 1 will be described with reference to FIGS. 1 to 3. FIG. 1 is an electrode pattern diagram of a surface acoustic wave filter device used in this embodiment. For convenience of explanation, the two input / output terminals of the symmetric lattice circuit section 23 will be described as an input terminal 21 and an output terminal 22, respectively. Whichever the input terminal 21 and the output terminal 22 are, the corresponding resonators are replaced with each other, and the structure is equivalent.

In the symmetrical lattice circuit section, the input terminal 21a
The resonator 20a is disposed between the input terminal 21b and the output terminal 22b. The resonator 20a is disposed between the input terminal 21b and the output terminal 22b. Also, the input terminal 21a and the output terminal 22b
There is a resonator 20c between them, and the same resonator 20d as this.
Has a symmetrical lattice structure arranged between the input terminal 21b and the output terminal 22a. Resonator 20a to 20d here
Are formed by surface acoustic wave resonators having reflectors at both ends of the IDT as shown in FIG. Here, the lattice circuit output terminal 22a and the lattice circuit output terminal 22b
Are connected to the IDT 11 and IDT 13 of the resonator filter section 24 composed of 3IDT. The surface acoustic waves are excited by these, and the IDT 12 converts them again into electric signals. Reflectors 7 and 8 are placed outside the IDT 11 and IDT 13 to reflect the surface wave propagating outward in the IDT 11 and IDT 13 to prevent loss deterioration in the pass band.

This filter is manufactured by forming a metal electrode on a piezoelectric substrate, and is sealed by an envelope in which the chip is put. The terminals of this package or the board to which the package is connected is generally set to a common GND for input and output. However, since the GND of the envelope is connected to the input side GND and one end of the center IDT of the 3IDT resonator filter for the filter of the present invention, it is not a common GND in the symmetrical lattice circuit section. This makes the common G
When it is set to ND, it is equivalent to short-circuiting the resonator 20b, and it is possible to prevent it from having characteristics equivalent to the LADDER type. FIG. 3 shows an example of frequency characteristics of the surface acoustic wave filter device obtained with this configuration. Good characteristics were obtained without deterioration of out-of-band attenuation at a distance from the pass band seen in the LADDER type.

Example 2 FIG. 4 is an electrode pattern diagram of a surface acoustic wave filter device of Example 2. In the second embodiment, a symmetrical ID filter is connected to each of the input and output IDTs around the 2IDT resonator filter. The resonator in the symmetrical lattice type filter section has a pattern as shown in FIG. 2 and is omitted in the figure. In this embodiment, two symmetric lattice filters are used, so that a larger out-of-band attenuation amount can be obtained and unbalanced input / output is possible.

Example 3 FIG. 5 is an electrode pattern diagram of a surface acoustic wave filter device of Example 3. In the third embodiment, a transversal surface acoustic wave filter is used instead of the resonator type surface acoustic wave filter used in the second embodiment. Therefore, it is possible to design a more delicate in-band frequency characteristic. At the same time, it can be used with unbalanced input / output while obtaining a larger out-of-band attenuation.

In the above embodiments, the symmetric lattice type circuit and the filter attached thereto are all formed on a single piezoelectric substrate as surface acoustic wave elements, but they are formed on separate substrates having different characteristics. Can also be connected by wire bonding or the like. As a result, the degree of freedom is increased in the bandwidth of the symmetric lattice type circuit using the surface acoustic wave resonator, which is restricted by the selection of the substrate, and the characteristics of the filter connected thereto.

[0021]

According to the surface acoustic wave filter device of the present invention, the surface acoustic wave resonators composed of at least one IDT formed on the piezoelectric substrate are arranged in a grid pattern having input and output terminals. I / O between at least one of the two terminals
Since it forms a DT and is not connected to this IDT by an electrode, and forms another IDT that converts the surface acoustic wave excited by that IDT into an electric signal, it is possible to input and output in non-equilibrium. In addition to having a high out-of-band attenuation, it is possible to maintain the low loss characteristic of the symmetric lattice type filter.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an electrode pattern of a surface acoustic wave filter used in Example 1.

FIG. 2 is a schematic diagram of an electrode pattern of a surface acoustic wave resonator forming a symmetrical lattice circuit.

FIG. 3 is a frequency characteristic diagram in the first embodiment.

FIG. 4 is a schematic diagram of an electrode pattern of a surface acoustic wave filter used in Example 2.

5 is a schematic diagram of an electrode pattern of a surface acoustic wave filter used in Example 3. FIG.

FIG. 6 is a schematic diagram of an electrode pattern of a conventional two-stage resonator type filter.

FIG. 7 is a frequency characteristic diagram of a conventional resonator filter having a two-stage configuration.

FIG. 8 is a structural diagram of a conventional LADDER type filter.

FIG. 9 is a frequency characteristic diagram of a conventional LADDER type filter.

FIG. 10 is a structural diagram of a filter in which impedance elements are formed in a symmetrical lattice type.

FIG. 11 is a frequency characteristic diagram of a filter in which an impedance element is formed in a symmetrical lattice type.

FIG. 12 is a frequency characteristic diagram of a symmetric lattice type filter having a common input / output GND.

[Explanation of symbols]

1 to 6 ... Pads, 7 to 10 ... Reflectors, 11 to 1
3 ... IDT, 20 ... Surface acoustic wave resonator, 21 ...
… Lattice circuit input terminal, 22 ……… Lattice circuit output terminal.

Claims (3)

[Claims] 1. At least one formed on a piezoelectric substrate
Surface acoustic wave resonators consisting of two comb-shaped transducers are arranged in a grid with input and output terminals, and a comb-shaped transducer is formed between at least one of the input and output terminals. A surface acoustic wave filter, which is not connected to the shape converter by an electrode, and which forms another comb-shaped transducer for converting the surface acoustic wave excited by the comb-shaped transducer into an electric signal. apparatus. 2. The surface acoustic wave filter device according to claim 1, wherein a comb-shaped converter formed between at least one of the input and output terminals is connected to the comb-shaped converter by an electrode. Surface acoustic wave characterized by forming a resonator type surface acoustic wave filter with another comb type transducer which converts the surface acoustic wave excited by the comb type transducer into an electric signal. Filter device. 3. The surface acoustic wave filter device according to claim 1, wherein the comb-shaped converter formed between at least one of the input and output terminals is connected to the comb-shaped converter by an electrode. Surface acoustic wave characterized by forming a transversal-type surface acoustic wave filter with another comb-type transducer that converts the surface acoustic wave excited by the comb-type transducer into an electric signal. Filter device.