JPH1065489A - Filter using surface acoustic wave element and antenna duplexer using the filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter having a ladder type filter structure using a surface acoustic wave resonator, in which a loss in a pass band is improved, and using the surface acoustic wave device. Provide a compact, lightweight and high-performance antenna duplexer. SOLUTION: A series arm is constituted by a surface acoustic wave resonator 1s and an inductance element 2 connected in series to the surface acoustic wave resonator, and is determined by an impedance characteristic of the surface acoustic wave resonator and an inductance value of the inductance element. A filter 5 using a surface acoustic wave element in which each element value is set such that the resonance frequency is located in a pass band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter using a surface acoustic wave element having a ladder type filter structure and an antenna duplexer provided with the surface acoustic wave filter.

[0002]

2. Description of the Related Art Surface acoustic wave devices are often used as high frequency filters in mobile communication equipment because of their small size, light weight, and excellent mass productivity. Surface acoustic wave devices used for mobile phones in mobile communications are required to have particularly low loss and sharp frequency characteristics.
Devices using a ted Interdigital Transducers) type filter or a surface acoustic wave resonator as an impedance element are used. As an example of the surface acoustic wave device constructed using the latter surface acoustic wave resonator among the above surface acoustic wave devices, the Institute of Electronics, Information and Communication Engineers technical report US92-5
2 (1992-09), pp. 9-16, and JP-B-56-197.
There is a technique described in Japanese Patent Publication No. 65-65. In this technique, a bandpass filter is configured by using a surface acoustic wave resonator as a series element and a parallel element of a ladder filter.
A similar technology is Electronics Letter 198.
5 years, Vol. 21, No. 25/26, pp. 1211-1212 (ELECTRONICS LETTERS 5th December 1985, Vol. 21, No.
25/26, pp. 1211-1212).

[0003]

As the simplest structure of the surface acoustic wave device having a ladder type filter structure using a surface acoustic wave resonator, the impedance element 1s of the series arm and the impedance of the parallel arm shown in FIG. Each element 1p has a band rejection type filter 5 using a surface acoustic wave resonator. By connecting an appropriate matching circuit 4 before and after the filter 1 as shown in FIG. 2, a low-loss filter can be realized. . The above surface acoustic wave device is used for an antenna duplexer of a mobile phone. For example, when the transmission band T and the reception band R are arranged on the frequency axis as shown in FIG. 11, the surface acoustic wave device operates in the reception band so that the transmission power does not go to the reception side as shown in FIG. It is configured to have blocking characteristics. The surface acoustic wave device for the transmission side filter used in the antenna duplexer needs to have a particularly low loss in the transmission band T as shown by a curve A in FIG. Is done. However, in the case of the conventional surface acoustic wave device, in order to obtain a sharp characteristic between the pass band and the stop band, the surface acoustic wave resonators 1 arranged in parallel are required.
It is necessary to reduce p, and there is a tendency for the loss to increase on the low frequency side of the passband T as shown by the curve B in FIG. Such an increase in the loss on the low frequency side of the pass band is not preferable because the transmission quality deteriorates.

[0004]

When the inductance element is connected in series to the surface acoustic wave resonator, the resonance frequency at which the impedance value becomes the minimum is determined when the impedance characteristic of the surface acoustic wave resonator is constant. It moves to a lower frequency depending on the size. This state is shown in FIG.
Shown in FIG. 9 shows a frequency-impedance characteristic of surface acoustic wave resonance, in which the horizontal axis represents frequency and the vertical axis represents impedance. A curve A indicated by a two-dot chain line indicates a change in impedance when no inductance is connected in series, and a curve B indicated by a solid line indicates impedance when an inductance is inserted in series. As is apparent from this figure, when an inductance is inserted in series, the lowest value of the impedance, that is, the resonance frequency moves to the lower frequency side. In order to solve this problem of the prior art focusing on this characteristic, the present invention provides an inductance element in series with a surface acoustic wave resonator of a series arm of a surface acoustic wave filter in which a surface acoustic wave resonator is connected to a ladder type filter structure. Was connected to form a filter. Therefore, by appropriately setting the impedance characteristics of the surface arm resonator of the series arm and the size of the inductance element to be connected so that the resonance frequency is located within the pass band, the impedance value of the entire element of the series arm is reduced. The surface acoustic wave device was configured so as to be minimum in the pass band and to reduce the pass loss.

The resonance frequency fr of a surface acoustic wave resonator in which the above-mentioned inductance elements are connected in series is approximately given by the following equations (1) and (2).

[0006]

(Equation 1)

Here, L is a resonance inductance value when the surface acoustic wave resonator is represented by a lumped equivalent circuit constant, and C is
Is the capacitance value when the surface acoustic wave resonator is represented by a lumped equivalent circuit constant, Cd is the braking capacitance value when the surface acoustic wave resonator is represented by a lumped equivalent circuit constant, and L ₁ is the value of the series inductance. Is shown.

From the above equation, values of a surface acoustic wave resonator, a series inductance element, and the like for obtaining a desired resonance frequency can be roughly set. More specifically, the value of the element is determined by simulation, experiment, or the like.
In an actual device, depending on the required frequency characteristics, the above-described structure of the series arm and the parallel arm is cascade-connected in several stages, and a matching circuit is connected to the input / output terminals so that a desired impedance value is obtained in a pass band. You.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. In the following description, 1 is a surface acoustic wave resonator, 2 is an inductance element, 3 is a package,
4 matching circuits, 5 surface acoustic wave filters according to the present invention, 6 piezoelectric substrates for surface acoustic wave elements, 7 piezoelectric substrates for inductance, 8 bonding wires, 9 bonding pads, 10 transmission filters, The receiving filter is denoted by 11, the antenna terminal is denoted by 12, the receiving terminal is denoted by 13, and the transmitting terminal is denoted by 14. FIG. 1 is a circuit diagram showing a basic configuration of a surface acoustic wave device according to a first embodiment of the present invention. The surface acoustic wave element 1 is used for a series arm 1s and a parallel arm 1p, respectively, and an inductance element 2 is connected to the series arm to form a surface acoustic wave filter 5.
The surface acoustic wave filter 5 having such a configuration can be used as a band-pass filter or a band rejection filter by connecting an appropriate matching circuit 4 to input / output terminals as shown in FIG. The matching circuit 4 is configured by combining inductance elements and capacitance elements in series or in parallel according to the input / output impedance of the surface acoustic wave filter 5.

FIG. 3 shows an example of the frequency characteristics of the band rejection type surface acoustic wave filter 5 shown in FIG. In the figure, the pass band is indicated by T and the stop band is indicated by R. For example, when applied to an antenna duplexer of a GSM mobile phone, this surface acoustic wave filter 5 should be used as a transmission filter. And a stop band R with a pass band T as a transmission band.
Is used as a reception band.

FIG. 4 is a diagram showing a basic configuration of a surface acoustic wave filter according to a second embodiment of the present invention. This embodiment has a configuration in which the filters shown in the first embodiment are connected in two stages. By connecting a matching circuit, a low loss and a good out-of-band suppression degree as shown in FIG. A simple band-pass filter. If such a band-pass filter is used as a reception filter of a transceiver, a signal in a transmission band can be blocked and only a signal in a reception band can be passed.

FIG. 6 is a perspective view showing a configuration of a surface acoustic wave filter using the surface acoustic wave element according to the present invention.
Although FIGS. 1 and 4 show only a circuit diagram as a basic configuration of the surface acoustic wave device, FIG. 6 more specifically shows a state where the surface acoustic wave device is mounted on a package.
After a conductive metal such as aluminum is formed on the piezoelectric substrate 6 such as a lithium niobate single crystal by vapor deposition or the like to a thickness of, for example, 1000 °, the surface acoustic wave resonators 1s, 1p is formed. For example, copper 8 is placed on an insulating substrate 7 such as a glass epoxy resin.
Inductance element 2 made of microstrip by plating to a thickness of 0 μm and using photolithography technology
To form The surface acoustic wave resonator substrate 6 having the first surface acoustic wave resonator 1 s and the second surface acoustic wave resonator 1 p formed on the surface thereof, and the inductance element substrate 7 on which the inductance element 2 is mounted are mounted on the package 3 made of ceramics. Each element is adhered and fixed to the bottom, and each element is connected to a bonding pad 9 provided on the package 3 by a wire 8 to form a surface acoustic wave device. In order to reduce the influence of the inductance of the wire 8, two wires are provided at one location.

FIG. 7 is a perspective view showing another specific configuration of a surface acoustic wave device according to the present invention. In this example, the inductance of the wiring wire 8 is positively used so that the wire 8 of the serial arm connecting between the first surface acoustic wave resonator 1s and the second surface acoustic wave resonator 1p is elongated. The substrate 6 was arranged, and one wire 8 was used so that the wire 8 itself effectively functions as an inductance element. According to this embodiment, since it is not necessary to use an inductance element having a large occupied area, there is a great effect in reducing the size of the surface acoustic wave device.

FIG. 8 is a conceptual diagram showing an embodiment of an antenna duplexer using a surface acoustic wave filter according to the present invention. A transmission filter 10 and a reception filter 11 are constituted by the surface acoustic wave filter according to the present invention housed in a package and a matching circuit 4 including an inductance element and a capacitance element, and these filters are integrated on one circuit board 15. By doing so, an antenna duplexer is configured. The transmission filter 10 is connected to the antenna terminal 12 and the transmission signal terminal 14, and the reception filter 11
Are connected to the antenna terminal 12 and the reception signal terminal 13.

[0015]

With respect to the surface acoustic wave filter of the present invention, the most important factor that determines the frequency characteristic of the surface acoustic wave filter is the impedance characteristic of the surface acoustic wave resonator. And the design conditions of the IDT. These piezoelectric substrates and IDTs are selected and designed in consideration of desired frequency characteristics, temperature characteristics, etc., but it is difficult to satisfy various conditions at the same time, and filter design is limited. Needless to say. For example, in the application example to the above antenna duplexer, in the case of the conventional surface acoustic wave filter shown in FIG. 10, in order to secure a desired amount of suppression of the stop band, a pass band as shown in a characteristic B of FIG. The loss on the low frequency side had to be sacrificed to some extent.

On the other hand, in the case of the surface acoustic wave filter of the present invention, since the inductance element is used, the degree of freedom in design is increased, and the impedance characteristics and the inductance element value of the surface acoustic wave resonator are appropriately set. As shown by the characteristic A in FIG. 11, the loss in the pass band can be improved as compared with the related art. Further, when an antenna duplexer is configured using the surface acoustic wave device according to the present invention, there is an effect of improving the performance by reducing the size and weight and reducing the loss.

[Brief description of the drawings]

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

FIG. 2 is a conceptual diagram showing a configuration of a surface acoustic wave filter to which a matching circuit is connected.

FIG. 3 is a characteristic diagram illustrating frequency characteristics of the device when a matching circuit is connected to the surface acoustic wave filter of the first embodiment to form a band rejection filter.

FIG. 4 is a circuit diagram showing a basic configuration of a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 5 is a characteristic diagram illustrating frequency characteristics of a filter when a matching circuit is connected to form a band-pass filter.

FIG. 6 is a perspective view showing a specific configuration of a surface acoustic wave filter according to the present invention.

FIG. 7 is a perspective view showing another specific configuration of the surface acoustic wave filter according to the present invention.

FIG. 8 is a conceptual diagram of an antenna duplexer configured using the surface acoustic wave filter according to the present invention.

FIG. 9 is a frequency-impedance characteristic diagram of a surface acoustic wave filter.

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

FIG. 11 is a diagram comparing the frequency characteristics of a surface acoustic wave filter according to the present invention with the frequency characteristics of a conventional surface acoustic wave filter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 surface acoustic wave resonator 2 inductance element 3 package 4 matching circuit 5 surface acoustic wave filter 6 surface acoustic wave resonator substrate 7 inductance substrate 8 bonding wire 9 bonding pad 10 transmission filter 11 reception filter 12 antenna terminal 13 reception Terminal 14 Transmission terminal 15 Circuit board

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazushi Watanabe 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the multimedia system development headquarters of Hitachi, Ltd. (72) Inventor Akinori Yuhara Yoshida, Totsuka-ku, Yokohama-shi, Kanagawa 292, Hitachi, Ltd. Multimedia System Development Division, Hitachi, Ltd.

Claims (4)

[Claims] 1. A filter using a surface acoustic wave element having a ladder-type filter structure in which a series arm and a parallel arm are formed by using a surface acoustic wave resonator in which interdigital electrodes are formed on a piezoelectric substrate. A filter using a surface acoustic wave element, wherein the serial arm comprises a surface acoustic wave resonator and an inductance element connected in series to the surface acoustic wave resonator. 2. The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave resonator has a series arm and a resonance frequency determined by an inductance element connected to the surface acoustic wave resonator is located within a pass band of the surface acoustic wave filter. A filter using a surface acoustic wave element in which element values of a wave resonator and the inductance element are set. 3. A filter using a surface acoustic wave element according to claim 1, wherein the surface acoustic wave resonator and the inductance element constituting the ladder type filter are contained in a single package. 4. An antenna duplexer using a plurality of filters using a surface acoustic wave element.