JPH04275710A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain a symmetric frequency feature by eliminating the difference of the sound speed in a weighting electrode and that in a single phase one-way electrode in a surface acoustic wave filter used for a communication unit and the like. CONSTITUTION:The weighting electrode 7 composed of an interdigital transducer 1c and a dummy electrode 6, which are provided on a piezoelectric substrate 3, and the single phase one-way electrode 8 composed of an interdigital transducer 1d and a reflector 2, which are provided on the piezoelectric substrate 3, are provided. Thus, the pitch size (m) of the interdigital transducer 1c is made longer than the pitch size (n) of the interdigital transducer 1d by a ratio between the speed of sound in the single phase one-way electrode and that in the weighting electrode, and therefore the filter feature is made symmetric.

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 with symmetrical filter characteristics used in communication equipment and the like.

0002

2. Description of the Related Art In recent years, with the progress of digitalization in the information and communication field, low loss and narrow band filters have been required for system convenience.

A conventional surface acoustic wave filter will be explained below. As shown in FIG. 3, in order to realize a low-loss filter, when a three-electrode configuration is adopted in which three single-phase unidirectional electrodes 11, 12, and 13 are disposed on the piezoelectric substrate 4, the shape Since this increases the size and cost, it is effective to use a unidirectional transducer to create a two-electrode configuration. 5 is a damping material.

As shown in FIG. 4, the two-electrode configuration
A dummy electrode 6 for correcting sound velocity deviation within the electrode is disposed on the piezoelectric substrate 4, and an interdigital transducer with a pitch dimension l.
ucer) 1a, and a single-phase unidirectional electrode 10 with an interdigital transducer 1b having the same pitch l as that of the interdigital transducer 1a and a reflector 2. This is a configuration in which . FIG. 5 shows the frequency characteristics of this two-electrode surface acoustic wave filter. The reason why the frequency characteristics are asymmetric is that there is a difference between the sound speed within the weighting electrode 9 and the sound speed within the single-phase unidirectional electrode 10.

[0005]

However, the conventional configuration described above has a problem in that the filter characteristics are asymmetrical, making it unsuitable as a channel filter for use in circuits such as digital communications.

The present invention solves the above-mentioned conventional problems, and aims to provide a surface acoustic wave filter with symmetrical filter characteristics.

[0007]

[Means for Solving the Problems] In order to achieve this object, the surface acoustic wave filter of the present invention includes a weighting electrode composed of an interdigital transducer and a dummy electrode disposed on a piezoelectric substrate, and a piezoelectric The pitch dimension of the interdigital transducer with weighted electrodes is smaller than the pitch dimension of the interdigital transducer with single-phase unidirectional electrodes. It has an elongated configuration.

[0008]

[Operation] With this configuration, the difference between the sound speed in the weighting electrode and the sound speed in the single-phase unidirectional electrode is corrected.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, on a piezoelectric substrate 3 of 35° Y-X crystal, an interdigital transducer 1c with a pitch dimension m, a weighting electrode 7 having a dummy electrode 6 arranged thereon, and a weighting electrode 7 with a pitch dimension n. It has a configuration in which a single-phase unidirectional electrode 8 on which an interdigital transducer 1d and a reflector 2 are arranged. 5 is a damping material. Electrode 7
, 8, the thickness of the aluminum film is 0.8 of the surface wave wavelength λ.
%, the sound speed on the weighting electrode 7 side is 3146 m
/s, the sound velocity on the side of the single-phase unidirectional electrode 8 having the reflector 2 is 3142 m/s. In order to correct this sound speed difference between the two electrodes, a difference in pitch between the interdigital transducers of the two electrodes is provided. That is, if the relationship between the pitch m of the interdigital transducer 1c of the weighting electrode 7 and the pitch n of the interdigital transducer 1d of the single-phase unidirectional electrode 8 is determined to be (Equation 1), as shown in FIG. The frequency characteristics of the surface acoustic wave filter of the example are symmetrical.

[0011]

[Math 1]

As described above, according to this embodiment, the relationship between the pitch m of the interdigital transducer 1c of the weighting electrode 7 and the pitch n of the interdigital transducer 1d of the single-phase unidirectional electrode 8 is expressed as (Equation 2). By determining as follows, the filter characteristics can be made symmetrical.

[0013]

[Math 2]

[0014]

Effects of the Invention As described above, the present invention makes it possible to determine the pitch dimension of the interdigital transducer of the weighting electrode from the pitch dimension of the interdigital transducer of the single-phase unidirectional electrode. With a configuration in which the length is increased by the ratio of the speed of sound within, it is possible to realize an excellent surface acoustic wave filter with symmetrical filter characteristics.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view showing the concept of the main parts of a surface acoustic wave filter according to an embodiment of the present invention.

[Figure 2] Frequency characteristic diagram of the surface acoustic wave filter

[Figure 3]
A schematic plan view showing the concept of the main parts of a conventional three-electrode surface acoustic wave filter

[Fig. 4] A schematic plan view showing the concept of the main parts of a conventional two-electrode surface acoustic wave filter.

[Figure 5] Frequency characteristic diagram of a conventional two-electrode surface acoustic wave filter

[Explanation of symbols]

1c Interdigital transducer 1d Interdigital transducer 2 Reflector 6 Dummy electrode 7 Weighting electrode 8 Single-phase unidirectional electrode

Claims (1)

[Claims] Claim 1: A single-phase unidirectional device comprising a weighting electrode composed of an interdigital transducer arranged on a piezoelectric substrate and a dummy electrode, and a weighting electrode composed of an interdigital transducer arranged on the piezoelectric substrate and a reflector. 1. A surface acoustic wave filter, comprising: an interdigital transducer of the weighting electrode; and a pitch dimension of the interdigital transducer of the weighting electrode is longer than a pitch dimension of the interdigital transducer of the single-phase unidirectional electrode.