JPH0241950Y2 - - Google Patents

Description

[Detailed explanation of the idea] (Industrial application field) The present invention relates to a surface acoustic wave filter.

(Prior Art) Surface acoustic wave filters with various configurations have been proposed and put into practical use. Figure 3 shows a conventional surface acoustic wave filter (hereinafter simply referred to as a 2-electrode filter) that is composed of one surface acoustic wave transducer (hereinafter simply referred to as a transducer) for each electrical input and output. It is a diagram showing an example. In the figure, 1 is a piezoelectric substrate, 2 is an input converter, 3 is an output converter, 4 is an input terminal, 5 is an output terminal, and 6
and 7 are surface acoustic waves whose input signals are output as elastic waves from the input transducer.

Generally, the converter has bidirectionality, so that the electrical signal supplied to the input terminal 4 is transmitted to the input converter 2.
The waves are converted into two surface acoustic waves 6 and 7 which travel in two opposing directions. The surface acoustic wave received by the output transducer 3 is an elastic wave 7 that propagates to one side.
Furthermore, only 1/2 of the input signal is converted into an electrical signal, so only 1/4 of the input signal is converted into an output signal and output from the output terminal 5. For this reason, it is known that a two-electrode filter has an insertion loss of 6 dB even under ideal conditions.

Conventionally, in order to reduce the insertion loss of surface acoustic wave filters, surface acoustic wave filters called multi-electrode filters have been proposed and used (Proceedings of the 1981 National Conference of the Institute of Electronics and Communication Engineers,
No.100,). For example, as shown in FIG.
and 3 are arranged alternately at equal intervals. This structure aims to reduce insertion loss by reducing the amount of surface acoustic waves that are not converted into output signals, and increasing the total number of transducers further reduces insertion loss.

(Problems to be Solved by the Invention) FIG. 5 shows the filter characteristics of this multi-electrode filter and the two-electrode filter. In FIG. 5, frequency is plotted on the horizontal axis and loss (dB) is plotted on the vertical axis, where ₀ indicates the center frequency. As is clear from the filter characteristics in Figure 5, compared to the two-electrode filter characteristics (shown by the solid line), the filter characteristics of the multi-electrode filter (shown by the broken line) have less loss in the passband, but However, on the other hand, there was a drawback that the loss decreased sharply and significantly in the attenuation region, making it impossible to obtain a sufficient amount of attenuation in the attenuation region. For example, a 5-electrode configuration filter, 7
The attenuation amounts in the attenuation range of the electrode configuration filter and the 9-electrode configuration filter are degraded by approximately 10 dB, 15 dB, and 20 dB, respectively, compared to the 2-electrode configuration filter.

As described above, the reason why a sufficient amount of attenuation cannot be obtained in the attenuation region of the filter characteristic is considered to be as follows.
That is, in general, in surface acoustic wave filters, in order to obtain a large amount of attenuation, the transducer is designed so that the attenuation poles of the filter characteristic are approximately equally spaced Δ ₁ , and the design also includes a two-electrode filter. It is being carried out.

On the other hand, in a multi-electrode filter, if l is the surface acoustic wave propagation distance between the centers of the transducer, λ is the wavelength of the surface acoustic wave, and ₀ is the center frequency of the filter, then the frequency band corresponding to the attenuation range of the filter is The period Δ ₂ at which the sharp loss decrease occurs is theoretically
It is known that ₀ λ/(2l).

However, when designing the converter, the filter characteristics of a multi-electrode filter constructed by arranging a large number of transducers were not considered, so the attenuation pole period _Δ1 of the converter and the multi-electrode structure Since there is a difference between the filter's sharp loss reduction period Δ 2 and the loss reduction period Δ ₂ of the filter, the loss deteriorates in the attenuation region as described above.

Therefore, by matching the attenuation pole period Δ ₁ and the sharp loss reduction period Δ ₂ , it is possible to maintain low insertion loss in the passband and reduce the deterioration of the attenuation amount in the attenuation region. It is hoped that this can be kept to a minimum.

The purpose of this invention is to suppress the sharp drop in loss in the attenuation region without impairing the filter characteristics in the passband, thereby improving the surface elasticity of a multi-electrode structure with sufficient attenuation and good filter characteristics. Our goal is to provide wave filters.

(Means for Solving the Problems) In order to achieve this purpose, according to this invention, a multi-electrode structure is constructed in which a plurality of surface acoustic wave transducers having a frequency interval of attenuation poles of filter characteristics with a frequency interval Δ ₁ are arranged. In the surface acoustic wave filter, each surface acoustic wave transducer 2, 3 is connected to the surface acoustic wave transducer 2, 3, and the distance l between the centers of these surface acoustic wave transducers 2, 3 is l= ₀ λ/(2Δ ₁ ), where λ: surface acoustic wave Wavelength ₀ : As the center frequency of the filter, it is characterized by being arranged in an array.

(Function) With this configuration, the period Δ ₁ of the attenuation pole of the converter can be made to match the period Δ ₂ of the sharp loss reduction of the multi-electrode filter.
In the frequency domain, it is possible to align the position where the loss sharply decreases in the attenuation region with the position where sufficient attenuation is not achieved, and it is possible to obtain good filter characteristics.

(Embodiment) An embodiment of the surface acoustic wave filter of this invention will be described below with reference to FIG. 1 and FIGS. 2A, B, and C. Components in FIG. 1 that are the same as those shown in FIGS. 3 and 4 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

The embodiment shown in FIG. 1 is a five-electrode filter, in which an output converter 3 and an input converter 2
It has a structure in which these are alternately arranged at a constant center-to-center spacing l. In this case, the period Δ ₂ in which the loss decreases sharply is Δ ₂ = ₀ λ/
Since it is theoretically given as ( _2l ), this _interval l is given by l = ₀ λ/(2Δ ₁ ).

Therefore, in the surface acoustic wave filter of this invention, the interval l is set to ₀ λ/(2Δ ₁ ), and the transducers 2 and 3 are arranged alternately at equal pitches.

FIG. 2a is a diagram showing the filter characteristics of this multi-electrode filter, and as is clear from this diagram, a sharp decrease in loss appears every period Δ ₂ . On the other hand, FIG. 2B shows the filter characteristics designed as a two-electrode filter, in which the attenuation pole occurs at a period Δ ₁ . Therefore, since Δ ₁ = Δ ₂ in the multi-electrode filter of this invention, the filter characteristics are obtained as the sum of the characteristics shown in FIG. 2A and B, and therefore, as shown in FIG. 2C. This results in extremely good filter characteristics in which the characteristics in the passband are not impaired and the decrease in loss in the attenuation region is minimized.

Furthermore, if the number of transducers is increased to reduce insertion loss, the loss degradation in the attenuation region will become even greater, but if the multi-electrode filter is configured with the electrode structure of this invention, The effect of suppressing the deterioration of the attenuation amount is remarkable, and moreover, it is improved more than the attenuation amount in the attenuation range in the case of the two-electrode configuration.

(Effect of the invention) As is clear from the above explanation, according to the multi-electrode filter of this invention, the center-to-center spacing of each transducer can be adjusted to match the period of the attenuation pole of the filter and the reduction of sharp loss. Since the periods are set to match, the characteristics in the pass band are not impaired, and furthermore, the decrease in the amount of attenuation in the attenuation band can be minimized. Therefore, the multi-electrode filter of this invention is a surface acoustic wave filter having low loss, sufficient attenuation, and good filter characteristics.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the surface acoustic wave filter of this invention, FIGS. FIG. 2C is a diagram showing the filter characteristics of the surface acoustic wave filter of this invention, and FIGS. 3 to 5 are diagrams for explaining conventional surface acoustic wave filters. 1...Piezoelectric substrate, 2...Input converter, 3...
Output converter, 4...input terminal, 5...output terminal.

Claims (1)

[Claims for Utility Model Registration] In a surface acoustic wave filter having a multi-electrode configuration in which a plurality of surface acoustic wave transducers each having a frequency interval Δ ₁ between poles of attenuation of filter characteristics are arranged, each surface acoustic wave transducer is The spacing l between the centers of the surface acoustic wave transducers is l = ₀ λ/(2Δ ₁ ) where λ is the wavelength of the surface acoustic wave ₀ is the center frequency of the filter. filter.