JPS6114688B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter that removes unnecessary signal waves.

Generally, as shown in FIG. 1, a surface acoustic wave filter has an input electrode 2 for converting an electric signal into a surface acoustic wave, a surface acoustic wave propagation path 4, and an elastic surface on the surface of a piezoelectric substrate 1. It consists of an output electrode 3 for converting waves back into electrical signals. Reference numeral 5 indicates a high frequency signal source supplied to the input electrode 2, and reference numeral 6 indicates a resistor connected to the output electrode 3.

As the input or output electrodes, interdigital electrodes having a structure in which interdigitated electrodes 7 and 8 cross each other as shown in FIG. 2 are usually used. The in-band characteristics of this surface acoustic wave filter are determined by the shape of the interdigital electrodes used as the input and output electrodes.
That is, it has already been shown by C.S. Hartman et al. (CS.
Hartmann et.al, IEEE Trans. MTT-21, 4,
162 (1973)). However, when a surface acoustic wave filter is created using the above method, a large unnecessary signal response occurs at the trap frequency. This is because the surface acoustic waves excited by the intersecting widths of the interdigital electrodes, which are equal to or smaller than the wavelength, do not propagate in only one direction, but approach omnidirectionality, causing diffraction effects. It is thought that this is due to the occurrence of
1, 35 (1971)) proposed the ASPW method (Angular
Spectrum of Plane Wave Method) is commonly used and provides a good explanation of the above-mentioned surface acoustic wave diffraction phenomenon. In order to compensate for this diffraction effect in surface acoustic wave filters, conventionally,
In Figure 1, the aperture length b of the input electrode and the aperture length c of the output electrode were set at an appropriate ratio (JD
Maines et.al.Electronics Lett.8, 17 (1972)),
This method has the disadvantage that the surface acoustic wave filter becomes long and large, making the device expensive.

An object of the present invention is to provide a surface acoustic wave filter that eliminates unnecessary signals caused by the above-mentioned surface acoustic wave diffraction phenomenon, improves the problems of the prior art, and has good frequency characteristics. .

The main points of the present invention will be explained below. When a high-frequency voltage is applied to the interdigital electrodes, an elastic strain with a width equal to the cross width of the electrodes is generated. However, in the conventional electrode configuration (Figure 2), the elastic strain along the opening surface of the electrodes is It becomes a uniform distribution. For this reason, when the electrode crossing width is less than or equal to the wavelength, a diffraction effect occurs as described above, causing deterioration of the attenuation characteristics near the trap frequency. Therefore, in the present invention, the electric field strength at the center is strengthened by providing a thick distribution of electrode width at the center of the electrode intersection portion along the aperture surface, and as a result, the elastic strain is also densely distributed at the center. Since the aggregated surface acoustic waves are concentrated at the center, the diffraction effect can be reduced and good frequency characteristics can be obtained.

FIG. 3 shows an electrode configuration according to an embodiment of the present invention.
In the figure, 9 has a thicker width at the center of the electrode.
This shows a so-called barrel-shaped electrode.

FIG. 4 shows the filter characteristics when the surface acoustic wave filter with the barrel-shaped electrode configuration according to the present invention is used as an intermediate frequency filter of a television receiver, and the filter characteristics with a conventional electrode configuration for comparison. . The electrodes in this case were 15 pairs of barrel-shaped electrodes for both the input and output electrodes, with an aperture length of 1700μ, and the input and output electrodes were placed 3400μ apart. Fourth
In the figure, a curve 10 shows a filter characteristic with a conventional electrode configuration, and a curve 11 shows a filter characteristic according to the present invention. As you can see from the figure, the audio trap frequency is 60.25MHz, and the video trap frequency is 60.25MHz.
The attenuation at 52.75MHz has been improved, especially at the audio trap frequency.
Therefore, the generation of unnecessary signals at the trap frequency is largely prevented.

Next, FIG. 5 shows an electrode configuration of another embodiment of the present invention. The electrode 12 shown in FIG. 5 has a barrel shape on only one side of the electrode, and even with this shape, the filter characteristics can be improved to a considerable extent. Furthermore, effects can be expected even if only a portion of the electrode is shaped like a barrel.

As described above, according to the present invention, the attenuation characteristics at the trap frequency can be significantly improved, and therefore, unnecessary signals at the trap frequency can be largely blocked.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the conventional general configuration of a surface acoustic wave filter, FIG. 2 is a diagram showing the structure of a conventional interdigital electrode, and FIG. 3 is a diagram showing the electrode configuration of an embodiment of the present invention. , FIG. 4 is a diagram showing the effect of the filter characteristics according to the present invention in comparison with the conventional filter, and FIG. 5 is a diagram showing the electrode configuration of another embodiment of the present invention. 1: piezoelectric substrate, 2: input electrode, 3: output electrode, 9: barrel-shaped electrode, 12: partially barrel-shaped electrode.

Claims (1)

[Claims] 1. In a surface acoustic wave device consisting of a comb-like electrode that converts an electrical signal into a surface acoustic wave, a material that propagates the generated surface acoustic wave, and a comb-like electrode that converts the surface acoustic wave into an electrical signal, A surface acoustic wave device characterized in that at least a part of the electrode has a barrel shape.