JPS622724B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to interdigital electrodes used in surface acoustic wave filters.

FIGS. 1 and 2 are diagrams for explaining the outline of interdigital electrodes. In FIG. It is provided. Interdigital interdigital electrode 2 represents a weighted interdigital electrode, and interdigital interdigital electrode 3 represents a normal interdigital interdigital electrode without weighting. . In FIG. 2, piezoelectric substrate 1
A multi-strip coupler 4 is provided between the interdigital electrodes 2 weighted above. A multi-strip coupler 4 is provided to effectuate the weighting of the electrodes 2. The interdigital electrodes of FIGS. 1 and 2 operate as input or output transducers, respectively. Further, the interdigital electrodes are explained in Japanese Patent Application No. 62679/1983.

The waveform response h(t) in the time domain when impulse-exciting the interdigital electrode used to convert an electric signal into a surface acoustic wave signal or vice versa in a surface acoustic wave filter is affected by diffraction, dispersion, loss, etc. If ignored, it is given by the following equation.

Here, δ is Dirac's δ function, Xn is the distance between the position of the n-th electrode finger and the observation position, v is the surface wave propagation velocity, and an is the weighting of the electrode fingers.
As a result, the frequency characteristic H(ω) of the interdigital electrode is It is indicated by. Therefore, the weighting an of the interdigital electrodes having the required frequency characteristics is the inverse Fourier transform of the frequency characteristics. For example, the weighted envelope waveform of an to obtain a general rectangular passband characteristic for a surface acoustic wave filter is a waveform expressed by SINX/X, that is, a waveform consisting of a main lobe and several side lobes.

If this weighting is performed by changing the excitation effective aperture length of each electrode finger of the interdigital electrode, the length of the electrode finger will change depending on the inverse Fourier transform waveform of the required frequency. In this case, phase distortion of the wavefront occurs due to the difference in the propagation speed of the surface acoustic wave depending on the presence or absence of the electrode fingers. Although this can be reduced to some extent by providing pseudo electrode fingers, even in this case distortion of the wavefront in the gap between the electrode fingers cannot be prevented. Therefore, in a structure in which the effective openings of the electrode fingers are arranged on a straight line coinciding with the surface wave propagation direction, the phase distortion due to the gap is concentrated in a part of the surface wave propagation path, which is not preferable. Therefore, in order to prevent this, there is a method of arranging the effective opening on a straight line inclined to the direction of surface wave propagation as shown in Figure 3, or a method of arranging the effective opening on a straight line inclined to the direction of surface wave propagation as shown in Figure 4. A method is known in which a part of the effective opening is moved in parallel in a direction perpendicular to the opening. However, in the method shown in FIG. 4, unnecessary coupling occurs at the crossed portion p in FIG. 4 at the boundary where the arrangement is shifted, and surface waves are excited in that portion as well, resulting in deterioration of the characteristics. Furthermore, since the method shown in FIG. 3 has a structure in which unnecessary coupling parts shown in FIG. 4 are distributed over the entire interdigital electrode, the characteristics are similarly deteriorated. Although it is possible to take the above-mentioned unnecessary joints into consideration during design, the calculations become complicated and this is not practical.

Therefore, an object of the present invention is to provide an interdigital electrode used in a surface wave filter in which propagation of a wave front is not concentrated in one part, unnecessary coupling between electrode fingers is reduced, and design is easy to meet specifications. An object of the present invention is to provide interdigital electrodes.

According to the present invention, in the case of the conventional electrode arrangement as shown in FIG. 4, by omitting a part of the electrode fingers, the amount of bonding at unnecessary bonding portions is reduced. An interdigital electrode can be obtained that reduces unaccountable effects and exhibits characteristics close to those of the design.

The present invention will now be described in detail with reference to drawings of embodiments of the invention. FIG. 5 is a diagram showing an embodiment of the present invention, which is a transducer-shaped electrode in which the electrodes that cause unnecessary coupling are omitted in order to reduce the coupling at the cross portion p in FIG. 4. Since the unnecessary crossed coupling part q that also occurs in Fig. 5 has a large coupling distance, the excitation of surface waves by this part is significantly reduced compared to the interdigital electrode shown in Fig. 4. There is.

In general, in the case of interdigital electrodes with rectangular passband characteristics, the envelope waveform of the electrode fingers becomes a waveform expressed by SINX/X, so as shown in Figure 4, the effective opening is If the effective aperture length is moved in parallel at the boundary of the side lobe when creating a structure in which a part of the aperture is moved in parallel, the effective aperture length will be sufficiently small at the boundary of the side lobe, and the electrode of that part will be shown as shown in Figure 5. Even when some fingers are omitted, the deterioration in characteristics is extremely small. For example electrode index
The degree of deterioration obtained when the four electrode fingers at the boundaries of all side lobes are omitted in a filter using 610 interdigital electrodes with a multi-electrode structure with a center frequency of 37 MHz and a bandwidth of 6 MHz is 6th.
As shown in the figure, the ripple increase is only about 0.3 dB, and since this ripple also has a long period, it is possible to perform a design that compensates for the ripple in advance. FIG. 7 shows the characteristics when this compensation is performed.

In addition, in order to reduce the phase distortion of the surface wave due to the difference in surface wave propagation velocity caused by the presence or absence of the electrode finger, which is caused by omitting a part of the electrode finger, it is possible to add a structure to the pseudo electrode finger. Preferably, FIG. 8 shows a second embodiment of the present invention in which a pseudo electrode 5 is provided.

In addition, in the case of interdigital electrodes with a double electrode structure, the repetition period of the electrode fingers is approximately 1/4 of the wavelength at the center frequency of the filter, so even in the pseudo electrode fingers, the reflected waves of adjacent electrode fingers are opposite. Deterioration of characteristics can be prevented because they are combined and canceled in phases.

As explained above, the present invention has the effect of suppressing unnecessary surface wave excitation and providing an interdigital interdigital electrode with small phase distortion without requiring a particularly complicated design.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining the outline of interdigital electrodes, FIGS. 3 and 4 are diagrams showing conventional interdigital electrodes, and FIG. 5 is a diagram showing a first embodiment of the present invention. 6 and 7 are diagrams showing the amplitude characteristics of the interdigital electrode shown in FIG. 5, and FIG. 8 is a diagram showing a second embodiment of the present invention. In the figure, 1... piezoelectric substrate, 2... weighted interdigital electrode, 3... regular interdigital electrode, 4... multi-strip coupler, 5... pseudo electrode finger.

Claims (1)

[Claims] 1. An interdigital electrode consisting of a main lobe and at least one side lobe, in which the position of the side lobe is shifted in a direction perpendicular to the surface wave propagation direction with respect to the main lobe, the electrode at the boundary between the main lobe and the sub lobe. A blind-shaped electrode with a part of the finger removed.