JPS6210046B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter using interdigital electrodes.

Generally, for an apodized interdigital electrode, an impulse response is determined by inverse Fourier transform of the desired frequency characteristic, the electrode pitch is defined in correspondence with the time of this impulse response, and the adjacent difference is determined in correspondence with the magnitude of the response. It is configured by specifying the crossing width between potential electrode fingers, and this is called crossing width weighting. In an actual impulse response, a set of envelopes connecting each response often intersects, and in this case, the magnitude of the response becomes very small near the intersection, and the width of the intersection between the corresponding electrode fingers also decreases. becomes very small. In such a pair of electrode fingers with a small intersecting width, the surface waves excited between the fingers are likely to diffuse, resulting in large diffraction loss. Largely affected by dimensional variations during manufacturing,
The resulting frequency characteristics deviate significantly from the desired frequency characteristics.

In addition, when an electrode pattern is designed using a reflection method, small responses occur not only in the area where a set of envelopes intersect, but also in other areas, and in this case, the same problem as described above occurs. happens.

Therefore, the present invention eliminates all of the above-mentioned conventional drawbacks, and by forming a zero-crossing width region, the diffraction loss is minimized and the influence of manufacturing variations is suppressed as much as possible, and the frequency characteristics obtained and the desired frequency are It is an object of the present invention to provide a surface acoustic wave filter that further reduces errors in frequency characteristics by reducing errors in characteristics and minimizing differences in surface wave sound speed due to the formation of zero crossing width regions. .

That is, in the present invention, the crossing width between a plurality of adjacent electrode fingers of different potentials corresponding to a response having a magnitude of 6% or less compared to the maximum response in an impulse response is set to zero, and the crossing width of the plurality of zero crossings is set to zero. This device includes interdigital electrodes whose widths are sequentially shifted in the length direction of the electrode fingers.

Embodiments of the present invention will be described in detail below with reference to the drawings.

As is already known, the impulse response of a bandpass filter is obtained by inverse Fourier transform of a desired frequency characteristic, as shown in FIG. 1, for example. In this impulse response, the response shows minimum values at multiple locations, and a set of envelopes 1 and 2 is 3,
Intersect at 4, 5, 6, 7, 8. For example, for a filter whose weighting function obtained by inverse Fourier transform is sin x/x and whose bandwidth is W, the time axis of the impulse response is
The minimum value of the response, or the intersection point, is at the positions of 1/W, 2/W, 3/W, ………………. Interdigital electrodes are configured to correspond to such impulse responses. The interdigital electrode is constructed by inserting a pair of comb-shaped electrodes into each other, and the comb-shaped electrode has a large number of electrode fingers. Each electrode finger is arranged such that the electrode pitch (the distance between the centers of adjacent electrode fingers having different potentials) corresponds proportionally to the time interval of the impulse response. and,
The width of intersection between adjacent electrode fingers of different potentials is determined by the magnitude of each response, except for minute responses that have a magnitude of 6% or less of the maximum impulse response (response indicated by symbol 9 in Figure 1). It is set in proportion to the Intersection points 3, 4, 5 of envelopes 1 and 2,
The electrode fingers corresponding to a plurality of minute responses having a magnitude of 6% or less existing around 6, 7, and 8 are provided so as not to intersect. An example is shown in FIG. This figure shows the vicinity of the intersection point 3 of the impulse responses in FIG. In the same figure,
In the area surrounded by the broken line, each electrode finger is formed to correspond to a response greater than 6% of the maximum response. Then, electrode fingers 10 and 11, electrode finger 11
and 12, electrode fingers 12 and 13, electrode fingers 13 and 14,
The electrode fingers 14 and 15 and the electrode fingers 15 and 16 correspond to a response with a magnitude of 6% or less, and opposing electrode fingers are spaced apart so that they do not intersect. Moreover, each adjacent electrode finger 10 and 11,
The zero-crossing width portions between 12 and 13, . ing. Other zero crossing width areas 4 to 8
Also, as shown in Fig. 3, within each region, the zero crossing width portions are sequentially shifted along a certain slope line (the line may be different from the above line) in the same way as above. . The regions 3 to 8 may also be shifted from each other as much as possible. Also. The portion other than the zero crossing width may also be shifted along a line inclined to the propagation direction. Note that in the above example, the zero intersection width portions of each area are shifted in the same direction, but by 6%.
Depending on the shape of the electrodes, a portion of the zero-crossing width portion may be shifted along a line that slopes in the opposite direction.In short, it is sufficient to shift the zero-crossing width portion so that they do not overlap as much as possible in the propagation direction. . However, depending on the electrode shape, some of the zero-crossing width regions may overlap in the propagation direction, and the zero-crossing width regions may partially overlap in the propagation direction, but as a whole, the effect of the sound speed difference is sufficiently minimized. can do. FIG. 3 shows an envelope connecting the finger tips of the interdigital electrodes corresponding to the impulse response in FIG.
4, 5, 6, 7, and 8 are indicated by a plurality of vertical lines.

In this example, only the intersection width corresponding to a response of 6% or less of the maximum response is set to zero because if the response is 6% or less, the surface wave diffraction is large and is easily influenced by dimensional errors. This is because if a value exceeding % is set to zero, the frequency characteristics may be greatly disturbed.

According to this embodiment, since there is no excitation source with a small intersecting width, the large diffraction loss caused by this is no longer generated. Furthermore, when forming electrode fingers by photo etching, etc., variations in finger dimensions tend to occur, and in the past, this had a relatively large effect especially on those with a small intersecting width. There is no impact. Therefore, the effects of diffraction loss and manufacturing variations can be suppressed as much as possible, and the desired frequency characteristics at the time of design can be brought close to the actual frequency characteristics obtained.

In general, when a surface wave has an electrode on a propagation substrate, the speed of sound there becomes relatively high, and conversely, the speed of sound in an electrode-free area becomes relatively slow.

On the other hand, when a zero crossing width portion is formed, an electrodeless portion is created, and the electrodeless portion tends to be concentrated at the center in the direction of the electrode fingers, resulting in a difference in sound speed and distortion in the wavefront. However, in this example, the zero crossing width portion is shifted in the direction of the electrode finger, so
The influence of changes in sound speed can be reduced, and desired characteristics and actual characteristics can be brought closer to each other more reliably.

Although the above-mentioned embodiment shows a single-type electrode finger, the present invention can be similarly applied to a split-type electrode finger. Additionally, when forming interdigital electrodes using a reflection method, a response with a magnitude of 6% or less compared to the maximum response may occur in areas other than the intersection of the impulse response envelopes. However, it is preferable to configure the intersection width to be zero for these responses as well, as described above. Further, in the above embodiment, the intersection width is set to zero in all cases corresponding to a response with a magnitude of 6% or less, but depending on the case, it may be possible to set it to zero only partially.

Furthermore, in order to equalize the speed of sound, it goes without saying that a dummy electrode may be added to the above embodiment. When installing dummy electrodes,
As shown in FIG. 4, dummy electrodes 18, 20, 2
2 and the electrode fingers 17, 19, 21 facing each other is G, and α is the distance by which the electrode fingers and dummy electrodes are sequentially shifted in order to disperse the zero crossing width portion, then the relationship G>α is established. need to be maintained.

Since the present invention has the configuration described above, diffraction loss is reduced, the influence of manufacturing variations is greatly reduced, and characteristics very close to desired frequency characteristics can be reliably obtained, which is a great practical effect. has.

[Brief explanation of the drawing]

The figures are for explaining the surface acoustic wave filter according to the present invention. Figure 1 is an impulse response diagram, Figure 2 is a diagram showing the main part pattern of the interdigital electrode, and Figure 3 is the envelope of the interdigital electrode. FIG. 4 is a diagram showing a main part pattern of an interdigital electrode according to another embodiment.

Claims (1)

[Claims] 1. In a surface acoustic wave filter equipped with interdigital electrodes with weighted crossing widths, the crossing width of adjacent electrode fingers of different potentials is selected in accordance with an impulse response based on desired frequency characteristics.
Compared to the maximum response in the impulse response,
The cross width of a plurality of electrode fingers corresponding to a response having a magnitude of 6% or less is set to zero, and the positions of the plurality of zero cross widths are sequentially shifted in the length direction of the electrode fingers. Surface acoustic wave filter.