JPS6210045B2 - - 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 is also very small. becomes smaller. 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. It is greatly affected by dimensional variations during manufacturing, and the resulting frequency characteristics deviate greatly from the desired frequency characteristics.

In addition, when an electrode pattern is designed using a reflection method, small responses occur not only near the intersection of a set of envelopes but also in other areas, and in this case, the same period of time as described above occurs. A problem arises.

Therefore, the present invention eliminates all of the above-mentioned conventional drawbacks, and provides a surface acoustic wave filter that has low diffraction loss, suppresses the influence of manufacturing variations as much as possible, and reduces the error between the obtained frequency characteristics and the desired frequency characteristics. The purpose is to provide.

That is, the present invention is equipped with interdigital electrodes that correspond to a response having a magnitude of 6% or less compared to the maximum response in an impulse response, and have zero crossing width between adjacent electrode fingers of different potentials. be.

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, in a filter whose weighting relationship is sin x/x after inverse Fourier transform and whose bandwidth is W, the response is at positions 1/W, 2/W, 3/W, etc. on the time axis of the impulse response. It has a minimum value or intersection point. Interdigital electrodes are configured to correspond to such impulse responses.

The interdigital electrode is composed of a pair of comb-shaped electrodes inserted 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. The crossing width between adjacent electrode fingers of different potentials is the width of each response, except for a minute response that is less than 6% of the maximum impulse response (response indicated by symbol 9 in Figure 1). It is set proportionally to the size. The electrode fingers corresponding to minute responses with a magnitude of 6% or less existing near the intersection points 3, 4, 5, 6, 7, and 8 of the envelopes 1 and 2 are arranged so that they do not intersect. It is provided. An example is shown in FIG. This figure shows the vicinity of the intersection point 3 of the impulse responses in FIG. In the figure, each electrode finger is formed in a portion surrounded by a broken line to correspond to a response greater than 6% of the maximum response. A plurality of vibration sources, including a vibration source that should essentially be a substantially zero vibration source, have a magnitude of 6% or less compared to the maximum response, and are considered to be zero vibration sources. Specifically, electrode fingers 10 and 11,
Electrode fingers 11 and 12, electrode fingers 12 and 13, electrode finger 1
3 and 14, electrode fingers 14 and 15, electrode fingers 15 and 16
The interval corresponds to a response with a magnitude of 6% or less,
Opposing electrode fingers are provided spaced apart so that they do not intersect. As a result, the electrode fingers 10 and 16 located on both sides of the electrode fingers with zero crossing width are necessarily at the same potential, and no surface waves are excited in the area between them. Figure 3 shows only the envelope connecting the finger tips of the interdigital electrodes corresponding to the impulse response in Figure 1, and shows 17,1
8, 19, 20, 21, and 22 intersect and have zero width. 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, surface wave diffraction is large and it is highly susceptible to dimensional errors. This is because if a value exceeding 6% 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 it is no longer generated. Furthermore, when forming electrode fingers by photo etching or the like, variations in the finger dimensions tend to occur, and in the past, this had a relatively large effect on those with a small intersecting width, but according to this embodiment,
There is no such effect. 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.

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. Furthermore, in the above embodiment, the intersection width is set to zero for all responses corresponding to a magnitude of 6% or less, but in some cases,
You can just partially reduce it to zero. Furthermore, in order to equalize the speed of sound, it goes without saying that a dummy electrode may be added to the above embodiment.

Since the present invention has the configuration as described above, it has great practical effects in that diffraction loss is reduced, the influence of manufacturing variations is greatly reduced, and characteristics close to desired frequency characteristics can be obtained.

[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.

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.
6 compared to the maximum response in the impulse response.
% or less, the intersecting width of a plurality of electrode fingers is set to zero, and the electrode fingers located on both sides of the electrode fingers with this intersecting width of zero are set at the same potential. surface acoustic wave filter.