JPS6219088B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in the electrode pattern of a regular transducer for a surface acoustic wave filter. In a surface acoustic wave filter made using a piezoelectric material with a high dielectric constant, the impedance of the interdigital transducer is relatively low, and it may be difficult to achieve impedance matching with circuits connected before and after the filter. Therefore, in order to increase the impedance, a method has been proposed in which the interdigital electrode of the transducer is divided into a plurality of parts in the surface wave propagation direction and electrically connected in series. Figures 1b and 1c show electrode patterns of a regular interdigital transducer constructed based on this method. Note that FIG.
3 are arranged at appropriate distances apart. Detailed electrode patterns of one of the interdigital transducers 3, which is constructed in a regular manner using the filter shown in FIG. 1A, are shown in FIGS. 2B and 2C. The transducer 3 shown in FIG. The structure is The first and third comb electrodes 4 and 6 constitute a first transducer portion on the left side, and the second and third comb electrodes 5 and 6 constitute a second transducer portion on the right side, Terminals 7 and 8 are led out from the first and second comb-shaped electrodes 4 and 5, respectively. Such a transducer 3 can obtain almost the same frequency characteristics as a normal regular type transducer before division, but since the terminals 7 and 8 are led out from the same direction, it is difficult to use a hermetic package. is difficult. On the other hand, the transducer 3' shown in figure c
Since the third comb electrode 11 is arranged in opposite directions on the left and right via the two electrode fingers 14 and 15, the first and second comb electrodes 9 and 10 are arranged in opposite directions, Terminals 12, 13 will also be placed on opposite sides of the transducer from each other. Although such a transducer 3' is suitable for hermetic packaging, it suffers from sidelobe attenuation adjacent to the passband, as shown by characteristics B, F, and J in Figures 3, 4, and 5. becomes small, which is unfavorable in terms of frequency characteristics. In each figure, characteristics A, E, and I show the results obtained with an ordinary transducer that is not divided. The present invention was made in view of the above-mentioned conventional situation, and is suitable for hermetic packaging.
Moreover, not only can deterioration of side lobes adjacent to the pass band due to division be prevented, but also the attenuation of the side lobes can be further improved. An object of the present invention is to provide a surface acoustic wave filter having a regular type transducer with high impedance. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 shows an electrode pattern of a regular interdigital transducer according to the present invention. The transducer electrode 20 is divided into transducer portions 21 and 22 via two adjacent electrode fingers 23a and 23b of the same potential, similar to the pattern of FIG. 1c. These parts 21 and 22 are electrically connected in series through fingers 23a and 23b, and the capacitance formed by part 21 and the capacitance formed by part 22 are connected in series, and the entire transducer 20 The capacity of the transducer is reduced to 1/4 compared to the capacity of the transducer before division. The fingers 23a and 23b constitute a zero vibration source 23. The first transducer part 21 has three successive fingers 24a, 24b and 24c, 25 at the same potential.
It is composed of a, 25b and 25c. Zero vibration source 2
It includes consecutive zero vibration source groups 24 and 25. 26,2
Reference numerals 7 and 28 indicate an oscillation source group having an appropriate number of positive and negative oscillation sources, and these oscillation source groups 26, 27, and 28 are sequentially arranged with the zero oscillation source groups 24 and 25 in between. The second transducer section 22 similarly has three consecutive fingers 29a, 29b and 2 at the same potential.
It includes two consecutive zero oscillation source groups 29 and 30 composed of 9c, 30a, 30b and 30c. Source group 31, 3 having an appropriate number of positive and negative sources
2 and 33 are arranged with zero vibration source groups 29 and 30 interposed between them. These source groups 29,3
0, 31, 32 and 33 are arranged symmetrically with the first part 21 with respect to the zero vibration source 23. The vibration source of this transducer 20 is shown in symbols as follows:
"1, -1, 1, 0, 0, -1, 1, 0, 0, -1,
1, -1, 1, -1, 1, -1, 0, -1, 1, -
1, 1, -1, 1, -1, 0, 0, 1, -1, 0,
0, 1, -1, 1''. Here, an oscillation source is generated between adjacent electrode fingers when an electrical signal or surface wave is applied to a transducer, and when a certain electric field is applied between the fingers, the oscillation source is An oscillation source that has an electric field in the opposite direction is called a ``negative oscillation source,'' and when adjacent fingers are at the same potential, no surface waves are excited or received, so it is called a ``zero oscillation source.'' . A "positive oscillation source" is indicated by a symbol "1", a "negative oscillation source" is indicated by a symbol "-1", and a "zero oscillation source" is indicated by a symbol "0". Assuming that the center frequency of the pass band is 58 MHz, the attenuation characteristics of this embodiment are as shown by characteristic C in FIG. 3, and the amount of attenuation of the side ropes adjacent to the pass band is as large as 23 dB. Incidentally, characteristic A indicates the characteristic of an ordinary regular type transducer which is not divided, and characteristic B indicates the characteristic of a transducer which is divided into two as shown in FIG. 1c. The side lobe attenuation of characteristic A is 13 dB, and that of characteristic B is:
It is 11.5dB. As is clear from comparing the characteristics A, B, and C, when dividing into two using the conventional method (Fig. 1 c), the side lobe attenuation is 2.5 dB worse than that of the original transducer, whereas this example Characteristic C not only prevents sidelobe deterioration due to division, but also reduces sidelobe attenuation by 10dB compared to the original sidelobe attenuation.
It also improves. In this way, when dividing the transducer into two parts via the zero vibration source, a zero vibration source group with two consecutive zero vibration sources is formed in each divided transducer part, and this zero vibration source group is sandwiched between the transducer parts. Since an oscillation source group having positive and negative oscillation sources is arranged, the amount of attenuation of the side lobe adjacent to the pass band becomes significantly large. As shown below, the inventor assumed various transducers and calculated the sequential attenuation. It was discovered that the amount of attenuation of the lobe becomes significantly large. Tables 1, 2, and 3 show the results of examining the amount of attenuation of side lobes adjacent to the passband for various transducers constructed based on the present invention. In the table, the symbol indicating the source is omitted from the display of a group of two consecutive zero sources, for example, the one shown in Figure 2 is expressed as "3-2-7-0", and any symbol other than 0 is used. The number of years indicates the number of positive and negative vibration sources, and since it is symmetrical, one side is omitted. Furthermore, the expressions ``undivided'' and ``normally divided into two'' indicate an original transducer that is not divided, and a transducer that is divided into two as shown in FIG. 1c, respectively.
In Table 1, the center frequency is 58MHz, and in Table 2, the center frequency is 58MHz.
45MHz, Table 3 shows 38MHz, and each table was examined under conditions such that the pass band width was almost the same. As is clear from each table, the products based on the present invention differ in degree compared to the conventional products, but
Both have improved sidelobe attenuation.

【table】

【table】

【table】

[Table] Figures 3, 4, and 5 show some attenuation characteristics appropriately selected from each table, and the correspondence between the figures and the table is indicated by an alphanumeric symbol. As described above, according to the present invention, it is possible to obtain a surface wave filter equipped with a regular interdigital transducer having high impedance and a large amount of sidelobe attenuation, and furthermore, hermetic encapsulation is extremely easy.

[Brief explanation of the drawing]

Figure 1a is a schematic diagram of a general surface wave filter.
Figures b and c both show conventional split-type electrode patterns, Figure 2 shows an example of the electrode pattern of a regular transducer in a surface wave filter according to the present invention, Figures 3 and 4. and the fifth
Both figures are diagrams showing attenuation characteristics.

Claims (1)

[Claims] 1 A regular type transducer of a surface acoustic wave filter is divided into two parts via a zero vibration source, and each divided transducer part has a group of two or more vibration sources including at least one positive vibration source or negative vibration source. A surface acoustic wave filter characterized in that two consecutive zero vibration sources are coupled through a group of zero vibration sources.